Excitation and desorption of physisorbed H via theΣu2 electron scattering resonance.

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