Configurational effects in the optical spectra of Ar and Kr on Mg and Al

We have investigated the optical spectra of Ar and Kr adsorbed on Mg and Al at coverages $\Theta$ between ${10}^{14}$ and 2×${10}^{15}$ ${\mathrm{cm}}^{-2\mathrm{}}$. The spectra of Kr on Mg in $s$ and $p$ polarization vanish as ${\Theta }^2$ for $\Theta \to 0$ in the spectral region ordinarily occupied by the Kr $4p^6\to 4p^{5}5s$ resonances. There remains an optical threshold shoulder blue shifted by about 1.7 eV. In contrast, the resonance lines of Kr on Al, and of Ar on Mg and Al, persist at low coverage. Similar phenomena have been reported by Cunningham, Greenlaw, and Flynn. These and other results from the literature are discussed using a Hartree-Fock description of the rare-gas—metal complex. The energies of the optical processes can be understood in reasonably quantitative terms. No quantitatively reliable criteria are as yet available, however, to determine which of the alternative spectral forms occurs in any particular case.