LOW-FREQUENCY DYNAMICS OF CO CU BREAKDOWN OF BORN-OPPENHEIMER APPROXIMATION/

Synchrotron infrared studies Of adsorbates on surfaces offer the possi bility of probing low-frequency vibrational modes and dynamics at high resolution (1 cm-1, 125 mueV). Extensive experiments have been perfor med for ordered submonolayers of CO on low index Cu surfaces [(square- root 3 X square-root 3)R30-degrees CO/Cu(111), c(2X2) CO/Cu(100) and ( 1X2) CO/Cu(110)], in the 100-2500 cm-1 (approximately 12.5-312.5 meV) range. These are the first IR experiments to directly probe the adsorb ate-substrate vibrations for this system. In addition, however, severa l surprising features are observed including the dipole-forbidden hind ered rotation and an unexpected broadband absorption. Two theories hav e been motivated by this work, and will be compared to our experimenta l results. The first theory is a cluster calculation by Tully, Gomez, and Head-Gordon, which takes into account the breakdown of the Born-Op penheimer approximation and predicts lifetimes and frequencies of the vibrational modes for CO/Cu(100). Another Drude-based theory, proposed by Persson, considers the coupling of the conduction electron current to the vibrations of the adsorbates, accounting for the observation o f both the broadband absorption and dipole-forbidden peak.