Optical excitation spectra of adsorbates on dielectric surfaces

Abstract

Optical absorption spectra of adsorbates on dielectric surfaces have been considered at low coverage. Radiative and nonradiative processes have been taken into account. Expressions for the spectral functions have been derived describing the spectra of the symmetric and antisymmetric modes, respectively. At low coverage and when the damping of the surface electron–hole pairs is much greater than the effective radiative damping, the relative intensities of the symmetric and antisymmetric modes take positive and negative values describing the physical processes of absorption (attenuation) and stimulated emission (amplification), respectively. Thus, peaks of the symmetric and antisymmetric modes arising from two nearby excited states of the adsorbed atom may cancel each other out, provided that the frequency profiles of the peaks in question overlap. Numerical calculations have been made for the spectra of rare-gas atoms He, Ne, Ar, Kr, and Xe which are physisorbed on graphite. The resulting excited geometrical configurations arising from the cancellation process between the spectra of the symmetric and antisymmetric modes have been computed as a function of the distance R from the atom to the dielectric surface, and they are graphically presented. The disappearance or the persistence of the excited geometrical configurations may help to determine the distance R at which the atom is physisorbed on the dielectric surface under investigation.