Infrared linewidths and vibrational lifetimes at surfaces: H on Si(100)

The temperature dependence of the natural linewidth of an adsorbate-substrate mode, Si-H, has been measured for the first time. Molecular-dynamics simulations of the infrared line shapes, with the use of an accurate ab initio force field and a novel stochastic method to include quantum effects, are in good agreement with experiment. Linewidths are shown to be dominated by pure dephasing above 250 K and by inhomogeneities at low temperature. The vibrational lifetime, computed to be ≃${10}^{\mathrm{-}8}$ s, contributes negligibly to the linewidth.