Investigation of quantum effects on vibrational excitation of CF3Br scattering from graphite

Artikel i vetenskaplig tidskrift, 2002

Mixed quantum–classical molecular dynamics simulations of vibrational excitation of CF3Br in collisions with graphite have been carried out. The quantum problem is made numerically manageable by a reduced dimensionality treatment of the internal degrees of freedom including only the C–Br stretch and the umbrella motion. These two coordinates are treated quantum mechanically as a 2D wavepacket whereas the translational and rotational degrees of freedom and the surface atoms are treated classically. A 3D wavepacket scheme where the distance to the surface is included among the quantum degrees of freedom is also considered as well as purely classical trajectories. It is found that the total vibrational excitation is quite low. Excitation of the C–Br stretch dominates with only a very small fraction of the excitation energy going into the umbrella motion. The 2D wavepacket calculations and the classical trajectories (initiated without zero point energy) give nearly identical results for the total excitation but the 2D wavepacket model excites the umbrella motion to a lesser extent. The 3D wavepacket method shows a considerable dependence on the initial width of the wavepacket due to the underlying mean-field approximation. All methods show a linear increase in vibrational excitation with increasing surface temperature, in agreement with previous experimental results.