Sharp pair spectra and excited states of rare-gas impurities in metals

Optical-absorption spectra in the photon energy range 6-11.5 eV are reported and interpreted for Kr and Xe impurities present at various concentrations in random rare-gas-alkali-metal alloys. The Xe spectra show structure at energies more than 2.5 eV above threshold, even in dilute alloys. This is interpreted as arising from d-like excited states, and provides evidence that the electron gas near impurities can be excited into a variety of persistent configurations. Sharp symmetrical absorption lines {~}200 meV wide that emerge with increasing impurity concentration in both Kr and Xe alloys are shown to originate from pairs of rare-gas atoms. The sharpness of these pair lines and the general form of the spectra are interpreted in terms of specific ground and excited configurations closely related to states of diatomic rare-gas molecules. Similar results are also presented for heteroatomic Xe-Ar pairs.