Electronic excitation and relaxation dynamics of the LUMO-derived level in rubrene thin films on graphite

Time resolved two-photon photoemission (TR-2PPE) spectroscopy has been performed for rubrene films on highly oriented pyrolytic graphite. When a second layer is formed on the first monolayer (ML), 2PPE intensity from the lowest unoccupied molecular orbital (LUMO)-derived level shows a clear resonance at a pump photon energy of 4.1 eV. In contrast, the resonance is very weak for sub-ML films. Substrate-molecule interaction blurs the intramolecular resonant transition for sub-ML films. The lifetime of electrons in the LUMO-derived level increases exponentially with increasing film thickness, for thickness up to 3 ML. The lifetime increase becomes more moderate for further increase in the film thickness. This change in the slope of the increase in lifetime suggests a transition in the relaxation mechanism, from electron tunneling to intramolecular relaxation medicated by the substrate. When ultraviolet photons of 4.45 eV are used to pump electrons to the LUMO-derived level, the decay profiles for films thicker than 1 ML deviate from a simple exponential decay. Such deviation is not significantly observed for sub-ML films. When visible photons of 2.97 eV are used for pumping, the decay profiles are well reproduced by a simple exponential decay, irrespective of the film thickness. The deviation from simple exponential decay is attributed to the relaxation of holes produced at deep occupied levels to the highest occupied molecular orbital-derived level.