Singlet-to-triplet conversion of metastable He atoms at alkali-metal overlayers

Energy distributions of electrons emitted from alkali-metal surfaces by impact of metastable He atoms reveal that there is a high probability for transformation of singlet atoms (excitation energy ${\mathit{E}}^{\mathrm{*}}$=20.6 eV) into triplet atoms (${\mathit{E}}^{\mathrm{*}}$=19.8 eV) prior to deexcitation into the ground state. The conversion probability (as expressed by the ratio R of the intensities of valence-band emission due to triplet and singlet ${\mathrm{He}}^{\mathrm{*}}$ deexcitation, respectively) increases with increasing alkali-metal coverage on a Ru(0001) substrate, and in turn decreases with increasing oxygen exposure at a fixed alkali coverage. These findings indicate that R is a qualitative measure for the degree of ‘‘metallization’’ of the adlayer. R also increases with temperature due to broadening of the nearest-neighbor distribution whereby, on the average, a larger part of the adlayer becomes metalliclike. For Cs overlayers exhibiting work functions <2 eV the mechanism of deexcitation changes and may proceed via ${\mathrm{He}}^{\mathrm{*}\mathrm{-}}$ (1${\mathit{s}}^1$2${\mathit{s}}^2$) formation as reflected by the R data as well as by the widths of the electron spectra.