Scattering of NO from a graphite surface at cryogenic temperatures

The scattering of NO molecules from a graphite surface at cryogenic temperatures (T_s<80 K) allows to study the molecular translation-rotation energy transfer without surface phonon contributions in the entrance channel. The angular distributions show that there are no diffusely scattered molecules at surface temperatures below T_s=70 K, whereas the signal of forward scattered molecules remains present down to T_s=20 K, the lowest temperature investigated. The rotational behavior of the scattered molecules can be described by a Boltzmann distribution characterized by a rotational temperature T_rot. It is nearly constant below T_s=80 K and is determined by the molecular kinetic energy in surface normal direction. The data are consistent with the formation of a short-lived collision complex (NO··C_n) between the NO molecule and a few surface atoms. The complex decomposes in a unimolecular fashion. The cryogenic surface temperatures require effective shielding of the crystal from the heat radiation of the surrounding experimental equipment. The data show that the heat radiation influences the crystal temperature, however, it has only negligible influence on the molecule–surface interaction.