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

TL;DR

This study calculates vibrational linewidths of chemisorbed hydrogen on molybdenum and tungsten surfaces, revealing mode-dependent electron coupling effects and coverage-dependent energy dissipation mechanisms relevant for hydrogen-metal surface interactions.

Contribution

It provides first-principles calculations of vibrational linewidths, demonstrating mode selectivity and coverage effects in electron-phonon coupling for hydrogen on metal surfaces.

Findings

Good agreement with experimental linewidths for Fano-shaped modes

Coupling strength varies with vibrational mode nature

Linewidths decrease with increasing hydrogen coverage

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.