| Synchrotron infrared studies of adsorbates on surfaces offer the possibility of probing low frequency vibrational modes and dynamics at high resolution (125 {dollar}mu{dollar}eV, 1 cm{dollar}sp{lcub}-1{rcub}).{dollar} We developed and used an infrared synchrotron source at the National Synchrotron Light Source which is 100-1000 times brighter than the standard 1000K globar ir source. Extensive experiments have been performed for ordered submonolayers of CO on low index Cu surfaces (c(2 x 2) CO/Cu(100), {dollar}(surd{dollar}3 x {dollar}surd{dollar}3)R30{dollar}spcirc{dollar} CO/Cu(111), and p(1 x 2) CO/Cu(110).), in the 200-2500 cm{dollar}sp{lcub}-1{rcub}{dollar} ({dollar}sim{dollar}12.5-312.5 meV) range. These are the first ir experiments to directly probe the adsorbate-substrate vibrations for this system. In addition, however, several surprising features are observed including the dipole-forbidden hindered rotation as an anti-absorption feature and an unexpected concomitant broadband absorption. Feasibility studies of mixed isotope experiments and bridge site occupation are also presented. Two theories have been motivated by this work, and will be compared to our experimental results. The first theory is a nonadiabatic (Born-Oppenheimer approximation breaks down) cluster calculation by Tully, Garcia and Head-Gordon, that predicts lifetimes of the vibrational modes for CO/Cu(100). Another Drude-based theory, proposed by Persson, considers the coupling of conduction electrons, excited beneath and parallel to the surface, to parallel vibrations of the adsorbates, accounting for the observation of both the broadband absorption and dipole-forbidden peak. Surprisingly, relaxation appears to occur not via multi-phonon excitations, as expected for these low frequency modes that are in close proximity to the phonon band, but through electron-hole pair creation. |
