Observation of a dynamical propensity rule in rotationally inelastic gas-surface scattering: NH3on Au(111)

We have measured rotational state distributions for N${\mathrm{H}}_3$ scattered from Au(111) using multiphoton-ionization laser spectroscopy. The N${\mathrm{H}}_3$ exhibits a strong propensity to scatter into the lowest-$K$ states; these are states in which the molecule is tumbling normal to the molecular symmetry axis as opposed to spinning about it. Additionally, the scattered rotational distribution exhibits a sharp feature near zero rotational energy, indicative of a rotational rainbow.