Formation and relaxation of rovibrationally excited H2 molecules due to plasma-surface interaction

We report on the interaction of hydrogen atoms and molecules under high flux conditions with a cooled copper surface and its impact on gas phase densities and internal excitation of the molecules. These densities were measured by means of laser-induced fluorescence using tunable radiation sources in the vacuum-ultraviolet (vuv). While H atoms were detected by two-photon absorption laser-induced fluorescence, the necessary vuv radiation for the detection of rovibrationally excited H2 molecules in the electronic ground state were produced by stimulated anti-Stokes Raman scattering. The results reveal a strong loss mechanism of H atoms and the formation of rovibrationally excited H2 molecules due to surface interaction. The surface reaction probability of H atoms under high flux conditions on copper was estimated. Surface collisions are shown to have a profound influence on the density distribution of rovibrationally excited H2 molecules: The distributions follow lower temperatures and are less Boltzmann-like, i.e., the distributions of the internal excitation of H2 molecules differ more from thermodynamic equilibrium.