Rotationally inelastic diffraction of molecular beams: H2, D2, HD from (001) MgO

In the coherent scattering of molecules from a crystal surface, energy of molecular rotation can be transferred to translational motion. This interchange has been studied for thermal beams of H2, D2, and HD diffracted from (001) surfaces of MgO prepared by cleavage in an ultrahigh vacuum. Strong new diffraction peaks are identified for in-plane scattering, beyond those predicted for elastic interactions. The location of these peaks is in agreement with calculations based on the assumption that phonons are not excited during the collision with the lattice. In the experiments, only those rotational transitions are detected for which there are extrema in the variation of the scattering angle with the wavenumber of the incident molecules. Scattering at these critical angles is analyzed and it is shown that for beams formed by effusion, these extrema lead to sharply peaked scattering maxima, which facilitate detection of rotational--translational interchanges. The efficiency of rotational transitions is found to be high, comparable to elastic scattering, and to depend strongly upon the angle between the incident beam and the scattering surface