Neutral excited configuration of rare-gas atoms adsorbed on alkali metals

We report optical-excitation spectra of rare-gas atoms adsorbed on alkali-metal surfaces. The data were obtained with the use of differential reflectance methods with synchrotron radiation. Optical absorption appears to increase almost linearly from 0 at a well-defined excitation threshold energy that agrees quantitatively with the theoretical prediction for rare gases adsorbed on metals. The absorption continuum above threshold, for adsorbates distributed at dilution over the surface, is broken by the spin-orbit partner of the $P_{\frac{3}{2}}$ threshold process and by excitation to higher atomiclike configurations. Additional features which grow with coverage originate from adsorbate pairs. Both the threshold profile and pair peaks bear noticeable resemblance to the properties of rare-gas atoms alloyed into alkali metals, which have been investigated in earlier work. No theory is currently available to describe either the linear threshold profile or the pair peaks. The data establish unambiguously that the neutral excited rare-gas adsorbate configuration is created by the optical excitation, rather than the ionic configuration produced in photoemission work. These various properties are systematically described by a quantitative phenomenological model.