Effect of the Ar-Ni(s) potential on the cross section for Ar+CH ₄/Ni{111} collision-induced desorption and the need for a more accurate CH ₄/Ni{111} potential

In a previous paper [L. Sun, P. de Sainte Claire, O. Meroueh, and W. L Hase, J. Chem. Phys. 114, 535 (2001)], a classical trajectory simulation was reported of CH ₄ desorption from Ni{111} by Ar-atom collisions. At an incident angle θ _i of 60° (with respect to the surface normal), the calculated collision-induced desorption (CID) cross sections are in excellent agreement with experiment. However, for smaller incident angles the calculated cross sections are larger than the experimental values and for normal collisions, θ _i=0°, the calculated cross sections are approximately a factor of 2 larger. This trajectory study used an analytic function for the Ar+Ni(s) intermolecular potential which gives an Ar-Ni{111} potential energy minimum which is an order of magnitude too deep. In the work reported here, the previous trajectory study is repeated with an Ar+Ni(s) analytic intermolecular potential which gives an accurate Ar-Ni{111} potential energy minimum and also has a different surface corrugation than the previous potential. Though there are significant differences between the two Ar+Ni(s) analytic potentials, they have no important effects on the CID dynamics and the cross sections reported here are nearly identical to the previous values. Zero-point energy motions of the surface and the CH ₄-Ni(s) intermolecular modes are considered in the simulation and they are found to have a negligible effect on the CID cross sections. Calculations of the intermolecular potential between CH ₄ and a Ni atom, at various levels of theory, suggest that there are substantial approximations in the ab initio calculation used to develop the CH ₄+Ni{111} potential. The implication is that the differences between the trajectory and experimental CID cross sections may arise from an inaccurate CH ₄+Ni{111} potential used in the trajectory simulation.