Infrared spectroscopy of CO on NaCl(100). II. Vibrational dephasing and band shapes

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Abstract
Temperature dependence of the fundamental vibrational transition of CO on NaCl(100) single crystal surfaces has been measured between 4 and 55 K using Fourier-transform infrared spectroscopy. The absorption profiles at various temperatures are very nearly symmetric. At 4 K, the transition is centered at 2154.93 cm−1, and the full-width at half-maximum (FWHM) is 0.17 cm−1; at 55 K, the transition shifts to 2155.14 cm−1, and the FWHM broadens to 0.39 cm−1. The temperature-dependent frequency shift and broadening of the fundamental transition are interpreted by a vibrational phase relaxation process, in which the coupling between the stretching mode of CO and a low frequency CO-surface mode causes the dephasing of the excited state vibrational motion. The Boltzmann factor associated with the spectroscopic temperature dependence allows us to determine that the surface mode has a frequency of 40 cm−1 . We associate this mode with the frustrated translational motion of CO over the NaCl(100) surface. The observed temperature-dependent absorption band undoubtedly is a consequence of collective motions among the CO stretching vibrations within the monolayer. However, the current model, that assumes this collective motion is coupled by the CO transition dipoles, cannot explain the data. Further theoretical work will be required to understand the infrared spectroscopy of CO on NaCl(100). Below 10 K, the CO fundamental absorption becomes temperature independent. However, the limiting bandwidth and bandshape depend on the NaCl(100) crystal surface preparation. For one set of crystals, the 4 K bandshape was Lorentzian with a FWHM of 0.17 cm−1 . For another set, the 4 K bandshape was Gaussian with a FWHM of 0.11 cm−1. We believe these bandwidths, narrower than those reported previously for any adsorbate system, are limited by heterogeneities of the monolayer and/or substrate. The 0.11 cm−1 bandwidth provides a lower limit of 45 ps for the lifetime of vibrationally excited CO on NaCl(100).