Adsorbates in phonon bath: moderate coverage of Cu(2 1 1) by CO molecules

Using molecular dynamics combined with semiempirical potentials, we have modeled, in the presence of a phonon bath, the adsorption of CO molecules on Cu(2 1 1) stepped surfaces at moderate coverages. Our calculations confirm the scanning tunneling microscopy results by Meyer et al. [Chem. Phys. Lett. 240 (1995) 379] that, at a coverage of 1/2 ML, the CO molecules are adsorbed on top, at the border of steps forming a (2 × 1) structure. At higher coverages, and/or temperatures, the overlayer structure is more complicated and CO molecules adsorb at step borders near a bridge position, seldom in the middle of a terrace. For the first time, relaxation of the substrate in the presence of the adsorbates is accounted for. At low temperature, the vibrational analysis of the (2 × 1) overlayer structure shows a mode at 61 cm−1 due to collective vibration of CO molecules. The CO molecules vibrate mainly perpendicular to the step borders of Cu(2 1 1) surface out of phase. Above the bulk cut-off frequency, the local phonon spectral density of the first two layers of the substrate displays a coupling mode between CO molecule and Cu(2 1 1) substrate, vibration consistent with the adsorption at the border of the steps.