Photoinduced ultrafast interfacial electron transfer probed with two-photon-photoemission

Abstract

Ultrafast heterogeneous electron transfer (HET) from the excited singlet state of the organic chromophore perylene into the inorganic semiconductor rutile TiO 2 was investigated with femtosecond time-resolved two-photon photoemission (2PPE). With 2PPE one can address adsorbates at coverages far below a monolayer on single crystal surfaces. With the same chromophore perylene fixed with different anchor and bridge groups at the surface of rutile TiO 2 (110) the corresponding 2PPE transients revealed the relevant parameters that characterize the contributing processes. Instantaneous optical injection on one hand and slow injection over a long distance on the other hand were realized. Direct optical charge transfer was realized with the chromophore catechol that is known to form a charge transfer complex with Ti atoms on the surface of TiO 2 . The slow injection cases were realized by inserting rigid molecular bridges. Comparison of the different 2PPE signals with corresponding transient absorption (TA) signals for the identical systems revealed the physical processes and time scales that control the 2PPE transients. On the surface of the single crystals only one long time constant was measured via 2PPE also in the case of a long rigid bridge/anchor group in contrast to a broad distribution of time constants observed for the same molecules anchored in the nm-size cavities of an anatase TiO 2 film measured via TA. The broad distribution of time constants in the latter measurements can be attributed to different microscopic environments giving rise to different distances between the chromophore and the nearest TiO 2 wall.