HYDROGEN DISSOCIATION ON COPPER - IMPORTANCE OF DIMENSIONALITY IN CALCULATIONS OF THE STICKING COEFFICIENT

The dissociative adsorption of H-2 on copper is studied using classical trajectory an quantum wave-packet calculations. A multidimensional effective-medium potential is used which takes the molecular orientation as well as the surface structure into account. The emphasis in this work is on the effects of including all six molecular degrees of freedom in calculations of the sticking coefficient and the implications on comparisons with experimental data. We find that six-dimensional effects, among other things, lead to substantial shifts of the sticking onset energies. The change of onset energy due to vibrational excitation or isotope exchange is also sensitive to six-dimensional effects. This means that a two-dimensional analysis of the relation between observed onset energies and potential barriers sometimes may be misleading. The agreement between calculated and experimental sticking curves is improved by including six dimensions. The surface atom motion, however, is found to have a negligible effect on the sticking coefficient at surface temperatures used in recent molecular beam measurements. By comparisons in reduced dimensionality we find that a quasiclassical calculation gives a reasonable account of the true quantum sticking coefficient in the present case. There are significant quantum effects but these appear small compared to the multidimensional effects. A multidimensional combined quantum and classical approach seems to be appropriate for the dissociation dynamics problem.