VIBRATIONAL SPECTROSCOPY AS A PROBE OF SURFACE HETEROGENEITY - CO ON NACL(100)

The fundamental (Delta v = 1 <-- 0) and the first overtone (Delta v = 2 <-- 0) transitions of monolayer physisorbed CO on NaCl(100) single crystal surfaces at 5 K have been investigated using polarized Fourier transform infrared spectroscopy. Comparisons of the transition frequencies, bandwidths and absorbances for these two transitions together with those of dilute isotopes allow us to isolate the contribution of dynamic and static coupling effects of molecules within the monolayer. Homogeneous and heterogeneous effects can also be distinguished from band profiles of these various transitions. We conclude that the residual line broadening of 0.1 cm(-1) at 5 K for the isolated CO isotopes arises from heterogeneous surface effects. We set an upper limit of 0.07 cm(-1) for the linewidth of single CO molecules on NaCl(100) due to surface heterogeneity. A lower limit of 1 x 10(-8) cm(-1) is provided by the vibrational lifetime of adsorbed CO molecules. Sharpness of the vibrational features shows that infrared spectroscopy of adsorbate is a sensitive method for probing surface and adlayer irregularities of adsorbed molecules on a single crystal.