The effects of alignment on the dissociation of H₂ on Pd(111)

The effects of rotational alignment in surface scattering phenomena are studied. Alignment means a spatial preference for the plane of rotation. Specifically, changes in the reflection probability for H2 reaction with Pd(111) were studied for different molecular rotational alignments (helicopters vs. cartwheelers) and different incident translational energies. Experimentally, molecules were prepared with well-defined, non-statistical alignments using a linearly polarized laser before interaction with the surface. After scattering, the molecules were probed with another laser to determine their internal state. We found that the reflectivity of helicopter molecules was less than that of cartwheeler molecules. This effect increased with increasing incident energy. The result is interpreted as meaning that helicopters are more likely to dissociate than to scatter. The results are further evidence of steric forces in the H2/Pd interaction.