The optical excitation spectra of neutral rare-gas atoms physisorbed on metal surfaces have been considered. Emphasis has been given to the dynamic effects of the surface plasmons on the lifetimes of the adsorbed atoms. At low coverage and when the damping of the surface plasmons is much greater than the effective radiative damping, the spectral functions of the symmetric and antisymmetric modes consist of asymmetric Lorentzian lines, whose asymmetry depends on the strength of the surface plasmons. At this limit 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. Hence, the occasional disappearance of the spectral lines of the optical absorption is due to a cancellation process, which takes place between the frequency profiles arising from two nearby excited states of the adsorbed atom. The red shifted peak of the symmetric mode of the higher excited state and the blue shifted peak of the antisymmetric mode of the lower excited state of the atom cancel each other out provided that their frequency profiles nearly coincide. This may be a possible explanation of the persistence-extinction phenomenon that has been observed for a number of rare-gas substrate systems in the low coverage limit, where it has been proposed that a charge-transfer instability exists. Numerical results indicate that the peaks of excited Xe on Al and excited Kr on Au vanish in the low coverage limit.
Quantum theory of spontaneous and stimulated emission of surface plasmons
