Estimation of direct and indirect interactions between CO molecules on Pd(1 1 0)

Interactions between CO molecules at coverages between 0.05 and 0.5 monolayers on Pd(1 1 0) below 100 K were investigated, using temperature-programmed desorption, low-energy electron diffraction, and high-resolution electron-energy-loss spectroscopy. We estimated individual interaction elements according to conceptual interaction classification, i.e., direct (or through-space) and indirect (or through-substrate) interactions. Based on the energy shifts of the Pd–CO stretch mode, the through-substrate interaction was estimated to be repulsive of about 60 meV at the half monolayer. On the other hand, the through-substrate interaction due to CO static dipoles was also estimated to be repulsive of about 30 meV from the work function change. From the observed superstructures, in addition, the existence of an attractive interaction between CO molecules was proposed and we discussed that origin.