Sputtering of condensed gases by nuclear stopping: Chemical aspects

Abstract

Sputtering of room-temperature solids has been studied now for nearly a century and a 1 half, and an enormous body of data on the various aspects of sputtering of these materials has been collected. Sputtering of the much more fragile and elusive condensed gases, however, has been studied systematically only for the last dozen years. The motivation for studying condensed gases comes primarily from the need to quantify erosion rates and chemical transformations of icy astronomical objects such as comets and certain Saturnian and 2-11 Jovian satellites, the need to understand erosion processes of pellets of solid hydrogen and deuterium injected into plasma devices, 12'13 and the need to understand the effects of high-energy particles impacting on cryopumping surfaces 14'15 ~ such as those found in fusion reactors. Additional motivation has been provided, particularly in our laboratory, by the discovery that the sputtering of these surfaces induces interesting chemical transformations in the residual solid 1619 and forms an excellent intense source of a broad range of cluster ions 16'19-32 which are difficult to generate by other means. Finally, an understanding of the detailed nature of the processes that lead to the ejection of ions from chemically simple frozen gases may provide insight into the mechanisms underlying the currently very popular tools of analytical chemistry, secondary ion mass spectrometry (SIMS) and fast-atom-bombardment mass spectrometry (FABMS). Considerable progress has been made in the relatively short time that condensed gases have been studied, and the physical and chemical processes specific to this type of target are beginning to be understood. Enough is now known that a number of general observations are commonly accepted. The sputtering yield is independent of temperature below a threshold; measured yields are as much as an order of magnitude greater than those predicted by standard linear cascade theory and increase rapidly with increasing stopping power; the kinetic energy distribution of the sputtered material retains the E 2 tail characteristic of a linear cascade but peaks at a very low energy; the large cluster ions which are observed preferentially retain polar or polarizable solvating units; only selective chemical transformations occur when reactive frozen gases or mixtures of gases are sputtered; and sputtering yields of condensed inert gases have been observed to decrease with increasing ion dose from a high initial value. Most of these observations have been summarized in a number of recent reviews which emphasize various areas of interest. Schou has discussed primarily the electronic sputter33 ing of condensed gases. Sigmund has presented the current state of theoretical work on