The potential energy surface, vibrational phase relaxation and the
order-disorder transition in the adsorption system Pt111-CO
Surf. Sci., 213 (1):
49 - 89 (1989)DOI: DOI: 10.1016/0039-6028(89)90252-5
Abstract
We report detailed measurements of the lineshape and intensity of
the C-O stretching vibration in the adsorption system Pt111-CO
using IR reflection-absorption spectroscopy. A simple analysis of
the data reveals interesting and useful information about the dynamics
of molecules at surfaces. In particular, from the coverage dependence
of the intensity for the on-top and bridging species we estimate
a difference in binding energy between the two sites of ~ 60 meV.
The #entropy# factor is found to be particularly important in determining
their relative occupation. A semi-quantitative potential energy curve
linking the on-top and bridge sites can be constructed but the operation
of lateral interactions in the c(4�2) structure causes it to be modified,
producing an elevated minimum (T * ) about 0.4 � off the on-top site.
From an analysis of the temperature dependence of the relative intensities
of the C-O stretches the bridge-to-T* excitation energy is determined
to be about 80 meV. The change in halfwidth and frequency as a function
of temperature up to 250 K can be explained in terms of the relaxation-dephasing
model. The dephasing mode for the on-top species is clearly the soft,
frustrated translation at 48 cm-1 identified in inelastic helium
scattering experiments by Lahee et al. The corresponding mode for
the bridging species is at ~ 300 cm-1, supporting a recent reanalysis
of the He scattering data which suggests that there are no soft frustrated
translations as in the case of the on-top species. Above 250 K the
rapid increase in linewidth indicates an order-disorder transition
which is confirmed by LEED intensity data. Finally, we speculate
as to the relevance of the information obtained here for solving
other interesting adsorption problems.