[en] Measurements of the rate of desorption of alkali atoms from the surfaces of Si crystals reveal surface phase transitions, a well known one for the (111) face at 1020 +- 40 K and a previously unreported one for Si(100) at 980 +- 20 K. The kinetics of the desorption can be interpreted with a model that has a relatively smooth surface above the transition and a less uniform one below. The latter has special sites, such as edges, where the atoms are bound more strongly than they are at the terraces between the edges. Tetramethyldioxetane molecules seeded into an excess of hydrogen in a nozzle beam can be caused to decompose to two acetone molecules by their impact on a surface. The reaction is followed by detecting blue light emitted by electronically excited acetone formed in the collision. The light increases approximately as the cube of the excess of the kinetic energy over a reaction threshold of 50 +- 10 kJ mol^-1. The amount of light varies with the nature of the surface and its temperature. On the eight surfaces tested it is largest for a polymer of perfluorinated ethylene and propylene. A simple model is proposed to account for the compensation effect that is observed when desorption occurs from surfaces that undergo a phase transition. Measurements are reported of the angular distribution of beams of He and Ar diffracted from crystals of LiF