Sponsored by the ACS Division of Physical Chemistry
If every chemist has a favorite molecule, it might be natural to assume that acetylene is the closest to Robert W. Field’s heart.
Field, 67, has spent more than three decades studying this four-atom molecule’s behavior in the gas phase. Over the course of his long career at Massachusetts Institute of Technology, Field has devised a stable of laser-based spectroscopic methods that have made it possible to extract information on the dynamics of acetylene and other small molecules.
“But you never forget your first love,” Field says, referring to the two-atom molecules on which he cut his teeth as a graduate student at Harvard University. Working under the guidance of William Klemperer, Field collected all of the spectroscopic information he could about the valence electronic states of carbon monoxide, one of many molecules with broken, or “perturbed,” spectral patterns. “Field did not invent spectral perturbations, but he surely understood their importance,” Klemperer says. Field eventually showed that spectral perturbation data similar to what he’d collected for CO could be used to provide a global description of intramolecular dynamics.
Later, during his postdoc at the University of California, Santa Barbara, Field used perturbations, ligand field theory, and newly available tunable lasers to characterize the electronic structure of alkaline earth monoxides such as CaO and alkaline earth monohalides such as CaF. Field continued to probe the structure and dynamics of these and other diatomics after starting his own lab at MIT in 1974.
Soon enough, however, he started to get interested in more complex molecules. “While there is great enjoyment in the detailed understanding of diatomic molecules,” Klemperer notes, Field realized that “it is the spectra and dynamics of polyatomic molecules that predominate the spectroscopic frontier.”
The move from molecules with two atoms to those with three or four comes with a dramatic increase in the complexity of spectra. Undeterred, Field developed a new technique—stimulated emission pumping—with the help of his MIT colleague James L. Kinsey, who is now at Rice University. In SEP, tunable lasers excite molecules to targeted vibrational levels with geometric structure far from that of their vibrational ground state.
Acetylene soon became SEP’s poster child. Field used the technique to examine, among other things, how bending one CCH bond in acetylene (HC≡CH) provides a map of its isomerization path toward highly unstable vinylidene (H2C═C:).
Today, SEP “has grown to represent a truly cornerstone tool of modern molecular spectroscopy,” says spectroscopist David J. Nesbitt of JILA at the University of Colorado, Boulder.
“I would have never dreamed I would still be working on acetylene 30 years later,” says Field, who notes that there’s still a place in his heart—and his lab—for diatomics.
Field, who earned his undergraduate degree in chemistry from Amherst College, previously won the American Physical Society’s H. P. Broida, E. K. Plyler, and A. L. Schawlow Prizes as well as the Optical Society of America’s Ellis Lippincott and W. F. Meggers Awards. He and Kinsey shared an ACS Nobel Laureate Signature Award with their graduate student Yongqin Chen in 1990. Field is also a fellow of the American Physical Society, the Optical Society of America, the Royal Society of Chemistry, and the American Academy of Arts & Sciences.
Field will present the award address before the ACS Division of Physical Chemistry.