Site-specific properties and dynamical dipole coupling of CO molecules adsorbed on a vicinal Cu(100) surface

Abstract

The dynamic and spectroscopic behavior of CO adsorption on a stepped Cu(100) surface is investigated using transient IR diode laser-reflection absorption spectroscopy. Disproportionate intensity behavior, defying Beer–Lambert’s law, is observed which makes it impossible to use spectral intensity for determining either the total or site-specific concentrations. A theoretical model, based on the Persson–Ryberg treatment of mixed isotope studies of CO at fixed coverage, is used here, with modifications added to allow for coverage dependence to account for dynamic dipole coupling between CO molecules and simulate the IR absorption spectra. This enables the spectral intensities and positions to be analyzed and the extraction of previously unattainable information on site-specific molecular spectroscopic parameters and concentrations on this CO/Cu(100) system. The CO stretching frequencies indicate that the Cu–CO bond is formed by transfer of the CO antibonding 5σ electron to copper and that the binding energy of CO at terrace sites decreases with increasing coverage. The model calculation shows that, as a result of dynamical dipole coupling, a 7% step-CO concentration, with a vibrational polarizability of 0.2 Å3, causes a 3 times larger IR absorption peak than the remaining 93% of CO at terrace sites. CO adsorption on this Cu(100) surface was found to be repulsive correlated with the order parameter determined as n=3/2. Concentrations determined from the dynamical coupling calculation show that CO occupies step and on-top terrace sites at all coverages at 90 K, with the more tightly bound step sites saturated at lower coverage. A simple model is devised to describe the equilibrium between the step and terrace CO populations and provide an estimate of the dynamical parameters governing CO motion between the step and terrace sites.