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Tribute to Michael D. Fayer.

M Fayer is one of the most influential physical chemists working today. Over the course of his 40-year career, he has made fundamental and often seminal contributions to our field, most notably as a leader in the development of ultrafast nonlinear optical spectroscopy toward understanding the properties of molecules. His research combines modern experimental laser techniques with theoretical and computational methods, often addressing wide-ranging questions pertaining to the nature of chemical reactivity, structure, and dynamics. As an Assistant Professor at Stanford, Fayer was interested in applying the latest state-of-the-art technologies to outstanding questions in physical chemistry. His curiosity was at least partially driven by his childhood background as the son of an electrical engineer growing up at the dawn of the electronics revolution. It is therefore not surprising that he was among the first to recognize the chemical significance of the first photon echo experiments, which were carried out in the late 1960s by a team of physicists led by Sven Hartmann. Inspired by this development, Fayer began to experiment with these and similar techniques, which used sequences of ultrafast visible pulses to tease out and to extract information about time-resolved molecular response functions. Among the methods he developed and contributed to are the optical transient grating, the photon echo and stimulated photon echo, the infrared (IR) photon echo, the transient optical Kerr effect, and timeresolved 2D-IR spectroscopy. Fayer’s experiments investigate a wide array of chemical phenomena, ranging from the transport of energy to the dynamics of electronic excitations in molecular liquids and solids − from the structural relaxations of lowtemperature glasses to the dynamics of nanoconfined water in proteins and in fuel-cell membranes. One of Fayer’s first major interests was the dynamics of electronic excitations in amorphous solids and liquids. The opposing roles played by static and dynamic site energy disorder and spectral diffusion were among the issues addressed by his experiments, which combined transient grating spectroscopy and theory. Studies of the “hopping” transport of electronic excitations among weakly coupled chromophore sites provided fundamental insights into the nature of structural and dynamic disorder in liquids and glasses and their effects on excited-state mobility. Fayer studied the structures of polymers by characterizing the Förster energy transfer in these systems, and he extended similar ideas to probe electronic charge and energy transport in a variety of complex systems such as micelles and liquid crystals. This body of work continues to influence contemporary studies of, for example, natural and artificial light-harvesting systems, which seek to understand the relationships between molecular structure, electronic−vibrational coherences, and dissipation. Fayer’s work on low-temperature glasses exemplifies his focused ability to tackle the most challenging chemical problems and to successfully obtain critical information to understand molecular phenomena. The dynamics of glassforming liquids involve the cooperative motions of molecules over a broad range of temporal scales and are responsible for the complex and useful properties of glassy materials. By performing a variety of nonlinear spectroscopic techniques, each sensitive to a specific time window, Fayer assembled a cohesive picture of the underlying relaxations of glass-forming liquids, ranging over time scales from picoseconds to 10 000 s. The results of these studies provided important experimental information against which leading theories of liquid relaxations, such as the mode-coupling theory, have been tested. Another of Fayer’s major interests was the development of time-resolved nonlinear optical spectroscopy in the IR regime to study the vibrational motions of molecules. Fayer collaborated with members of the physics department at Stanford to develop the Free Electron Laser (FEL), which was able to produce ultrafast optical pulses in the IR. Using the capabilities of the FEL, Fayer’s group performed the first IR photon-echo experiments on molecular vibrational transitions in glasses and in liquids. This milestone experiment precipitated the rapid development of 2D-IR methods, which are now widely implemented in laboratories across the world to probe the interactions between local molecular vibrational modes, similar to the way 2D-NMR experiments probe the couplings between nuclear magnetic spin transitions. Fayer subsequently applied 2D-IR to study chemical reaction dynamics on time scales inaccessible by other methods, such as the picosecond association and dissociation of solute−solvent complexes and molecular isomerization processes. Fayer is currently engaged in research using 2D-IR techniques to better understand the properties of water and