Dynamics of atomic adsorbates: hydrogen on Cu(111)

We have measured the absolute changes induced in the far infrared reflectivity by H and D atoms adsorbed on Cu(111). The strongest features in the spectra are the dipole forbidden parallel vibrational modes which give rise to anti-absorption peaks with an intensity almost 10 times higher than that of the dipole allowed perpendicular modes. The dynamics can be explained by excitation of electron-hole pairs in the substrate. The energy relaxation time of the parallel vibration is 1 ps, but the linewidth is larger by a factor of 10 probably due to lateral tunneling of the adsorbates and dephasing processes.