Excitation Mechanism in the Collision-Induced Dissociation of Methane Molecular Ion at Kiloelectronvolt Translational Energy

Abstract

Collision-induced hydrogen loss from methane molecular ion has been studied at kiloelectronvolt laboratory translational energy by using mass-analyzed ion kinetic energy spectrometry (MIKES). The kinetic energy release (KER) distribution evaluated from the MIKE profile is bimodal: a small KER component arising from vibrational excitation and a large KER component from electronic excitation. CH4•+ has been generated by electron ionization and by charge exchange ionization to vary its internal energy content. It has been found that the collision-induced dissociation (CID) via electronic excitation becomes more efficient at higher incident energy, while the efficiency of the vibrational excitation is hardly affected by the incident energy. Most importantly, it has been found that the electronic excitation is the main mechanism in CID at kiloelectronvolt incident energy. Vibrational excitation also occurs but supplies only a few vibrational quanta and hence is important for the least endoergic process only.