Formation ofH−andD−ions by hydrogen and deuterium particle backscattering from alkali-metal surfaces

A model has been developed for the generation of negative ions by particles backscattered from alkali-metal surfaces. The negative-ion secondary-emission coefficient (NISEC) is taken to be the product of the backscattered particle energy and angular distribution, a negative-ion formation probability, and a survival probability. The backscattered distributions are calculated using the Marlowe Monte Carlo reflection code. The formation and survival probabilities have exponential forms with adjustable parameters $\alpha$, $\beta$. A least-squares fit of the NISEC integral to the experimental data provides semiempirical values for the $\alpha$, $\beta$. The survival probabilities are found to vary inversely with surface work function. The formation probabilities are dependent upon work function and valence electron density. The semiempirical survival probabilities are in fair agreement with earlier model calculations. The quality of the least-squares fits together with the survival probability comparisons represents a first point of contact for theory and experiment.