Effect of nonconservative forces on the neutralization of sputtered and other desorbed ions

We discuss the effect of nonconservative forces on ions near surfaces and apply it to recent experimental work of Yu on the sputtering of O^- from Nb and Va. These data displayed a significant deviation from the usual exponential dependence of yield on 1/v_⊥. This deviation which occurs at low velocities was partially explained by Lang using a model in which the sputtered ion interacts with a single surface atom via a Morse potential. Although his model suggests that the effective velocity of the ion is larger than that observed it does not explain the full deviation of the effective velocity, which goes through a minimum and then turns up as the observed velocity goes to zero. In our paper we show that just by including a nonconservative force in the Lang model the full deviation can be explained nicely.