Excitation and desorption of physisorbed H₂ via theΣu2 electron scattering resonance

Our high-resolution electron energy-loss measurements concern physisorbed H₂ and comprise differential cross sections for the excitation of the internal H₂ modes and the H₂-surface bonding mode and their combinations and extend over the electron impact energy range of the classical low-energy H₂ Σu2 resonance. Comparison with corresponding data for the excitation of the internal modes of gas phase H₂ reveals that strong elastic electron reflectivity from the Cu(100) substrate profoundly distorts the inelastic scattering pattern for physisorbed H₂. We find that this influence can be corrected for and that the resulting peak cross sections agree with the H₂ gas phase data, in accordance with theoretical predictions for the excitation of the internal H₂ vibration. We have used corrected cross sections for the rotational mode spectra of physisorbed H₂, HD, and D₂ 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. H₂ and HD can desorb from all levels while D₂, 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 H₂ and HD reveals that direct bound-free transitions also contribute to the electron induced desorption.