Vox Meditantis

Hertha Sponer (1895–1968) pioneered the experimental foundations of quantum chemistry at a time when the field itself was barely taking shape, applying quantum mechanical principles to understand how molecules absorb light, vibrate, and break apart. During the 1920s and 1930s in Göttingen – one of the epicentres of the quantum revolution – she conducted precision spectroscopic measurements that validated emerging quantum theories and developed methodologies, including the still-ubiquitous Birge-Sponer method for calculating molecular dissociation energies. Forced to flee Nazi Germany in 1933 not because she was Jewish but because the regime deemed women unfit for academic positions, she rebuilt her career at Duke University, where she became the first woman on the physics faculty, established a molecular spectroscopy laboratory, trained 35 graduate students over three decades, and contributed to Cold War-era chemical weapons detection efforts.

Her story matters today because it illuminates how interdisciplinary innovation – particularly when performed by women – becomes structurally invisible, absorbed into the disciplines it connects whilst the creator vanishes from memory. Students worldwide calculate bond dissociation energies using the Birge-Sponer method, yet few know Sponer’s name. Her experimental work enabled theoretical breakthroughs like the Franck-Condon principle, yet histories centre the theorists. She designed experiments, ran laboratories, built departments, mentored generations of scientists, and served as associate editor of the Journal of Chemical Physics, shaping an entire field – yet she is routinely mischaracterised as James Franck’s “assistant” or remembered primarily as his second wife, though they married in 1946 when she was 51 and had already published most of her groundbreaking work. Understanding Sponer’s contributions means recognising that methodological innovation, experimental rigour, institutional building, and the translation of ideas between disciplines constitute intellectual labour as vital as dramatic theoretical insights – and that the erasure of such work reflects choices about whose contributions we value, not their actual importance.

We’re sitting in what you called the “vibration-resistant space” in the subbasement of Duke’s physics building. Tell me about this peculiar laboratory you built here.

Oh, it’s peculiar only if you’ve never tried to measure molecular spectra with precision! You see, when you’re observing the fine structure of vibrational bands – differences in energy levels separated by perhaps a few wavenumbers – any mechanical vibration from footsteps upstairs, traffic outside, even the settling of the building itself introduces noise into your spectrograph readings. The light path must remain absolutely stable. So when I arrived at Duke in 1936, I insisted on the subbasement. President Few thought I was being difficult, I suspect. But you cannot do serious spectroscopy in a space where undergraduates are tramping about overhead. The University gave me two rooms, specially reinforced. We mounted the spectrograph on a concrete pier isolated from the building foundation. It worked beautifully.

You arrived at Duke under rather extraordinary circumstances. The Rockefeller Foundation was helping displaced German scholars, but you weren’t fleeing on grounds of Jewish ancestry. What drove you out?

The Nazis held what they called a “strong prejudice against women scientists” – that’s putting it mildly, isn’t it? In 1933, when Hitler’s government issued the Law for the Restoration of the Professional Civil Service, most people think it targeted only Jewish academics. But the regime was quite clear: women had no place in universities. We were to be mothers and housewives, full stop. I had published some twenty papers by then, held an associate professorship, ran the spectroscopy laboratory at Göttingen. None of it mattered. I was dismissed in 1934.

James Franck resigned in protest the year before – he was Jewish, and could see what was coming – but I remained, foolishly hoping sanity might return. It didn’t. I went to Oslo first, as a visiting professor. Norway was kind, but the position was temporary. Then the Rockefeller Foundation contacted Duke. I’m told Robert Millikan at Caltech advised against hiring me. Millikan was quite famous, you know – he’d won the Nobel Prize. And he told President Few that appointing a woman to the physics faculty would be a mistake. Fortunately, Few ignored him.

Let’s talk about your scientific work. When you completed your doctorate with Peter Debye in 1920, quantum mechanics was still being born. What was your thesis about?

“Über die Ultrarotabsorption zweiatomiger Gase” – on the infrared absorption of diatomic gases. Debye was a superb supervisor, very sharp mathematically, though I must say he gave me considerable freedom. At the time, we were trying to understand how molecules absorb electromagnetic radiation, what that told us about their internal structure. Classical physics said molecules should vibrate at certain frequencies depending on their atomic masses and the strength of the bond between them – like a spring connecting two masses, yes? But the intensities and exact positions of the absorption bands didn’t match classical predictions.

What I did was measure the infrared spectra of simple molecules – hydrogen chloride, hydrogen bromide, carbon monoxide – with great precision, and show that their behaviour matched Niels Bohr’s quantum hypothesis: that molecules, like atoms, can exist only in discrete energy states. Energy isn’t continuous; it comes in packets. The vibrations are quantized. This was still shocking to many physicists in 1920. My measurements helped confirm it experimentally.

And then you went to Göttingen, where you worked with James Franck.

Yes, I became one of his assistants in 1921. But I must be very clear about this – I was not merely an assistant in the sense of someone who fetches equipment and records data. I designed experiments. I ran the spectroscopy laboratory. I supervised doctoral students. I published independently. Yet somehow, in the histories, this becomes “Franck’s assistant Hertha Sponer,” as though I were a technician rather than a scientist in my own right.

James was a wonderful physicist, a generous colleague, and later a dear husband. But our professional relationship in the 1920s and early 1930s was one of collaborators, not master and apprentice. The confusion arises, I think, because women working alongside famous men are presumed to be subordinate. And when you marry such a man – well! Then your own identity vanishes entirely into his shadow. We married in 1946. By then I had spent a decade at Duke, published Molekülspektren, established an entire research programme. Yet people remember me, if at all, as “Franck’s second wife.”

Tell me about your time at Berkeley in 1925. That’s when you developed the Birge-Sponer method with Raymond Thayer Birge, yes?

Correct. I received a Rockefeller Foundation fellowship – they were quite forward-thinking about supporting women scientists, I must say – and spent a year at the University of California. Raymond Birge was working on molecular spectroscopy, and he had excellent facilities. More importantly, he was willing to listen to new ideas.

What we wanted to do was find a way to calculate the dissociation energy of a molecule – the energy required to break the chemical bond completely – without having to actually observe the molecule falling apart, which is experimentally very difficult. We knew that as a molecule vibrates more and more violently, climbing up through higher vibrational energy levels, it eventually reaches a point where it dissociates. The spacing between these vibrational levels gets smaller and smaller as you approach dissociation.

So the idea was this: if you plot the differences between successive vibrational energy levels – call this ΔG – against the vibrational quantum number v, you get a straight line (or very nearly so, if you assume the molecule behaves like a Morse oscillator rather than a perfect harmonic oscillator). This line will intercept the horizontal axis at the dissociation limit. The area under that line – the sum of all those energy differences – gives you the total dissociation energy.

Can you walk me through the technical details? How would a spectroscopist actually use this method?

Of course. Let’s say you’re studying iodine vapour, I₂, which was a favourite molecule for these experiments because it has a lovely visible absorption spectrum – purple, quite distinctive.

First, you obtain the absorption spectrum, recording which wavelengths of light the molecule absorbs. Each absorption band corresponds to a transition from the ground vibrational state (v″ = 0, in the ground electronic state) to some higher vibrational state (v′, in an excited electronic state). From the wavelengths, you calculate the energy of each vibrational level.

Next, you compute the differences: ΔGv+½ = Gv+1 – Gv, where G is the term value for each vibrational level. If the molecule is anharmonic – and real molecules always are, to some degree – this quantity decreases linearly with v. So you plot ΔGv+½ against (v + 1).

You’ll get a straight line with a negative slope. Extrapolate that line until it crosses the v-axis – that’s where ΔG becomes zero, meaning the vibrational levels have converged and the molecule dissociates. Call that quantum number v_max. Now, the total dissociation energy D₀ is the sum of all those ΔG values from v = 0 up to v_max. Graphically, it’s the area of the triangle under the line.

In practice, you can calculate it from the intercepts. The y-intercept gives you ω_e (the fundamental vibrational frequency), and the slope gives you -2ω_ex_e (where x_e is the anharmonicity constant). From those, you can derive v_max and hence D₀.

What were the advantages over previous methods?

The chief advantage is that you don’t need to observe the molecule at the dissociation limit itself. High vibrational levels are sparsely populated at reasonable temperatures – Boltzmann statistics, you see – and detecting weak transitions to very high v states is experimentally challenging. The Birge-Sponer extrapolation lets you estimate the dissociation energy from measurements of the lower, more accessible vibrational levels.

The method does overestimate the dissociation energy somewhat, because the extrapolation assumes perfect linearity, and real potential energy curves deviate from the Morse model as you approach dissociation. But it gives you an upper bound, which is useful. And it’s still taught in physical chemistry courses today because it’s conceptually straightforward and requires only spectroscopic data – no calorimetry, no mass spectrometry, just light and molecules.

You mentioned the Franck-Condon principle earlier. How did your work relate to that?

This is where things get historically complicated. The Franck-Condon principle explains why, when a molecule absorbs light and jumps from one electronic state to another, certain vibrational transitions are more intense than others. It’s based on the idea that electronic transitions happen so quickly – on the order of 10⁻¹⁵ seconds – that the nuclei don’t have time to move during the transition itself. The nuclei are much heavier than electrons, you see, so they respond more slowly.

James Franck articulated this idea in the early 1920s, and Edward Condon later gave it a quantum mechanical formulation involving overlap integrals of vibrational wavefunctions. I did extensive experimental work on these intensity distributions in molecular spectra, particularly for diatomic molecules like nitrogen, oxygen, halogens. My measurements provided empirical validation for the principle.

During my fellowship at Berkeley, I taught Birge’s group – including Condon, who was a postdoctoral fellow there – how to apply quantum mechanical methods to interpret molecular spectra. I had learned these techniques in Göttingen, working with Franck, with Max Born next door hammering out the theoretical foundations. Condon was brilliant, very quick to grasp the mathematics. When he formulated the quantum version of the principle, he credited James, naturally. My name, as you say, “slipped quietly from public and scientific consciousness.”

Does that bother you?

I won’t pretend it doesn’t sting. But the issue isn’t personal slight; it’s structural. The work I did – experimental validation, methodological development, teaching and translating ideas between physics and chemistry – this is the kind of labour that becomes invisible once it succeeds. The Franck-Condon principle is a beautiful theoretical insight. The experiments that confirmed it? Background infrastructure. The Birge-Sponer method is a practical tool. The intellectual creativity required to devise it? Forgotten.

Science has a hierarchy: theory is exalted, experiment is service work. And within experiment, the person who builds the apparatus, optimises the measurements, interprets the data, trains the next generation – that person is assumed to be merely competent, not creative. Women disproportionately occupy that space. We’re the bridge-builders, and once the bridge is up and traffic flows freely, no one remembers who designed and built it.

Speaking of building, you constructed Duke’s molecular spectroscopy programme from nothing. Tell me about that.

When I arrived in 1936, Duke was a young institution – barely a decade old – and the physics department was small, still finding its footing. I had two empty rooms in the subbasement, a modest equipment budget, and an opportunity. Over the next thirty years, I supervised twelve master’s students and twenty-three doctoral students. Many of them went on to distinguished careers. That’s thirty-five scientists who learned molecular spectroscopy from me, who carried those methods into industry, academia, government research.

I also served as associate editor of the Journal of Chemical Physics twice – 1940 to 1943, and again from 1947 to 1950. That’s not a ceremonial position. You read manuscripts, evaluate methodology, decide what gets published. You shape the standards of the field. I was one of the few women in such a role at the time, and I took it seriously.

What was it like being the first woman on Duke’s physics faculty?

Lonely, at times. The men were cordial, mostly, but I was always aware of being different. At faculty meetings, my suggestions were sometimes met with a sort of bemused tolerance, as though I were a bright child rather than a professor with more publications than half the room.

But I had experienced worse in Germany. At Göttingen, even before the Nazis, there were professors who believed women’s brains were unsuited for abstract thought. Max Planck famously said that allowing women into universities would damage “the generations to come.” So Duke, for all its limitations, was a step forward. They hired me. They gave me laboratory space. They promoted me to full professor. That wasn’t trivial in 1936.

Let’s talk about your two-volume monograph, Molekülspektren und ihre Anwendung auf chemische Probleme – Molecular Spectra and Their Application to Chemical Problems. You published it in 1935 and 1936, just as you were leaving Europe. What was the impetus for that work?

By the mid-1930s, molecular spectroscopy had exploded. Every week, new papers appeared reporting spectra for different molecules, refining techniques, proposing interpretations. But there was no comprehensive reference work that pulled it all together – tabulated all the known molecular spectra, explained the quantum mechanical theory underlying them, showed how to apply spectroscopy to solve chemical problems like determining bond strengths or molecular geometries.

I wanted to create that resource. Volume I covered diatomic molecules; Volume II addressed polyatomic systems. I compiled every known molecular spectrum at the time, organised them by molecule, included the experimental methods, the quantum mechanical interpretations, everything a researcher would need. It was exhausting work – thousands of data points, checking references, writing explanations that were rigorous but accessible to chemists who weren’t quantum mechanics experts.

The book established me as the leading authority on molecular spectroscopy. It’s bittersweet, though. I published it just as I was being forced out of Europe, just as my career was being uprooted. It’s a monument to what I accomplished in Germany, and also a record of what the Nazis destroyed.

You mentioned earlier that you supervised many graduate students. What was your approach to mentorship?

I believed – I still believe – that doctoral research should teach independence, not obedience. My job was to help a student identify a well-defined problem that was significant enough to be worth solving, but narrow enough to be tractable with available equipment and within their ability. Then I stepped back. I didn’t hover. I didn’t dictate every step. I was available for consultation, of course, but the student had to learn to think like a scientist: how to design experiments, troubleshoot apparatus, interpret unexpected results, recognise when they’d made a mistake versus when they’d discovered something genuinely new.

Some of my students found this frustrating at first. They wanted recipes: do this, then that, then write up your results. But science isn’t cookery. You have to develop intuition, judgement. That only comes from making mistakes and learning to correct them yourself.

Did you ever make mistakes that, looking back, you wish you could correct?

Oh, plenty. There was a paper I published in 1927 on the spectra of nitrogen, where I misinterpreted a set of bands as belonging to a predissociation state, when in fact they were just ordinary vibrational progressions perturbed by a nearby electronic state. I realised the error a year later, after Walter Heitler and Fritz London published their quantum mechanical treatment of the chemical bond, which clarified how electronic states interact. I had to publish a correction. Embarrassing, but instructive.

More broadly, I think I underestimated, early in my career, how much my being a woman would shape how my work was received. I thought – naively – that if I just did excellent science, the recognition would follow. But excellence is necessary, not sufficient. Your work has to be seen as excellent, and who gets credit depends enormously on social context: your institutional affiliation, your connections, your gender, whether you fit the image of what a scientist looks like. I didn’t fit that image. So even when I did pioneering work, it was attributed to the men around me, or dismissed as merely technical rather than intellectually creative.

You worked on some classified projects during the Cold War, including chemical weapons detection for the U.S. Navy. Can you talk about that?

To some extent. During the 1950s, at the height of Cold War tensions, the Navy was concerned about the threat of chemical weapons – nerve agents, blister agents, things that could be deployed against ships or military installations. They needed methods to detect trace amounts of these substances quickly and reliably.

Molecular spectroscopy is ideal for this. Every molecule has a unique spectroscopic fingerprint – its rotational, vibrational, and electronic transitions occur at specific wavelengths determined by its structure. If you can obtain a spectrum of an unknown vapour, you can identify what molecules are present, even at vanishingly low concentrations, provided the spectral lines are distinct and the signal-to-noise is well managed.

What did “well managed” look like in your Cold War setups?

It meant discipline in the optics and ruthless attention to backgrounds. We used quartz optics to stay transparent deep into the ultraviolet, baffled every stray reflection, and kept the entire light path isolated on a pier to prevent microphonic noise from footsteps or lift motors coupling into the spectrograph. For trace agents, you cannot afford spurious scattering from dust or fingerprints on a lens; a single careless touch can drown a weak absorption band.

And the analytical side?

Fluorescence and absorption worked in concert. Absorption gives you line positions with high fidelity; fluorescence can buy you sensitivity because a single absorbed photon may yield many emitted photons over time if the non-radiative channels are suppressed by cooling. We chilled samples – down to liquid helium temperatures when warranted – to narrow linewidths and quench thermal broadening, which sharpens the spectral fingerprint of an analyte and separates nearly isomeric species. With clean calibration lamps and known standards, one can identify a nerve agent precursor against a noisy matrix by a handful of lines that sit precisely where they must.

You once distinguished two trichlorobenzene isomers by their patterns alone. That sounds simple in retrospect, but not in the moment.

Simple only after the work is done. The spectra of positional isomers differ subtly because the electron distribution and vibrational modes shift with the substitution pattern. If your prism or grating is properly characterised, and your plates or photomultipliers are within linear response, the differences in band origins and progressions are diagnostic. We built apparatus to make those small differences legible – long optical paths for absorption, careful slit functions, and, where needed, fluorescence excitation-emission maps to isolate the tell-tale transitions.

If you have time, please could you give a step-by-step “explain it to an expert” on one of your core procedures?

Gladly. So here’s the Birge–Sponer procedure in practice, with real constraints

• Objective: Determine D0 for a diatomic from an electronically excited state’s vibrational progression when high-v lines are weak or absent.
• Data acquisition: Record the electronic band system with a calibrated spectrograph; assign v′ using combination differences and rotational contour analysis. Keep effective spectral resolution such that Δσ on band origins is better than one tenth of the fundamental spacing ωe.
• Data reduction: Compute ΔGv+1/2 = G(v+1) − G(v) from the origin positions corrected for isotopic shifts and perturbations. Estimate uncertainties from wavelength calibration and assignment ambiguities.
• Linear regime: Fit ΔGv+1/2 ≈ ωe − 2ωexe(v+1/2) over the unperturbed range; exclude bands near avoided crossings or predissociation onsets. Use robust regression if needed to downweight outliers.
• Extrapolation: Determine vmax at ΔG → 0 from the fit; integrate the fitted line from v = −1/2 to vmax − 1/2 to obtain D (vibrational sum). Correct to D0 by subtracting ZPE of the lower state as appropriate for the thermochemical definition required.
• Bias control: Report an upper-bound character due to linear extrapolation; where possible, bracket with Morse-fit or RKR-potential integration to quantify overestimation. Document perturbations and selection rules affecting observed intensities, to justify band selection.
• Trade-offs: Linear extrapolation is fast and robust with sparse data but overestimates D0; Morse and RKR-based approaches reduce bias at the cost of more parameters and higher sensitivity to measurement noise and misassignments. My view was pragmatic: do the simple thing honestly, then refine if the application demands tighter thermochemistry.

You worked in the wake of the Born–Oppenheimer separation. How did that enter your day-to-day?

It entered through restraint. We treated electrons and nuclei on separated timescales for line assignment and intensity analysis, but looked for where the approximation frayed – Herzberg–Teller contributions, vibronic coupling, predissociation. In assigning bands, assume separability, then interrogate the exceptions; the exceptions often teach you the molecule. The experimentalist’s vocation is to find the seam and tug.

Was there an undocumented trick you relied on and never published?

Many little ones. One I taught all my students: if you suspect a faint overlapping band system, rotate the cell a few degrees and watch how fringes from etalon effects move while molecular lines do not; it is a fast way to separate instrumental artefacts from physics. And keep a “dark ledger” – systematically record your backgrounds at each slit setting and plate batch. The most expensive hour in a spectrograph is the hour you spend believing a ghost.

You described yourself as a liaison between physics and chemistry. In practice, how did you maintain credibility on both sides?

By speaking each language without apology. With physicists: selection rules, perturbations, line shapes, instrument functions. With chemists: bond energies, heats of formation, isomer identification, kinetics. The bridge holds if it is anchored firmly on both banks. If one side hears only jargon of the other, they will blame the bridge for their discomfort.

Let’s address the “assistant” narrative head-on. What would you correct for the record?

That I “assisted” in the minor sense. I ran a laboratory, designed apparatus, supervised theses, founded a programme. Collaboration is not servitude. Marriage, when it came, did not retroactively erase authorship. Credit is not a marital property regime.

You trained thirty-five graduate students. In today’s metrics-driven environment, that labour is often undervalued relative to splashy discoveries.

Institutions eat their seed corn when they disparage cultivation. Methods, instruments, temperaments – these are the soil from which discoveries grow. A field withers without them. I was content to be a gardener.

A moment of self-critique beyond the nitrogen misassignment you mentioned earlier?

I sometimes clung to a spectral interpretation longer than was wise because I had invested months in the apparatus that “proved” it. One must learn to grieve an idea quickly. Instruments tempt you to love the answers they can give; it is better to love the questions.

Contemporary critique from your era: some theorists argued that intensity patterns could be extracted without laborious band-by-band measurements, by appealing to general quantum principles and a few parameters. How would you respond?

Principles are excellent servants and poor masters. Global fits hide local lies. Without detailed spectra, you risk fitting ignorance with elegance. Show me the lines, then we shall speak of parameters.

How do you see your work refracted through today’s quantum chemistry – density functional theory, ultrafast lasers, single-molecule spectroscopy?

It delights me. The bridge we built carries heavier traffic now – materials, catalysis, medicine. Yet the essentials persist: careful experiments, honest approximations, an eye for where the model fails. If my name is not always remembered, I take comfort that the methods endure and the students’ students still speak the language.

Advice to a young scientist, especially a woman, traversing disciplinary boundaries?

Choose problems that matter to both disciplines; otherwise, each side will call you derivative of the other. Document your contributions relentlessly, because citation customs lag behind reality. Find allies who respect your judgement and not merely your results. And keep a record of your budgets, your apparatus, your assignments – archives are a future form of self-defence.

One moment of unexpected humour to close?

When I asked Duke for the subbasement, someone quipped, “Professor Sponer prefers to work underground.” I replied, “Only because quantum effects are subtle; the noise above ground is far too classical.” They laughed – and gave me the keys.

If history hands you one more correction, one sentence long, what would it be?

That the bridge was not accidental; it was engineered.

Letters and emails

As responses to the interview continue to arrive, we’ve chosen five letters and emails from our growing community, each offering a distinct perspective. These readers are eager to learn more from Hertha Sponer herself – seeking insight into her scientific craft, her personal journey, and the wisdom she might share with those pursuing similar paths today.

Lucia Romano, 34, Computational Chemist, Milan, Italy
Dr Sponer, I run density functional theory calculations daily to predict molecular properties – bond lengths, vibrational frequencies, dissociation pathways – and get results in hours that would have taken you months of painstaking spectroscopic measurement. But I’m curious: when you were interpreting those spectra by hand, plotting vibrational progressions on graph paper, did the slowness of the process reveal patterns or anomalies that computational speed might cause us to miss today? Do you think there’s intellectual value in the friction of manual analysis that we’ve lost in the age of automated fitting algorithms?

Dr Romano, your question touches something I’ve thought about considerably since arriving in this curious future of yours. You ask whether the slowness revealed things that speed might obscure – yes, absolutely, though perhaps not in the way you expect.

When you spend three weeks photographing a single band system, developing each plate in the darkroom, measuring line positions with a comparator whilst your eyes ache under the lamp, you develop an intimacy with the spectrum that no algorithm can replicate. You notice when a line is slightly broader than its neighbours, or when the spacing between progressions shifts by half a wavenumber in a way the theory doesn’t predict. These anomalies don’t announce themselves; they accumulate gradually in your awareness, like recognising a friend’s gait from a distance before you can see their face.

I remember working on the nitrogen afterglow spectrum in 1926 – ghastly work, really, because the bands were faint and overlapping. I had to expose plates for hours, and even then the lines barely emerged above the grain. But precisely because I was staring at those same features night after night, I noticed that certain bands appeared in the afterglow that didn’t appear under direct discharge excitation. That asymmetry told me something about the kinetic pathways, about which states were populated by collisional energy transfer versus direct electron impact. If I’d had your computational speed, I might have fit the whole mess to a Boltzmann distribution and moved on, missing the non-equilibrium physics entirely.

The friction, as you call it, enforces a kind of discipline. You cannot afford to be careless when each data point costs hours. So you think harder about which measurements actually matter, which bands to pursue, where the information content truly lies. I’ve seen students – even in my own laboratory – who, given too much ease, measure everything and understand nothing. They collect data like stamp collectors, without asking what question each spectrum actually answers.

But I don’t want to romanticise difficulty for its own sake. Much of the slowness was simply tedious: aligning optics, coating plates with emulsion, calibrating wavelength standards against iron arcs. If your computers eliminate that drudgery, good. What concerns me is whether they also eliminate the contemplative space – the long stretches where you’re waiting for a plate to expose or a spectrum to develop, and your mind wanders around the problem from oblique angles.

Here’s a concrete example: the Birge-Sponer extrapolation came to me not whilst I was measuring, but whilst I was plotting. Raymond and I were sitting in his office at Berkeley with sheets of graph paper spread across his desk, physically drawing the ΔG values against quantum number, watching the line emerge point by point. At some moment – I can’t even say precisely when – we both saw that if you simply extended that line to zero, you’d have your dissociation limit. It seems obvious in retrospect, but the insight came from the physical act of drawing, from watching the pattern materialise under our pencils.

Your algorithms do the fitting instantaneously, which is powerful. But do they show you the residuals in a way that makes you suspicious? Do they flag the point that doesn’t quite belong, or do they simply weight it down and move on? The danger of automation is that it answers the question you asked, not the question you should have asked.

I’ll tell you what I’d recommend: occasionally, work through a spectrum the old way. Pick one molecular system, obtain the computational result, then go back and analyse it manually – assign each line, plot each progression, calculate the spacings by hand. Not because the computer is wrong, but because the act of reconstruction teaches you where the method is fragile, where assumptions hide, where the model might be lying to you politely.

There’s another aspect, more philosophical perhaps. When I measured a dissociation energy, I knew exactly what went into it – which lines I’d observed, which I’d extrapolated, what the error bars meant. The number had provenance. It was mine in a deep sense; I could defend every digit. With your calculations, the number emerges from a black box of basis sets and functionals. You can cite the method, but do you truly understand where the number comes from? Can you sense when it’s trustworthy?

I’m not arguing against progress – how could I? Your tools let you study systems we could only dream of: transition states, excited surfaces, molecules too unstable to isolate. But I do think there’s value in what I’ll call “manual literacy” – the ability to work a problem from first principles when the algorithm fails or when you need to judge whether its output is physically reasonable.

So yes, Dr Romano, slowness revealed things. Not because virtue resides in suffering, but because intimacy with data requires time, and insight often comes not from the measurement itself but from the space around it – the plotting, the pondering, the moments when your hands are busy and your mind is free to notice what doesn’t fit.

Ahmed Benali, 29, Science Policy Researcher, Tunis, Tunisia
You mentioned that Robert Millikan advised Duke against hiring you because you were a woman, yet Duke went ahead anyway – and you spent three decades building their programme. I work on research funding equity in North Africa, where we see similar patterns of bias against women scientists and researchers from the Global South. What specific institutional structures or individuals at Duke actually protected your work and allowed you to thrive despite the prejudice? Were there particular policies, or was it more about finding individual allies? I’m trying to understand what concretely changes outcomes, not just rhetoric.

Mr Benali, your question cuts to the practical heart of things, which I appreciate. You’re quite right that institutional rhetoric about diversity means little without concrete mechanisms that actually shift power and resources. Let me tell you what worked at Duke, what didn’t, and what I learned about navigating institutions that are ambivalent about your presence.

First, understand that Duke hired me not out of progressive conviction but out of opportunism – and I mean that descriptively, not bitterly. They were a young university desperate to build scientific prestige quickly. The Rockefeller Foundation offered to subsidise my salary for the first few years as part of their Emergency Committee in Aid of Displaced Foreign Scholars. So Duke got an established researcher, trained at Göttingen, with an international publication record, at reduced cost. President William Preston Few was a pragmatist. Yes, Millikan told him hiring a woman would be a mistake, but Millikan wasn’t writing the cheques. The Foundation’s money created a window where my gender became a manageable inconvenience rather than a disqualifying factor.

Lesson one: external funding bodies can override internal bias when they tie resources to specific hires. Your work in North Africa might explore how international fellowships or development grants could be structured to require institutions to hire women or researchers from marginalised regions, with financial penalties for non-compliance. Make the bias expensive.

Once hired, what protected me? Three things, in order of importance.

First, I delivered results. Within two years, I had re-established my laboratory, published in Physical Review and Journal of Chemical Physics, and attracted my first doctoral students. I made Duke look legitimate. Institutions tolerate what they cannot afford to lose. My graduate students went on to positions at Oak Ridge, at industrial laboratories, at other universities – they became advertisements for Duke’s programme. Every time one succeeded, it justified my position retroactively. This is exhausting, of course – you must perpetually prove your worth whilst your male colleagues are presumed competent – but it’s the reality.

Second, I cultivated allies strategically, particularly in chemistry. The physics department was my home, but chemists needed spectroscopy for molecular structure determination, and I provided a service they valued. When budget discussions happened, I had advocates outside my own department arguing that Sponer’s laboratory was essential infrastructure. Diversify your support base; don’t depend solely on your immediate colleagues, who may resent you or see you as competition.

Paul Gross, who chaired Chemistry at Duke, was particularly important. He wasn’t a crusader for women’s rights – nothing so grand – but he recognised competence and had no patience for nonsense. When someone suggested my teaching load should be lighter because women couldn’t handle advanced courses, Gross pointed out that I’d been teaching quantum mechanics at Göttingen whilst some of the full professors at Duke were still learning it. That kind of ally – someone who defends you on meritocratic grounds rather than sentimental ones – is worth ten well-meaning liberals who sympathise but don’t fight.

Third, I built institutional infrastructure that outlasted individual prejudices. The spectroscopy laboratory wasn’t just my research space; it was a facility that other faculty and graduate students used. I trained technicians, maintained equipment catalogues, established protocols. I made myself embedded in the university’s operational fabric, not merely a individual researcher who could be dismissed without disrupting anything. When you build infrastructure, you build dependency, and dependency is a form of power.

What didn’t work? Appealing to fairness or justice. I tried that exactly once, early on, when I was excluded from a faculty committee on graduate admissions. I argued that as an associate professor supervising doctoral students, I should participate in admissions decisions. The response was polite dismissal – something about committee size and established procedures. I learned then that moral arguments fail when they threaten entrenched interests.

What worked instead was demonstrating that my exclusion produced worse outcomes. I pointed out that the committee had rejected two applicants with strong spectroscopy backgrounds because no one on the committee understood the field well enough to evaluate their potential. One of those applicants went to MIT and did excellent work. The following year, I was added to the committee – not because they’d suddenly discovered gender equality, but because they’d discovered that ignorance was costly.

Lesson two: frame your inclusion in terms of institutional self-interest, not abstract justice. Show how excluding you produces measurable failures – missed opportunities, poor decisions, wasted resources. Make the bias legible as incompetence.

Now, let me be honest about the costs. I was promoted to full professor in 1946, ten years after I arrived. My male colleagues with equivalent records were promoted in five or six. I was never offered the department chairship, though I had seniority and administrative experience. My salary lagged behind men at my rank – I discovered this only late in my career when a dean accidentally showed me the wrong spreadsheet. I had to accept these injustices strategically, deciding which battles were winnable and which would merely mark me as “difficult,” making the next battle harder.

This is the devil’s bargain of institutional survival: you succeed by being indispensable, but being indispensable requires you to overwork, to absorb slights, to spend energy on politics that your colleagues spend on research. It’s grinding. Some days I wondered whether I’d have contributed more science if I’d remained in Europe despite the Nazis – at least there I’d have starved quickly rather than being slowly exhausted by institutional resistance.

But here’s what I want you to understand for your policy work: individual resilience is not a solution. Yes, I survived and even thrived, but that required extraordinary effort, strategic thinking, fortunate allies, and external funding that most women scientists never accessed. The fact that I succeeded doesn’t prove the system works; it proves I was unusually lucky and unusually stubborn. Policy must aim not to produce more Hertha Sponers who navigate hostile institutions brilliantly, but to transform institutions so that navigation isn’t necessary.

Concrete recommendations from my experience: tie funding to measurable equity outcomes, not aspirations. Require institutions to report salary data disaggregated by gender and origin. Establish independent review panels with power to reverse hiring and promotion decisions where bias is evident. Create grant programmes that fund the researcher directly, not the institution, so that talented women can move to universities that actually want them rather than being trapped by institutional inertia.

And finally, document everything. I kept meticulous records – correspondence, meeting notes, publication dates, evidence of my contributions to collaborative work. When my name was omitted from a review article on molecular spectroscopy in 1952, I wrote to the editor with a list of my ninety-three publications in the field and politely inquired whether ninety-three papers constituted a sufficient contribution to merit a citation. The correction appeared in the next issue. Institutions have short memories for women’s work; you must build your own archive.

Saanvi Iyer, 42, Quantum Information Scientist, Bengaluru, India
My team is working on molecular qubits – using molecular spin states and vibrational modes for quantum computing applications. You pioneered understanding how molecules occupy discrete quantum states and transition between them. If you could design one experiment today using modern ultrafast lasers, cryogenic control, and single-molecule detection that was simply impossible with 1930s spectrographs, what molecular system would you interrogate, and what question would you finally be able to answer that frustrated you then?

Miss Iyer, just hearing there is talk of molecular qubits and quantum computing gives me pause – truly, the world has sped on since my days plotting spectra in the lamplight. Our greatest ambitions, back in the Göttingen laboratory, barely scratched at such grand possibilities: we hoped only to extract the hidden information in light and matter, never imagining leaps so profound as computation itself at the scale of a molecule.

If I could summon forth modern apparatus, one experiment I dreamed of in the 1930s would finally come to fruition. During my work with the nitrogen molecule, I was forever frustrated by the fleeting lifetimes of certain excited states – those that decayed before our plates could even register their passage, whose absorbance was lost among the noise. We could measure vibrational progressions well enough for long-lived states, but the dynamics of predissociation – how energy leaked from molecule to fragments in a quantum blink – remained obscured, tantalising but always on the boundaries of possibility.

Given your ultrafast lasers, cryogenic manipulation, and single-molecule detection, I would design a study on the real-time evolution of a predissociative electronic state in diatomic nitrogen. Here’s how I imagine it: We’d prepare an ensemble of nitrogen molecules, cooled to near absolute zero to sharpen all transitions. Then, with a femtosecond laser pulse – your “ultrafast” device, faster than any spark or discharge I ever commanded – we’d excite the molecules into that elusive state whose lifetime is measured not in seconds, nor milliseconds, but femtoseconds.

Next, rather than passively recording the spectrum, we’d track the state’s evolution directly – measure its vibrational wave packet as it oscillates and decays, observing precisely when and how the bond snaps. If the technology permits, we would do so at the level of individual molecules: seeing not merely the average but the peculiarities, capturing quantum signatures of decoherence and tunnelling, features impossible to see in a bulk measurement.

In my own era, we could only infer predissociation from the broadening or vanishing of spectral lines, and even that required great patience and, frankly, luck with the apparatus. We argued endlessly over why certain bands disappeared, whether it spoke to energy thresholds or subtler coupling with continuum states. The theory was evolving too – the work of Born, Oppenheimer, and Herzberg guiding our understanding, but the direct observation was out of reach.

With your tools, you might answer questions about time-resolved vibronic coupling: how does the interplay of nuclear and electronic motion govern whether a molecule survives or breaks? Is dissociation strictly statistical, or do specific quantum coherences favour one path over another? Could we, by careful timing or coherent control, suppress the fragmentation and preserve the molecule longer, opening doors to true quantum manipulation at the molecular level?

Such an experiment would teach us about the boundaries between determinism and chance in the quantum world – matters that exercised my colleagues and myself late into many evenings. To glimpse the molecular bond as it quivers, to intervene in its fate with precision, would be a realisation of many frustrated ambitions.

And if, beyond pure science, you harness such control in your quantum computing efforts, I hope you will remember that the path from spectroscopy to information science ran through many underlit rooms, many careful measurements, and many hands. To manipulate molecules for computation is high art; but first, let us rejoice that the secrets once locked far beyond our reach are now yours to open. May your marvels be built atop honest measurements and careful thought, for that is the true heart of our craft.

Rodrigo Espinoza, 51, Secondary School Physics Teacher, Santiago, Chile
I teach the Birge-Sponer method to my advanced chemistry students every year, and they learn the equation, do the calculations, get the right answer – but I’m not sure they grasp why it mattered. You lived through the moment when scientists were first proving that quantum mechanics wasn’t just abstract mathematics but actually described real molecular behaviour. How would you explain to a sixteen-year-old why calculating a dissociation energy from a spectrum was profound in 1925, not just useful? What was the philosophical shift, not just the technical achievement?

Mr Espinoza, I’m honoured that you teach the method Raymond and I developed, though your concern about students grasping why it mattered is well-founded. Let me try to convey what it felt like to stand at that particular moment in history, when the quantum world was just becoming real to us.

Before quantum mechanics, chemistry was largely empirical – a catalogue of reactions, properties, affinities. We knew that molecules had certain strengths, that some bonds required more heat to break than others, but the why remained mysterious. Classical physics told us atoms were held together by electrical forces, but it couldn’t explain the stability of matter itself, nor why hydrogen always formed H₂ rather than H₃ or some other combination. The old physics had failed utterly at the molecular scale.

Then came the quantum hypothesis – Planck’s energy packets, Bohr’s quantised orbits for hydrogen, de Broglie’s matter waves. These were astonishing ideas, almost mystical in their strangeness. Electrons didn’t orbit smoothly but jumped between discrete levels. Energy came in lumps, not continuous flows. Matter itself had wavelike properties. Many physicists were sceptical, and chemists – practical people who worked with flasks and flames – were even more dubious. It sounded like mathematical philosophy, not reality.

What the Birge-Sponer method demonstrated was this: quantum mechanics wasn’t merely abstract formalism. It made concrete, testable predictions about real chemical bonds. When we plotted those vibrational energy differences and extrapolated to find the dissociation energy, we were showing that molecules actually behaved as quantum theory predicted – that the strange mathematical machinery of wavefunctions and quantised states corresponded to something you could measure in a laboratory with light and glass.

Here’s how I’d explain it to your sixteen-year-old: Imagine you’re told that a staircase exists, but you’ve never seen one – you’ve only walked on ramps your entire life. Someone draws you a diagram showing discrete steps, each at a fixed height, and claims that’s how certain staircases work. You might think, “That’s an odd idea, but perhaps it’s just a simplification, an approximation of a ramp.”

Now suppose you have a way to measure the staircase from a distance – by bouncing a ball down it and listening to the rhythm of the bounces. If it’s truly a staircase, the intervals between bounces will be regular, corresponding to the step heights. If it’s a ramp pretending to be a staircase, the intervals will be uneven, continuous. When we measured molecular vibrations through spectroscopy, we heard the rhythm of quantum steps – regular, discrete intervals that matched the quantum predictions precisely. That’s when the staircase became real.

The philosophical shift was from description to understanding. Before quantum mechanics, we could describe that a carbon-carbon bond had a certain strength, but we couldn’t explain why it had that particular value and not another. Quantum theory told us that molecular energy levels were solutions to Schrödinger’s equation, determined by the masses of the atoms, the electronic structure, the fundamental constants of nature. The dissociation energy wasn’t an arbitrary number; it was a consequence of the quantum architecture of matter.

When Raymond and I sat in Berkeley plotting those points, watching the line form and realising we could extrapolate to the dissociation limit, we weren’t just inventing a computational shortcut. We were demonstrating that the quantum picture was complete – that it described not only the low-energy states we could observe directly but also the high-energy states near dissociation, where the molecule was vibrating so violently it was about to fly apart. The continuity of that line, the fact that the extrapolation worked, meant quantum mechanics governed the entire journey from stable molecule to separated atoms.

I remember the moment distinctly. Raymond looked at the graph and said, “If this holds, we’ve got dissociation without ever reaching it.” And I replied, “If this holds, quantum mechanics isn’t just for atoms anymore – it’s for chemistry.” That’s what made it profound.

For your students, perhaps this approach: Ask them to consider what it means that we can predict, from measurements on the lowest few vibrational levels, exactly how much energy is required to destroy the bond entirely. That’s prophecy of a sort – using information about gentle vibrations to foretell the violence of rupture. Classical physics offered no such power. Only quantum mechanics, with its discrete states and selection rules, made such extrapolation possible.

The practical utility – that chemists could determine bond strengths more easily – was valuable, certainly. But the deeper achievement was epistemological: we had confirmed that the quantum description of molecules wasn’t a convenient fiction or mathematical game. It was the actual architecture of chemical reality, governing everything from the faint absorption lines in a spectrum to the energy required to tear matter apart.

When your students calculate that dissociation energy, they’re participating in an intellectual revolution that transformed chemistry from craft to science, from empirical observation to predictive theory. The number they obtain isn’t merely a datum; it’s evidence that the universe, at its most fundamental scale, operates according to quantum rules – rules we can measure, test, and trust.

Tell them that in 1925, calculating a dissociation energy from a spectrum wasn’t just useful – it was proof that human beings could comprehend the hidden logic of matter itself. That’s what made it profound. And if they can carry that sense of wonder alongside their competence with the equation, then they’ll understand not just how science works, but why it matters.

Isla Rangi, 38, Patent Examiner (Chemistry), Wellington, New Zealand
You said earlier that you sometimes held onto an interpretation too long because you’d invested months building the apparatus that supported it – that instruments ‘tempt you to love the answers they can give.’ In my work, I see this all the time: researchers over-claiming based on limited data because they’re attached to a particular technique or platform. But there’s a flip side – what if you had abandoned your spectroscopic approach earlier when theorists said global quantum principles were enough? How did you distinguish between productive stubbornness that defends a valid method against premature dismissal, versus unproductive stubbornness that clings to a failing approach? Where’s the line?

Miss Rangi, you’ve identified one of the most difficult judgements in experimental science – knowing when to defend your method against premature dismissal and when stubbornness has curdled into foolishness. I struggled with this throughout my career, and I cannot claim to have always chosen correctly. But I can tell you what I learned about finding that line.

The first question I ask myself is this: Am I defending the method itself, or am I defending my investment in it? These feel similar but are fundamentally different. When theorists in the late 1920s argued that global quantum principles were sufficient and that detailed spectroscopic measurements were unnecessary labor, they were wrong – demonstrably so. The principles were correct in outline, but the parameters within those principles could only be determined empirically. You cannot calculate a dissociation energy from first principles if you don’t know the anharmonicity constant, and you cannot know the anharmonicity constant without measuring it. My stubbornness in continuing precision spectroscopy was defending the method because the method was answering questions the theory alone could not.

Contrast that with my nitrogen misassignment I mentioned earlier. I had interpreted certain bands as predissociation based on my apparatus’s resolution and my theoretical expectations at the time. When Heitler and London’s work on chemical bonding emerged, it became clear my interpretation was strained – the energy levels didn’t align properly with the new understanding. But I resisted revising my conclusion for nearly a year because I had spent months perfecting the photographic plates, and admitting error felt like admitting all that labor was wasted. That was defending my investment, not the method. The apparatus was fine; my interpretation was wrong. I should have corrected course immediately.

Here’s a practical test I developed: Can the method answer a question independent of the one I’m emotionally attached to? If your spectroscopic technique can only validate the one interpretation you favour, that’s a warning sign. If it can, in principle, produce results that would falsify your hypothesis – and you’d trust those results – then the method itself is sound, even if your current application is flawed. When I was certain predissociation explained my nitrogen bands, I should have asked: what measurement would prove me wrong? If I couldn’t design such a measurement, or if I found reasons to dismiss every contrary result, that was stubbornness speaking, not rigour.

Your work as a patent examiner gives you an advantage here – you see many researchers over-claiming from limited data. What I suspect you observe is that they’ve conflated capability with validity. An instrument may be capable of producing data, but that doesn’t mean the data validates their interpretation. I encountered this with fluorescence measurements in the 1940s. Fluorescence spectroscopy was becoming fashionable, and several groups claimed they could determine molecular geometries from fluorescence lifetime measurements alone. Technically, fluorescence lifetimes do contain geometric information – but extracting it requires assumptions about radiative transition probabilities, collisional quenching, and energy transfer that were poorly constrained at the time.

I was sceptical, perhaps excessively so. My stubbornness in favouring absorption spectroscopy over fluorescence methods delayed my laboratory’s adoption of techniques that eventually proved valuable. But the fluorescence enthusiasts were also wrong to over-claim. The line I should have walked – and didn’t – was this: recognise that fluorescence contains genuine information (don’t dismiss the method), but insist on rigorous uncertainty quantification and cross-validation with independent techniques (don’t accept premature claims). Neither blind defence nor reflexive rejection serves science.

Another heuristic: Listen to competent critics from outside your immediate circle. When physicists at Göttingen questioned my interpretations, I sometimes dismissed them because they weren’t spectroscopists – they hadn’t spent hours in the dark room, they didn’t understand the apparatus’s subtleties. But outside perspective is valuable precisely because it’s unencumbered by your investment. If someone who understands quantum mechanics but not your specific technique asks a question you cannot answer satisfactorily, that’s information. Either your explanation is inadequate, or the method has limitations you haven’t acknowledged.

I learned this most painfully in 1933, just before I left Germany. I had been measuring band intensities in oxygen spectra, trying to extract transition dipole moments. A visitor from Copenhagen – a student of Niels Bohr – asked me how I accounted for saturation effects in my absorption measurements, since intense lines might be optically thick even in dilute samples. I brushed the question aside, saying I’d calibrated carefully. But I hadn’t actually checked for saturation; I’d assumed my samples were dilute enough. When I finally did check, using a variable path-length cell, I discovered that my strongest lines were indeed saturated, which meant my intensity ratios were systematically wrong. The Copenhagen student was correct. My attachment to my published values – and my resentment at being questioned by someone younger – had blinded me to a real problem.

Where’s the line between productive and unproductive stubbornness? I think it lies here: Productive stubbornness defends the legitimacy of a question or method against those who claim it’s unnecessary. Unproductive stubbornness defends a particular answer against those who show it’s wrong.

When I defended spectroscopy against theorists who thought experiments were obsolete, I was defending the legitimacy of empirical measurement – the method could, in principle, produce many different results, and I’d accept whichever ones emerged. That’s productive. When I defended my nitrogen assignment beyond the evidence, I was defending a particular answer because I’d grown attached to it. That’s unproductive.

One more marker: Does revising my position require abandoning the apparatus and expertise I’ve built, or merely reinterpreting the data? If a new understanding requires you to throw out your entire laboratory and start over, scrutinise that understanding carefully – it may be wrong, or at least premature. But if a new understanding merely requires you to reanalyse your existing data with corrected assumptions, and you resist because it’s embarrassing or laborious, that’s unproductive stubbornness. The apparatus hasn’t failed; your interpretation has.

In your patent work, perhaps you could apply this distinction: Is the researcher defending the legitimacy of their experimental approach (which may be valid even if unfashionable), or are they defending a specific claim that the data doesn’t actually support? The former deserves space to mature; the latter deserves scepticism.

Finally, I’ll confess something I rarely admitted during my career: sometimes I couldn’t tell the difference myself until years later. In the moment, productive and unproductive stubbornness feel identical – both involve standing firm against pressure, both require confidence, both can be rationalised. The clarity comes retrospectively, when the community’s judgement settles or when better data arrives. That’s humbling, but it suggests we should build into our practice mechanisms that force re-evaluation: mandatory re-analysis after fixed intervals, required engagement with critics, explicit documentation of assumptions so they can be revisited.

I wish I could offer you a formula, Miss Rangi, but the line remains blurry, drawn differently for each circumstance. What I can offer is this: cultivate discomfort with your own certainty. When you find yourself utterly convinced, that’s the moment to seek the sharpest critic you can find and genuinely listen. The method may still be sound, but your attachment to it has likely grown beyond what the evidence warrants. And that attachment, however natural, is the enemy of clear thinking.

Reflection

Hertha Sponer died on 27th February 1968 at the age of seventy-two, having returned to Germany only shortly before her passing. One cannot ignore the calendar: her career spanned convulsive decades for Europe and for science itself, from the first shattering advances of quantum mechanics through exile and adaptation in a new land, to the slow, sometimes grudging, acceptance of her field – and her presence within it – in the United States. The arc of her life, outlined so patiently in these conversations, tells the story of both breakthrough achievement and stubborn gaps in recognition.

Listening to Dr Sponer, what stands out is her persistent ingenuity – her ability to turn displacement and exclusion not into bitterness, but into the foundations of a new laboratory, a new discipline, a new way of thinking about molecules and light. She transformed hardship into experiments with the same precision she applied to her spectra, and her formidable intellect was yoked to a deeply pragmatic sense of what could be built from whatever materials, budgets, or colleagues were at hand. The interviewer’s modern perspective often expects to find heroes born for adversity, but Sponer’s replies were never adorned with illusion. For her, resilience was not an abstract virtue. It was a mode of working when every professional meeting and each line of a spectrum could carry resistance or misunderstanding.

Across our conversations, recurring themes surface: perseverance, technical creativity, the often invisible labour required to bridge disciplines, and the recurrent erasure of women’s contributions by those who write scientific histories. Sponer’s frank reflections about the assistant narrative and marital eclipse offer a marked correction to many recorded accounts, which too readily folded her ingenuity under the shadow of James Franck or reduced her role to technical support. Her own testimony makes equally clear how she competed not just for recognition, but for the meaning of scientific work – choosing pragmatism over grandstanding, mentoring over notoriety, and the steady tending of a laboratory over the “heroic” myth-making that often orbits her male contemporaries.

Naturally, there remain uncertainties in the historical record. Accounts vary on who made which conceptual leap, whose data informed which theoretical synthesis, and how disciplinary credit was partitioned as quantum chemistry emerged as a field. Even Sponer could not always distinguish productive from unproductive stubbornness in herself until years later. This is the ambiguity that lingers in stories of exile and innovation: institutional prejudice and the accidents of memory blur the lines between the solitary scientist and the collective making of knowledge.

And yet, the afterlife of her work is undeniable. The Birge–Sponer method is a staple in textbooks; today’s computational chemists and spectroscopists inherit infrastructure she designed – if not always with her name attached. While her direct students carried her rigor into several branches of American science, later generations, from quantum computing researchers to specialists in ultrafast molecular dynamics, have circled back to cite her foundational studies, sometimes unknowingly walking trails she had quietly marked. Recognition, when it came, was sometimes posthumous: the Hertha Sponer Prize was established by the German Physical Society in 2001 to honour early-career women physicists, an overdue gesture that invites new generations to seek her example.

For young women in science today, Sponer’s legacy is quietly revolutionary. She modelled how to persist in slow, careful craft, how to mentor and lift up students even when recognition was uneven, and how to bridge disciplines – to inhabit the often-invisible spaces between. Her insistence on the value of building, of maintaining, and of asking the questions no one else thought costly enough to pursue, stands as a bulwark against the cycles of obscurity that still, far too often, engulf the work of women in STEM.

Hertha Sponer reminds us that the architecture of modern quantum chemistry, with all its power to explain and manipulate the fabric of matter, rests not just on the flash of theoretical insight but on years of quiet, measured experimentation, brought to life and kept alive in the generosity of teaching and the courage to insist on one’s own authorship. For everyone working or learning in science – in the shadows or the spotlight – her story continues to offer a radiant thread of possibility to be caught and woven into the present.

Who have we missed?

This series is all about recovering the voices history left behind – and I’d love your help finding the next one. If there’s a woman in STEM you think deserves to be interviewed in this way – whether a forgotten inventor, unsung technician, or overlooked researcher – please share her story.

Email me at voxmeditantis@gmail.com or leave a comment below with your suggestion – even just a name is a great start. Let’s keep uncovering the women who shaped science and innovation, one conversation at a time.

Editorial Note: This conversation with Hertha Sponer is a dramatised reconstruction, not a transcript of an actual interview. Dr Sponer passed away in 1968, decades before this piece was conceived. The dialogue presented here has been carefully crafted based on historical sources, including her published scientific work, archival materials, biographical accounts, and the broader context of her life and career in molecular spectroscopy and quantum chemistry. Whilst every effort has been made to remain faithful to her documented achievements, perspectives, and the scientific and social circumstances of her era, the specific words, anecdotes, and reflections attributed to her are the product of imaginative reconstruction informed by research, not verbatim records of her speech.
The purpose of this format is to bring Hertha Sponer’s contributions to life in a vivid and accessible way, honouring her legacy whilst acknowledging the gaps and silences in the historical record. Where uncertainty exists – about her private thoughts, specific laboratory practices, or interpersonal dynamics – creative interpretation has been employed responsibly, guided by what is known about her character, her era, and the structural forces she navigated. Readers should approach this interview as an evidence-based dramatisation: a bridge between rigorous history and engaging narrative, designed to illuminate both her scientific brilliance and the enduring challenges faced by women in STEM fields across generations.

Bob Lynn | © 2025 Vox Meditantis. All rights reserved.