Comparing quantum and classical dynamics:: H2 dissociation on W(100)

A quantitative comparison of the dissociation probabilities of H-2 On the W(100) surface obtained from quantum and classical dynamical simulations is presented. Four dimensions are employed, and the vibrationally adiabatic approximation is used to circumvent problems in classical mechanics associated with nonconservation of zero-point energy. For normal incidence, we find good agreement between the classical and quantum dissociation probabilities. Both are dominated by strong steering into geometries favorable for dissociation. Exciting either initial rotations or parallel translations reduces the effectiveness of steering, thus reducing the dissociation probability. For the former, the good agreement between classical and quantum still holds, however. for the latter this is not so, and the disagreement becomes progressively greater as the initial parallel momentum is increased. To understand this, we employ two new visualization techniques. For the classical dynamics, we use swarms of color-coded trajectories to illustrate the detailed motion of an ensemble. While for the quantum system, we project time-dependent wave packets onto a local basis set, adiabatic in the internal (rotational and parallel translational) degrees of freedom, plotting the results on the correlation diagram. Examination of these shows that the behavior of adiabatic states populated for normal incidence is dominated by a combination of steering and orientational hindering, both classical phenomena. In contrast, there are very large avoided crossings in the states populated at off-normal incidence, leading to motion which cannot be reproduced by a classical system. (C) 1998 American Institute of Physics.