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Scientific Contributions of Robin M. Hochstrasser Robin Hochstrasser has made major contributions to chemical and biological physics through a rich research career now spanning over 40 years, primarily at the University of Pennsylvania. His work has been characterized by a constant stream of experiments of lasting significance. The impact of Hochstrasser’s Group is evident in many research areas, including photochemistry, solid-state chemistry, ultrafast chemistry, laser spectroscopy, and protein dynamics. As importantly, Robin has established a research group at Penn that has spawned a generation of scientists and has made the University of Pennsylvania a truly international center in these fields. In the 1960s Robin spearheaded the use of ultrahigh magnetic and electric field measurements on crystals at low temperature. From high-resolution absorption and emission studies, he uncovered new molecular states and measured important properties of excitons and pair states in molecular crystals. In addition to the optical probing, Robin introduced the use of Zeeman and Stark spectroscopy to map out the magnetic and electric properties of solids. Zeeman effect studies were key for understanding the spin quantization in solids and electron exchange-mediated energy transfer, and Stark effect studies formed the basic framework for much of the subsequent experimental and theoretical work on the influence of electric fields on molecular spectra and on the measurements of dipole moments. In the 1970s, Robin’s group discovered an important interplay between the structure and the effective dimensionality of solids, which provided the first clear topology for 1-D solids, a classic contribution! Robin recognized the power of laser technology for molecular studies. He immediately understood that it presented an opportunity to obtain previously inaccessible information from new spectroscopies. The applications were broad and numerous. Robin and his co-workers made seminal contributions to molecular two-photon spectroscopies in crystals and in the gas phase. He also recognized the significance of directly measuring coherence decay of excitations in crystals, for diatomic and polyatomic molecules, and obtained the lifetimes of vibrational levels, thus unraveling the pathways for energy relaxations by intra- and intermolecular interactions. Robin’s group observed dephasing-induced-coherent-emission in liquids and solids, which allowed studies of transitions that are not populated in the usual sense and permitted direct studies of pure dephasing in the condensed phase. Robin’s deep understanding of the interaction of radiation with molecules led, with the help of these linear and nonlinear spectroscopies, to the development of the theoretical understanding of the interrelationships among Raman, resonant Raman scattering, and fluorescence processes. Robin realized the new opportunities for time-resolving nonradiative processes, phenomena of long standing interest to him, by what was called at the time “fast kinetics.” Robin has continued to be at the forefront of the field of ultrafast laser spectroscopy and has made important contributions in many applications utilizing the state of the art in laser technology at picosecond and femtosecond resolution. These include direct measurements of intersystem crossing from singlets to triplets,
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determination of vibrational relaxation times for simple ions in water and measurement of electronic dephasing rates. He carried out the first picosecond time-resolved measurements of chemical reactions in the gas phase under collision-free conditions. (These experiments involved stilbene cis-trans isomerization, which was to remain a favorite of Robin’s.) Robin’s wide-ranging contributions to condensed-phase dynamics have made it possible to study a variety of processes and to examine the molecular description of such phenomena as energy transfer, friction, structural influences, and chemical reactivity. The work stimulated many theoretical studies, and Robin’s group played a key role not only in the experimental studies but also in helping to formulate the theoretical understanding. Robin’s group has continued to explore new avenues for probing the dynamics. Most recently, he advanced the ability to time resolve the dynamics with infrared pulses, opening the door for new studies using the vibrations as the signature of molecular structure changes. In addition to these contributions to chemical physics and physical chemistry, Robin has also maintained a strong interest in biological applications, beginning with his studies in the early 1960s in collaboration with Mike Kasha on excitation energy transfer in photosynthesis. This work was followed by spectroscopic investigations of protein crystals and the in-depth investigation of resonance Raman spectra of biological chromophores. Robin’s most important contributions have been in the area of experimental studies on the picosecond and femtosecond dynamics of heme proteins, a field that was effectively originated by him and has evolved into a flourishing area in the biophysical sciences. His measurements of a transient absorption spectrum of a protein (hemoglobin) in the subnanosecond time regime signaled the beginning of modern research on the chemical dynamics of proteins. Robin’s efforts in this area have led to an impressive series of contributions, and these include: the subnanosecond geminate recombination of oxygen in hemoglobin, the framework for understanding the role of spin-orbit coupling and orbital correlations in influencing the geminate rates for oxygen, carbon monoxide, and nitric oxide, and the time-resolved infrared studies that have led to the first glimpse of the motion of a small molecule inside a protein. Most recently, he has undertaken the challenging problem of understanding the ultrafast dynamics of protein folding and unfolding. On a personal level, as graduate students, we have both benefited from Robin’s contagious enthusiasm for science. He inducted the concept of versatile and broad thinking, and he demanded the highest quality research. Robin will not rest on his laurels, and his happiness will continue to come from science and the advancement of new frontierssRobin, stay as a young, competitiVe assistant Professor! We wish you many more successful and healthy years ahead and happy celebration for these outstanding contributions. William A. Eaton Ahmed H. Zewail JP963477Z
© 1996 American Chemical Society
