Vibrons in a disordered monolayer: Application to the determination of theinfrared and sum frequency generation spectra of (CO)-C-12 and (CO)-C-13 isotopic mixtures adsorbed on NaCl(001)

The internal dynamics of a disordered monolayer formed by a mixture of (CO) -C-12 and (CO)-C-13 isotopes is studied using a description in terms of vib ron, propagation and scattering by isotopic defects randomly distributed in the layer. The spectral densities connected to the infrared and sum freque ncy generation vibrational spectra of the disordered monolayer are expresse d in terms of the average Green propagator of the vibrons using a generaliz ed t-matrix approximation to describe the collision of vibrons with the dis tributed defects. Our model can interpret the experimental infrared spectra of CO monolayers adsorbed on NaCl(100) and explain the behavior of the lin ewidth with temperature and isotopic concentration. Differences in the infr ared and sum frequency generation line shapes and intensities are discussed . Nonlinear spectroscopy experiments on this system should evidence these f eatures. [S0163-1829(99)08043-1].