Excitation of Na D-Line Radiation in Collisions of Sodium Atoms with Internally Excited H2, D2, and N2

Abstract

Excitation of D-line radiation in collisions of Na atoms with vibrationally excited N2, H2, and D2 has been studied in two modulated crossed beam experiments. In both experiments, the vibrational excitation of the molecules was provided by heating the molecular beam source to temperatures in the range of 2000–3000°K, which was assumed to give populations according to the Boltzmann expression. In the first experiment, a total rate coefficient was measured as a function of molecular beam temperature, with absolute calibration of the photon detector being made using the black body radiation from the heated molecular beam source. Since heating affects both the internal energy and the collisional kinetic energy, the first experiment could not determine the relative contributions of internal energy transfer vs collisional excitation. The second experiment achieved partial separation of internal vs kinetic energy transfer effects by using a velocity-selected molecular beam. Using two simple models for the kinetic energy dependence of the transfer cross section for a given change in vibrational quantum number, the data from both experiments were used to determine parameters in the models. Evidently the most probable situation is that most of the Na excitation energy comes from internal vibrational energy, with the remainder coming from kinetic energy. The paper discusses the consequences of the detailed results with respect to the excitation of sodium radiation in aurorae.