Dynamics of Ar+CH4/Ni{111} collision-induced desorption

Classical trajectory simulations were used to study Ar+CH4/Ni{111} collisio n-induced desorption and compared with experiment. To perform the simulatio ns, analytic potentials were determined for Ar/CH4 and CH4/Ni{111}. An accu rate form for the former potential was derived by carrying out a series of nb initio calculations at various levels of theory, while previously publis hed ab initio calculations were used to develop the latter CH4/Ni{111} pote ntial. Overall the simulation and experimental desorption cross sections ar e in excellent agreement, except at Small incident angles theta (i) (with r espect to the surface normal) and low initial Ar translational energies, E- i, where the simulation cross sections are approximately a factor of 2 too large. Most of the desorption occurs by trajectories in which Ar first stri kes CH4, but for both large theta (i) and E-i, a small fraction of the deso rption occurs by trajectories in which Ar first strikes the Ni surface. Exc itation of the CH4 vibrational modes is negligible and CH4 rotation receive s less than 10% of the available energy. Most of the available product ener gy is partitioned to CH4 translation and to the Ni surface and Ar atom. At low E-i, CH4 translation receives the majority of the available energy, wit h the effect greater for large theta (i). At high E-i, approximately 40% of the available energy goes to CH4 translation, independent of theta (i). Th e CH4 translational energy distribution is multimodal and its peaks may be associated with trajectories in which the Ar atom rebounds off or sticks to the Ni surface and collisions in which Ar strikes CH4 with small and large impact parameters. (C) 2001 American Institute of Physics.