Pressure and target gas effects in the collision-induced dissociation of methane molecular ions

Abstract

An approach to describing the decomposition patterns of polyatomic ions as a function of target gas pressure is presented. The kinetics of a decomposition scheme can be described employing a set of simultaneous first-order differential equations. From the pressure resolved data, the different contributions to the fragment ion intensities arising from single or multiple collision events can be calculated. Provided certain conditions hold, a high target pressure MS/MS spectrum can be predicted from data acquired at low target gas pressure. The decomposition pattern of 6 keV CH 4 + ions has been measured as a function of pressure for the three target gases He, Ar and Xe in order to illustrate the utility and limitations of the approach. The target gas pressure was varied from 0 to 13 Pa. An estimate of the absolute cross-section for collision-induced dissociation of the methane molecular ion is given. Furthermore, we conclude that, for the targets investigated, the internal energy deposition per collision follows the order He &>; Ar &>; Xe.