Adsorption and desorption kinetics of CO on Cu(110) studied by optical differential reflectance

Calibration of oblique-incidence reflectivity difference for label-free detection of a molecular layer

Oblique-incidence reflectivity difference (OI-RD) is a form of polarization-modulation ellipsometry that measures properties of thin films on a solid surface through the change in polarization state of light upon reflection from the surface. The measurement accuracy depends on the precision of the phase modulation amplitude and azimuthal alignments of key polarizing optical elements and, thus, requires careful calibration. In the present work, we describe robust methods of such calibrations that enable precise determination of the modulation amplitude and static retardation of a phase modulator and azimuths of key polarizing optics in an OI-RD system.

Microkinetic modeling of CO TPD spectra using coverage dependent microcalorimetric heats of adsorption

CO adsorption on the ternary methanol synthesis Cu/ZnO/Al2O3 catalyst was studied in detail by means of adsorption microcalorimetry and flow temperature-programmed desorption (TPD). Based on these experimental data, we established a microkinetic analysis method, which provides information about the adsorption kinetics of CO on the catalyst surface. Experimentally derived microcalorimetric heats of adsorption were applied in a microkinetic model to simulate TPD curves with varying initial coverage. Two approaches were used: an integral approach based on evaluation of the integral heats of adsorption which predicts the experimental TPD curves roughly and provides first approximations for the preexponential factors. The second, more detailed approach was based on the simulation of the adsorption isotherm taking the experimentally determined coverage-dependence of the heat of adsorption into account. This approach led to a significantly improved agreement between experimental and simulated TPD curves. Moreover, it was possible to derive the standard entropy of adsorption. The general applicability of our approaches is demonstrated by analyzing the CO TPD and microcalorimetry data obtained with a binary ZnO-free Cu/Al2O3 catalyst.

Optical response of the copper surface to carbon monoxide deposition

The optical response of the Cu surface upon CO deposition is investigated from the clean Cu(110) to the reconstructed CO/Cu(110)-p(2x1) geometry through ab initio electronic structure calculations. We ascribe the relevant structures in the calculated reflectance anisotropy spectrum of the reconstructed phase to the persistence of surface states transitions. These are excited by light polarized along the direction perpendicular to the one found at the clean surface. We devise a simple model for the evolution of the optical response in the adsorption process, identifying three different regimes. The impurity regime, at very low coverages, is characterized by a critical coverage that enhances the actual one by a factor of approximately 30, close to the value estimated experimentally.