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  • Writer's pictureDale DeBakcsy

Probing the Ultraviolet: The Spectroscopic Marvels of Emma Perry Carr

In its externals the life of Emma Carr (1880-1972) bears many similarities to that of fellow physicist Margaret Maltby (1860-1944). Both were born in the latter half of the 19th century, in the state of Ohio (Carr was born about 50 miles southwest of Akron, and Maltby about 50 miles northeast of it), to families who had roots in the American colonies stretching back to the 17th century. Both had families who were actively supportive of women’s education, and both went on to decades’ long careers based at one particular university, where they built up world-class scientific departments, Maltby at Barnard, and Carr at Mount Holyoke. 

The big difference between the two Ohian physicists lies in the length of their research careers. While Maltby largely gave up her promising and globally respected research upon becoming head of the Barnard physics department in 1903 in order to concentrate all of her resources on building up the department, supporting the students, and raising the child she claimed was her nephew, Carr found a way to continue her research through the proliferation of her administrative responsibilities, racking up a string of publications into the 1940s that established her as a world authority on the subject of ultraviolet absorption spectroscopy. 

Before we get to the subject of her fascinating research, however, a moment should be taken to appreciate her path as an undergraduate, if for no other reason than to have an encouraging example in our pockets to share with the high school and college age women in our lives who are worried about the non-linearity of their academic path, and what that might mean for their future. Carr matriculated at the University of Ohio in 1898, stayed there for a year, then for whatever reason transferred to Mount Holyoke, where she stayed for two years, and then decided that, rather than finishing her degree, she would continue on as a paid assistant at Holyoke for a couple of years, and then, when she finally decided to complete her degree, she did so not at Mount Holyoke, but at the University of Chicago, receiving it at last in 1905. The moral here being, everything is okay, there are lots of ways to get where you want to go, and no particular rush to do so - if Emma Carr could become an international scientific celebrity after taking seven years to get her undergraduate degree via courses spread across three different institutions, you can do a few things outside of the norm and end up just fine.

For the next five years of her life, she split her time between teaching at Mount Holyoke, and working on her PhD at the University of Chicago, which she earned in 1910 for work on aliphatic imidoesters. Receiving her PhD in 1910 in physical chemistry, she returned once more to Mount Holyoke, now as a full professor, and three years later, as the head of the chemistry department she would oversee until her retirement 33 years later in 1946. She was a tentpole figure at the university, not only one of its most popular lecturers, but a beloved residence hall head who frequently ate with the students, leading them in conversations like whether it was better, on balance, to be a monkey or a cow (it’s got to be monkey, right?). She was a personal confidant, but also an individual who worked tirelessly to create cross-generational research groups where undergrads, grad students, and faculty could all work together on original projects to the mutual benefit of all. 

As she was developing Holyoke’s chemistry program, Carr was carrying out her own research, which soon came to be dominated by the study of absorption spectroscopy. Longtime readers of the Archive will remember spectroscopy as a branch of science employed in astronomy to identify the chemical composition of gas clouds and stars, but here on the ground we have been employing it since the early 20th century to minutely probe the structure of molecular compounds. Every compound has unique frequencies of electromagnetic radiation that it will absorb, using the absorbed energy to change something about itself. Physicists and chemists, by subjecting molecules to different wavelengths of radiation and recording which are strongly absorbed, can then use the pattern of absorptions to say things about what the molecule must be like, a very powerful tool in an era when x-ray crystallography was in its infancy and modern imaging methods were a full half-century away. 

Lower frequencies of radiation, with their correspondingly lower energy, are able to make molecules vibrate in different ways, with the number of possible vibrations determined by the number of atoms in the molecule. By knowing what energies cause different vibrational effects, we can find out stuff like the length of the bonds in a molecule, or the angles between atoms, which went a long way to allowing scientists to construct models of those molecules they could not directly witness. Carr was interested in what could be learned from the higher frequencies in the UV portion of the electromagnetic spectrum. Photons of UV radiation have enough energy to not just make atoms jiggle a bit, but to promote electrons to new energy levels. Different molecules preferentially absorb different frequencies of UV light, and that surely meant something physically, but the question facing researchers in those years when molecular orbital theory was just getting itself worked out was, what?

In 1930 Carr proposed a formula in the pages of Nature relating a molecule’s frequencies of absorption in the UV spectrum, the number of double and triple bonds in the molecule, and its heat of combustion, a tour de force of scientific intuition grounded in years of experience that, even if it didn’t prove as generalizable as she expected, showed a profound inventiveness in seeing cross-linkages between different areas of chemical research. The fecundity of her insights and rigor of her experimental technique earned her in 1924 an appointment working for the International Critical Tables as an expert on absorption spectra, charged with regularly preparing tables that compiled the best data from across the world about characteristic absorption frequencies of different molecules. While others who had been so appointed looked upon the work as a non-critical task that could always be shoved aside to make room for their own research, Carr was devoted to the importance of the project, and made time to prepare her reports assiduously, while performing her own research, while visiting foreign labs to observe new techniques and get experience with new spectrographic machinery, while lobbying, usually successfully, for Mount Holyoke to purchase the machinery she found most useful for its own growing labs, while serving her administrative and mentoring roles at her college.

Carr was a dynamo, one of the most effective chemists of her era in securing funding, and using that funding to concentrate equipment and personnel for the achievement of clearly defined research goals, but yet an individual not so consumed by administration that she didn’t have time to carry out her own research. In 1946 she presented a talk outlining some of the work she had done in the study of unsaturated hydrocarbons (strands or rings of carbon atoms that contain double bonds) that explained the presence of some absorption frequencies in the Schumann region of the electromagnetic spectrum as the result of electrons in double bonds absorbing energy and jumping to a higher molecular orbital. The molecular orbital theory of bonding, which explained the relative stability of different molecular combinations in terms of the energy levels and locations of the molecule’s electrons in a manner analogous to the energy levels that had been worked out for the electrons of individual atoms, had only been established in 1927, and Carr’s work of the 1930s and 1940s proved a perfect experimental vindication of its model, demonstrating the energy consistently required to move an electron located in a stable double bond into one of the more exotic states hypothesized by molecular orbital theory. 

Carr finally retired in 1946, after 33 years of service as the Mount Holyoke chemistry chair. Nine years later, the college recognized her decades of foundational service by dedicating the chemistry building in her name, and the Carr Laboratory building today houses three different halls, one of which is also named after Carr. In 1971, the college and its president again expressed its appreciation to her lifetime of work by helping defray the cost of Carr’s care at the Presbyterian Home where she resided after a decline in her health made living alone no longer practicable. Emma Carr’s heart, which had faithfully seen her through a lifetime of administrative, scientific, and professional challenges that might have overcome a character less resolute, finally gave way in January of 1972.


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