Time-resolved Spectroscopy of Anharmonic Systems

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Keith Nelson of the Massachusetts Institute of Technology is supported by the Experimental Physical Chemistry Program in his continuing investigations of large amplitude lattice vibrations using time- and space-resolved femtosecond optical pulse-shaping techniques. Initially, he will focus on crystals that undergo structural phase transitions, in particular the switching among different ferroelectric phases. In addition, he will look at semiconducting materials whose bandgaps may be influenced and perhaps closed by optically induced large-amplitude coherent lattice vibrations. Nelson also plans to examine the behavior of molecular crystals in which the optical pulses induce motion along reaction coordinates to affect crystalline chemical reactions. He will investigate both bulk and thin film perovskites to study structural phase transitions and phonon-polariton cavities; diacetylene, perylene and pyrene to study molecular crystals; and Ti2O3 and GeTe as semiconductors. Prof. Nelson has demonstrated that significant nuclear displacements can be induced in molecular crystals using high energy pulses. In addition, if the wavelength is long enough, damage to the crystal is minimized. In these studies Nelson will investigate alternative strategies for achieving large nuclear displacements and what types of effects the molecular properties have on the optimum strategy. Prof. Nelson will apply techniques developed in his laboratory that exploit femtosecond optical pulses to a wide variety of materials such as ferroelectric crystals and small gap semiconductors, in order to determine if it is possible to effect a chemical change. In addition to the direct advances this work will likely offer to the fields of lattice spectroscopy and optical control of molecular processes, it can be anticipated that the planned research will be of technological interest and utility to designers of prototype devices for optical switching and information transmittal.