Singlet to Triplet Conversion of Metastable He Atoms during Deexcitation at a Cs-Covered Surface

Electron emission from a Cs monolayer on a Cu(110) surface by impact of metastable ${}_{}{}^3{}_{}{}^{}S$ ${\mathrm{He}}^{*}$ atoms is caused by Auger deexcitation as demonstrated by identification of Cs $5p$- and $6s$-derived levels in the electron energy distribution curves. Such curves recorded with ${}_{}{}^1{}_{}{}^{}S$ ${\mathrm{He}}^{*}$ are almost identical, and it turns out that only a small fraction of the singlet atoms undergo direct Auger deexcitation while the major part is converted into the triplet state from where Auger deexcitation occurs. A novel mechanism for this spin-flip process is proposed; it is accompanied by the creation of an electron-hole pair in the target for conservation of energy and total spin.