Optical reflectivity changes induced by adsorption on metal surfaces:
The origin and applications to monitoring adsorption kinetics
The Journal of Chemical Physics, 112 (2):
923-934 (2000)DOI: 10.1063/1.480722
Abstract
It is observed that when a monolayer of CO and acetylene is chemisorbed
on the Cu(100) surface, the reflectivity of the metal surface at
the He�Ne laser wavelength of 632 nm is reduced on the order of 1%,
while the physisorption of water, methanol, and acetone induces a
reflectivity change on the order of 0.01%. The small reflectivity
change induced by physisorption can be described by a three-layer
model taking into account the molecular layer refractive index. The
much bigger reflectivity change induced by the chemisorbed adsorbates,
on the other hand, is a result of bonding perturbations to the electronic
structure of the metal surface layer. The latter is supported by
an electron scattering model description of the reflectivity change
up to 1.96 eV on Cu. For both CO and acetylene, the optical reflectivity
change is found to be linearly proportional to the submonolayer coverage.
The phenomenon thus offers an excellent method to measure surface
kinetics. It is found from the reflectivity change measurements that
the initial sticking coefficient for both adsorbates is nearly unity
at 110 K; 0.85 for CO and 1.0 for acetylene. The temperature and
coverage dependence of the sticking coefficient shows that the adsorption
behavior of both molecules is well described as direct adsorption
mediated with an extrinsic precursor. For acetylene adsorption, the
sticking coefficient shows little dependence on the substrate temperature
suggesting that the "extrinsic precursor" is not a thermally equilibrated
species. For CO, the transition into a compression phase beyond 0.5
ML results in a corresponding change in the sticking coefficient
deduced from the reflectivity data.