Trajectory dynamics study of collision-induced dissociation of the Ar+CH4 reaction at hyperthermal conditions:: Vibrational excitation and isotope substitution

We investigate the role of vibrational energy excitation of methane and two deuterated species (CD4 and CH2D2) in the collision-induced dissociation (CID) process with argon at hyperthermal energies. The quasiclassical trajectory method has been applied, and the reactive Ar + CH4 system has been modeled by using a modified version of the CH4 potential energy surface of Duchovic et al. (J. Phys. Chem. 1984, 88, 1339) and the Ar-CH4 intermolecular potential function obtained by Troya (J. Phys. Chem. A 2005, 109, 5814). This study clearly shows that CID is markedly enhanced with vibrational excitation and, to a lesser degree, with collision energy. In general, CID increases by exciting stretch vibrational modes of the reactant molecule. For the direct dissociation of CH4, however, the CID cross sections appear to be essentially independent of which vibrational mode is initially excited. In all situations studied, the CID cross sections are always greater for the Ar + CD4 reaction than for the Ar + CH4 one, the Ar + CH2D2 being an intermediate situation. A detailed analysis of the energy transfer processes, including their relation with CID, is also presented.