Knowledge Bank

Vibrational spectra of CO physisorbed on NaCl(100) single crystal surfaces mounted in an ultrahigh vacuum chamber were obtained using a Fourier transform infrared interferometer. Structures of CO from a monolayer to thin slabs of multilayers have been explored. By probing with polarized light, monolayer CO molecules are shown to be adsorbed perpendicularly to the $surface1$. Polarized multilayer spectra show that CO molecules stacked above the monolayer are also aligned but in arrangements consistent with the bulk crystal structure of $\alpha -CO$. This structure is a consequence of the near perfect match of the $\alpha -CO$ lattice constant and that of the NaCl(100) substrate. Features of both longitudinal and transverse modes of multilayer CO were observed and the results imply that these two modes have different damping constants and consequently different vibrational relaxation times. The dependence of absorption profiles on sample thickness was also examined systematically. Theoretical formalism to account for the frequency shift, band splitting and broadening will be discussed.