FORMATION OF H- IN THE COLLISION OF H(3S) WITH HELIUM AND NEON

The formation of H- in the collision of excited (3s) atomic hydrogen with helium and neon is investigated. The ratio (sigma3s/sigma2s) of the cross sections for electron capture on H(3s) and H(2s) is measured through an analysis of the kinetic energy spectrum of H-, which, in the case of helium, also allowed the measurement of the ratio (sigma2s/sigma1s). A resonant cavity is used, in the adiabatic rapid passage configuration, to increase the laser power used to excite the 3s state from the metastable state. An excitation yield as high as 90% is obtained. The measurements cover the range 300-3000 eV and confirm the important growth of the cross section ratio at the low energy side.