The molecular and atomic states of oxygen adsorbed on Rh(100): Adsorption

The adsorption of oxygen at 100 K on Rh(100) has been studied with temperature‐programmed desorption and photoelectron spectroscopy. Oxygen adsorbs with a sticking probability of about 0.8 up to a near saturation coverage of 1.1×10¹⁵ atoms/cm² or three‐fourths of a monolayer coverage on Rh(100). Initial adsorption is dissociative. However, for coverages above half a monolayer a low temperature molecular state, denoted α, which does not exhibit isotopic exchange, desorbs at 150 K. Dissociated oxygen recombines and desorbs in three high temperature or β states. At low coverages oxygen desorbs in a second order state, β₃, between 1200 and 1325 K with an activation energy for desorption of 360 kJ/mole (85 kcal/mole). At coverages above half a monolayer, two first order states, β₁ and β₂, appear at 820 and 920 K, respectively. Their respective heats of desorption are 210 kJ/mole (50 kcal/mole) and 260 kJ/mole (62 kcal/mole). In contrast with previously reported results, no bulk diffusion of adsorbed oxygen atoms into Rh is observed for temperatures up to the onset of oxygen desorption.