The role of the methyl ion in the fragmentation of CH42+

The fragmentation of the CH42+ dication, formed upon C1s inner shell ionization and the following Auger electron decay, has been studied by means of Auger electron-ion and Auger electron-ion-ion coincidence spectroscopies at three different kinetic energies of the Auger electron, which correspond to the three main configurations (1t(2)(-2), 1t(2)(-1) 2a(1)(-1) and 2a(1)(-2)) of the dication. For each ion pair detected in coincidence with the Auger electron, the dynamics of the dissociation is discussed. The results show that the fragmentation process is stepwise and mainly occurs via the formation and dissociation of the CH3+ intermediate. The orientation of the methyl ion, induced by a dipole moment during the first step of the fragmentation, is suggested to play a key role. Moreover, the experiments together with ab initio CAS calculations of the equilibrium geometries of the methane dication and the methyl cation provide information on the structure of the intermediate methyl ion.