SINGLET-TO-TRIPLET CONVERSION IN LOW-ENERGY METASTABLE HELIUM METAL-SURFACE COLLISIONS

Metastable de-excitation spectroscopy studies of alkali coated metals revealed the existence of a very strong conversion of the He(2(1)S) metastable into the 2(3)S metastable state occurring prior to the metastable de-excitation on a low work function surface. Hemmen and Conrad [Phys. Rev. Lett. 67 (1991) 1314] and Brenten et al. [Z. Phys. D 22 (1992) 563] suggested that the formation of a He- (1s2s22S) resonance state could be an efficient intermediate step in this conversion. Using the coupled angular mode method, we study the energy position and widths of a He ion in front of a metal surface. This allows us to study quantitatively this conversion mechanism. It is found to be very efficient and to be associated with a strong electron emission corresponding to the decay of the He- ion to yield a ground state He atom.