Probing of Photocarrier Electrons and Excitons at an Organic Monolayer Film Studied by Two-Photon Photoemission Spectroscopy

The present study characterized the molecular orientation and photoexcited states of an organic polyaromatic molecule, namely, triphenylene (TP), deposited on a two-dimensional layered material substrate of graphite. Scanning tunneling microscopy and low-energy electron diffraction identified a periodic monolayer adsorption phase where the TP molecules are oriented with a standing-up configuration. The photoexcited electronic states of the monolayer system were probed by two-photon photoemission (2PPE) spectroscopy with variable photon energies (h nu). By tuning and detuning h nu for a resonant intramolecular excitation, we detected both excited photocarrier electrons and hot excitons. At the nonresonant 2PPE, specifically at h nu <= 4.5 eV, the photocarrier electron in the lowest unoccupied molecular orbital at 2.7 eV above the Fermi level was resolved, where the photocarriers were injected from the graphite substrate. In contrast, the presence of hot excitons (S-3) dominated the spectral features of the 2PPE signal at the resonant h nu of >4.5 eV. Additionally, strong fluorescence due to the recombination of the lowest singlet exciton (S-1) was observed during 2PPE spectroscopy with the resonant h nu, indicating that the generated hot excitons relax into S-1 (S-3 -> S-1) without quenching to the substrate.