Vibrational line shapes of low-frequency adsorbate modes: CO on Pt(111)

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Abstract
We present a study of the coverage and temperature dependence of the Pt-CO stretch vibration. As a function of coverage, the width of the absorption peak exhibits local minima at the ordered (4×4), (8×8), and c(4×2) structures. For the c(4×2) structure the temperature dependence of the peak width and position is in good agreement with the exchange model of vibrational phase relaxation. The exchange mode is found to be the parallel frustrated translation, i.e., the same mode that gives the dominating dephasing contribution to the C-O stretch vibration. For T>250 K, the linewidth increases much stronger than predicted by the exchange model which is attributed to a thermally induced order-disorder transition. Finally, we present Monte Carlo simulations as a function of coverage, in order to study some aspects of the disorder-induced contribution to the linewidth.