The ionization of thermal and hyperthermal beams of Na, K, and Cs on Si(111) surfaces

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Abstract
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 T are 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 z=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±0.1 s− 1 with n equal to 1.