Experimental Investigation ofβ1-H on W(100) Using High-Resolution Electron-Energy-Loss Spectroscopy: Bond Distances, Scattering Mechanisms, and Impact-Scattering Selection Rules

High-resolution electron-energy-loss studies of ${\beta }_1$-H and ${\beta }_1$-D on W(100) are reported for experimental conditions under which the first-order diffracted beam emerges from the crystal. A new vibrational mode is observed and assigned to the asymmetric stretch mode of ${\beta }_1$-H. The results yield an independent new determination of the H-W bond distance. The impact-energy dependence of the new asymmetric stretch mode does not agree with theoretical predictions related to cross sections and selection rules that govern them.