A quantitative comparison of the dissociation probabilities of H-2 On
the W(100) surface obtained from quantum and classical dynamical simul
ations is presented. Four dimensions are employed, and the vibrational
ly adiabatic approximation is used to circumvent problems in classical
mechanics associated with nonconservation of zero-point energy. For n
ormal incidence, we find good agreement between the classical and quan
tum dissociation probabilities. Both are dominated by strong steering
into geometries favorable for dissociation. Exciting either initial ro
tations 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 gre
ater as the initial parallel momentum is increased. To understand this
, we employ two new visualization techniques. For the classical dynami
cs, we use swarms of color-coded trajectories to illustrate the detail
ed motion of an ensemble. While for the quantum system, we project tim
e-dependent wave packets onto a local basis set, adiabatic in the inte
rnal (rotational and parallel translational) degrees of freedom, plott
ing 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 c
rossings in the states populated at off-normal incidence, leading to m
otion which cannot be reproduced by a classical system. (C) 1998 Ameri
can Institute of Physics.