Vibrational Linewidth of a Chemisorbed Polyatomic

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In this project in the Physical Chemistry Program, Buch will calculate the vibrational line shapes of molecules which are chemically absorbed on the surface of platinum. The calculated line shapes for the ethylidyde molecule will then be compared with the detailed experimental results of Trenary et al. This study will examine the influence of the high frequency adsorbate modes on the vibrational broadening mechanisms. The results of the calculations will then be used to construct simplified models of a "system/bath" partition concept. Line broadening mechanisms due to the interaction of the vibrational modes of the adsorbate with those of the solid will be studied. An atomic level approach will be used in which IR line shapes are derived quantum-mechanically from the microscopic force field. The focus will be on ordered overlayers of adsorbate which will take advantage of system periodicity to reduce the dimensionality of the problem. Zero order motion of the system will be described by the adiabatic approximation, in which the low frequency substrate and adsorbate modes move in the average field of the high frequency adsorbate modes. The resulting shifts in the low frequencies will be calculated for both the ground and excited states of the adsorbate, and used to evaluate the dephasing contribution to the linewidth. The vibrational decay contribution will be calculated with the help of the golden rule. The calculations will include no adjustable parameters; the data on the microscopic force field will be adopted from the literature. The markedly different widths and temperature dependencies of the three spectral lines measured experimentally by Trenary et al., will offer a unique opportunity to study mode specific effects in a chemisorbed polyatomic. This will be the first quantum-mechanical atomic level study of IR line shapes of a polyatomic adsorbate, including both decay and dephasing contributions for a realistic microscopic potential.