VIBRATIONAL OVERTONE SPECTROSCOPY AND COUPLING EFFECTS IN MONOLAYER CO ON NACL(100)

Monolayer physisorbed CO on NaCl(100) single crystal surfaces has been investigated with polarized Fourier transform infrared (FTIR) spectroscopy at 5 K. While the fundamental (upsilon = 1 <-- upsilon = 0) transition of the monolayer has been studied previously, we report for the first time the overtone (upsilon = 2 <-- upsilon = 0) transition. Its integrated absorbance is very weak, being only 0.5% of the fundamental. Comparisons of the transition frequencies, the 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. We can also distinguish homogeneous and heterogeneous effects from band profiles of these various transitions. The method of analysis of these data is the exciton model appropriate to treating the coupling of the electric multipoles of molecules within the adlayer.