Electronic Surface Resonance Enhancement of Vibrational Loss Intensities

Abstract

A high resolution EELS study of the electron reflectivity lineshape and (E,$\vec{k}$) dispersion relations of surface barrier resonances on W(100) is reported. Their effect on the impact energy dependence of the syumuetric stretch-vibration excitation cross-section of chemisorbed hydrogen is to produce resonance enhancement. Results are presented for both the low coverage c(2 × 2)H and high coverage p(l × 1)H chemisorbed layers. The energies of these resonances would appear to fit a simple Rydberg type series of levels for a smooth monotonic image potential surface barrier. However, the dispersion behaviour of the intense “ground state level” is shown to be anomalous and to deviate strongly from two-dimensional free electron behaviour. Strong resonance intensity is found only for energies corresponding to energy gaps in the surface projection of bulk states. The effect on adsorbate vibrational selection rules is discussed in terms of the degree to which the resonance electron density overlaps the short range scattering potential of the vibrating atoms.