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 c rystal surfaces at 5 K have been investigated using polarized Fourier transform infrared spectroscopy. Comparisons of the transition frequen cies, bandwidths and absorbances for these two transitions together wi th those of dilute isotopes allow us to isolate the contribution of dy namic and static coupling effects of molecules within the monolayer. H omogeneous and heterogeneous effects can also be distinguished from ba nd profiles of these various transitions. We conclude that the residua l line broadening of 0.1 cm(-1) at 5 K for the isolated CO isotopes ar ises 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 s urface heterogeneity. A lower limit of 1 x 10(-8) cm(-1) is provided b y 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 a dsorbed molecules on a single crystal.