Extremal Effects in Rotationally Inelastic Diffraction from Crystal Surfaces

Molecules diffracted from a crystal can exchange energy between translation and rotation. Such an interchange can lead to an extremum in the scattering angle as a function of the incident wavelength; this in turn concentrates scattered intensity at the extremal angle, causing a singularity in the differential cross section. Extremal scattering in rotationally inelastic collisions is explored quantitiatively for the diffraction of hydrogenic molecules and shown to affect significantly experiments with thermal beams.