THE IONIZATION OF THERMAL AND HYPERTHERMAL BEAMS OF NA, K, AND CS ON SI(111) SURFACES

Measurement of the energy dependence of the yields of Na+, K+, and Cs+ ions formed when beams of the corresponding alkali atoms M hit a Si(111) surface and of Na+ for Na striking a Pt surface at a temperature Tare those expected for equilibrium for thermal kinetic energies E of the incident atoms. Above a threshold energy of 0.5-1.0 eV the yields rise rapidly to maxima greater than 0.1 and then remain approximately constant as the energy increases to 100 eV. They are nearly independent of T in the range 300-1100 K. The results are represented well by a classical model having (1) a rate of electron transfer that varies exponentially with the distance z of M from the surface, (2) potentials that slow the incoming atom down as it nears the surface, and (3) energy transfer by elastic two-body collisions with the surface represented as hard cubes, each having the mass of an integral number n of surface atoms. At x= 5 Å the rate on Si for Na and K is 1012.5 ±0.2 s-1 with n equal to 3 and 5, respectively, while for Cs it is three times less with n equal to 20. For Na on Pt it is 1012.9 ±0.1s-1 with n equal to 1. © 1990 American Institute of Physics.