Mode selectivity in electron promoted vibrational relaxation of chemisorbed hydrogen on molybdenum and tungsten surfaces

Abstract

Electron-phonon coupling in atoms and molecules adsorbed at metal surfaces gives rise to finite vibrational linewidths in infrared or electron energy loss spectra. When it is the dominant contribution to the vibrational lifetime, it manifests itself in the form of a Fano line shape. Here, we report the linewidths of vibrational modes of chemisorbed hydrogen on the (100) and (110) surfaces of molybdenum and tungsten calculated from first-order time-dependent perturbation theory. For those modes with a Fano line shape, our results are in good agreement with the experiment. We further observe that the coupling strength between vibrations and electrons depends on the nature of the mode: for Lorentzian-shaped peaks, the experimental linewidths are always larger than those predicted based on pure electron-phonon coupling. The calculated linewidths exhibit a strong coverage dependence, decreasing towards higher coverages. This finding has important implications for nonadiabatic energy dissipation in hydrogen dynamics at metal surfaces. While electron-hole pair excitation is the dominant energy-transfer mechanism between hydrogen and pristine metal surfaces, other channels for energy dissipation, such as adsorbate-adsorbate interactions, may become more significant on metal surfaces densely covered with hydrogen.

Figures (3)

• Figure 1: Orthographic view on the $p(1\times1)$H/bcc(100) surface shown in panel a) and the $p(1\times1)$H/bcc(110) surface in panel b). The black arrows define the Cartesian coordinates parallel to the surface. The black and white opaque areas define the primitive cell and a $3\times3\times1$ supercell of the H/metal surfaces, respectively.
• Figure 2: Adsorbate vibrational modes for the $p(1\times1)$ phases of hydrogen adsorbed on molybdenum and tungsten. Panels a)--c) display the vibrations on the (100) facet, whereas panels d)--f) shows the vibrations of the (110) facet. Note that the wagging mode in panel a) and the asymmetric stretch mode in panel c) have a two-fold degeneracy.
• Figure 3: Electron-phonon-coupling-induced vibrational linewidths of the three different collective modes of hydrogen adsorbed on Mo(110), shown in panel a), and W(110), shown in panel b) as a function of hydrogen coverage $\Theta$. The $1/n^2$ coverages were realised by placing one hydrogen on the three-fold side of a $p(n\times n)$ surface slab.