Vibrational lifetimes and friction in adsorbate motion determined from quasi-elastic scattering

Abstract

The vibrational excitation of molecules adsorbed on a surface is typically probed by spectroscopic techniques such as infrared or Raman spectroscopy. In the present article we demonstrate an alternative method to determine vibrational lifetimes of adsorbate molecules using quasi-elastic helium atom scattering (QHAS). As a probe of diffusive motion of molecules on surfaces QHAS is well established. Here, we demonstrate that QHAS can also be used to probe the vibrational lifetime of a molecule in its adsorption well. Measurements of cyclopentadienyl, C5H5, on Cu(111) allow us to distinguish two substrate phonon modes as well as two molecular vibrational modes, perpendicular and parallel to the surface. We further find that the dephasing of the vibrational motion corresponds to the friction determined in previous diffusion measurements.

Energy Dissipation during Diffusion at Metal Surfaces: Disentangling the Role of Phonons versus Electron-Hole Pairs

“…Corresponding trajectories ( ) are conveniently obtained from Langevin MD simulations, in which the overall friction coefficient is varied until optimum agreement with the experimental decay rates is obtained [34,35,41]. Specifically, in the current work we employed a system of atoms = 200 adatoms in a supercell consisting of an (49 × 82) array of rectangular Cu(111) unit cells and used = 155 K to match the experimental Na coverage and temperature.…”

Measuring surface phonons using molecular spin-echo

“…Using a simple classical model, termed the tilted projection picture 7 for designing and interpreting the measurements, ultra-high resolution measurements of surface phonons have been used to measure the low energy part of dispersion curves, anharmonicity, and phonon lifetimes of a metal surface. 5,7 Helium spinecho measurements of surface vibrations have also been used to determine the dispersion relations and lifetimes of adsorbate vibrations, [8][9][10] and even phasons. 11 While often involving a very similar setup to HAS, molecular scattering experiments offer quite different and complimentary insight to atom scattering.…”

The intermediate scattering function for quasi-elastic scattering in the presence of memory friction

“…To date the Langevin equation has been used extensively to model HeSE measurements of adsorbate dynamics. The frictional coupling γ can be quantified by optimising the simulation to reproduce inelastic [23] or quasi-elastic [24] features in the experimental line shapes. The general theory behind the HeSE measurements has been set out and reviewed previously [22].…”