Excitation and desorption of physisorbed H2 via the 2Σu electron scattering resonance

Our high-resolution electron energy-loss measurements concern physisorbed H-2 and comprise differential cross sections for the excitation of the internal H-2 modes and the H-2-surface bonding mode and their combinations and extend over the electron impact energy range of the classical low-energy H-2 (2)Sigma(u) resonance. Comparison with corresponding data for the excitation of the internal modes of gas phase H-2 reveals that strong elastic electron reflectivity from the Cu(100) substrate profoundly distorts the inelastic scattering pattern for physisorbed H-2. We find that this influence can be corrected for and that the resulting peak cross sections agree with the H-2 gas phase data, in accordance with theoretical predictions for the excitation of the internal H-2 vibration. We have used corrected cross sections for the rotational mode spectra of physisorbed H-2, HD, and D-2 in a model concerning electron induced desorption via rotation-translation energy conversion. These spectra include transitions from the ground state as well as excited levels of the physisorption potential well. H-2 and HD can desorb from all levels while D-2, for energetic reason, can only desorb from the excited levels. This model gives a satisfactory account of the observed desorption cross sections and predicts characteristic velocity distributions of the desorbing molecules. The cross section data for H-2 and HD reveals that direct bound-free transitions also contribute to the electron induced desorption. Published by AIP Publishing.