Theoretical study of vibrational excitation of ammonia scattered from Cu Original Research Article

A time-dependent quantum-mechanical method is used to study the vibrational excitation of ammonia scattered from the Cu(111) surface. The two-dimensional model includes the translational coordinate and the umbrella vibrational coordinate of ammonia. Empirical potential energy surfaces are used in the calculation. It is found that the vibrational excitation probabilities of the scattered NH3 increase linearly with the normal incident kinetic energy above the excitation threshold. However, the vibration-translation correlation for the ND3 deviates somewhat from linearity. A much larger overall vibrational inelasticity is found for the deuterated isotope, which is attributed to its heavier mass and lower vibrational frequency. The mechanism for vibrational excitation is analyzed by the scattering of single channel wave packets. It is shown that vibrational excitation occurs almost exclusively for the initially nitrogen-up wave packet. Our theoretical results are in reasonably good agreement with experimental observations, in support for an electronically adiabatic, translational-to-vibrational energy transfer mechanism.