Rotational Alignment in the Associative Desorption Dynamics of Hydrogen Molecules from Metal Surfaces

We report that the alignment factors of hydrogen molecules desorbing in the vibrational ground state from Cu(111) and Pd(111) vary nonmonotonically with the final rotational state j depending on the translational energy. Upon averaging over the Boltzmann distribution of total kinetic energies at surface temperatures relevant to existing experiments, the resulting alignment factor value corresponds to a small preference for helicopter-like rotation.