Desorption of physisorbed molecular oxygen from coronene films and graphite surfaces

We present a study on the adsorption and desorption of molecular oxygen (O₂) on highly oriented pyrolytic graphite and coronene films deposited on it. To this end, density functional theory calculations were performed and experiments were made using the FORMOLISM device, which combines ultra-high vacuum, cryogenics, atomic or molecular beams, and mass spectrometry techniques. We first studied the desorption kinetics of dioxygen (O₂) on a coronene film and graphite at 15 K using the thermally programed desorption technique. We observed that the desorption of O₂ occurs at a lower temperature on coronene than on graphite. We deduce the binding energies that are 12.5 kJ/mol on graphite and 10.6 kJ/mol on coronene films (pre-exponential factor, 6.88 × 10¹⁴ s^-1). The graphite surfaces partially covered with coronene show both adsorption energies. In combination with theoretical density function theory (DFT) calculations using graphene and coronene as surfaces, we observe that the experimental results are in good agreement with the theoretical calculations. For the adsorption of the O₂ molecule, two orientations are possible: parallel or perpendicular to the surface. It seems that O₂ is best bound parallel to the surface and has a preference for the internal sites of the coronene.