The advances in femtosecond laser techniques facilitate the investigation of ultrafast electron dynamics at surfaces directly in the time-domain. We employ time-resolved two-photon-photoemission (2PPE) spectroscopy to study the electron dynamics of the unoccupied electronic states in hexafluorobenzene (C6F6) on Cu(111) serving as a model system for charge transfer across organic/metal interfaces. Our coverage dependent study reveals a lifetime of the lowest unoccupied molecular resonance of 7 fs for a single monolayer (ML) which increases to 37 fs above 3 ML coverage. We find that the population build-up of the excited state is delayed by a characteristic time of about 10 fs with respect to the exciting laser pulse. By angle-resolved 2PPE the mechanism of the delayed population rise is identified as intraband decay in the adsorbate band structure. The actual electron-transfer to the metal substrate occurs through interband scattering between the molecular resonance and substrate states on comparable timescales. Therefore the present study demonstrates that relaxation of hot electrons at the molecule-metal interface includes - even in the presence of strong electronic molecule-substrate interaction - decay channels within the adlayer.
PACS: 73.90.+f, 78.47.+p, 79.60.Dp
The publication is available at link http://stacks.iop.org/1367-2630/7/113