No abstract

The ultrafast injection dynamics, early recombination dynamics, and spectral signatures of four systematically varied dye-metal oxide hybrid systems were investigated using transient absorption spectroscopy techniques. First, photoinduced electron transfer from two different perylene derivatives into zinc oxide (ZnO) colloidal films is reported. Here, the electronic coupling of the perylene chromophore 2,5-Di-tert-butyl-perylene-9-yl-propionic acid (1) to the ZnO colloids was weaker than the electronic coupling of the chromophore 2,5-Di-tert-butyl-perylene-9-yl-acrylic acid (2). Second, the photoinduced electron transfer of the same two molecules attached to TiO2 colloids was measured and compared to the results for the ZnO colloids using the same techniques. The temporal traces at both the excited-state and the cationic state of the chromophores attached to the semiconductor surfaces were measured simultaneously and showed very good agreement, which indicated a direct injection into the semiconductor. The overall injection times for the ZnO samples was as short as 190 fs, which suggested a strong electronic coupling element for these systems. This injection time is short compared to reports on similar ZnO hybrid systems, but it is still longer than the injection times reported for the TiO2 hybrid systems. The transient absorption spectra of molecule 2 attached to TiO2 showed a large negative signal at 530–550 nm, which indicated the presence of a direct charge transfer state contribution in this system.

No abstract

Electron injection from photoexcited chemisorbed dyes into zinc oxide is known to proceed in a stepwise manner, yet the origin of the injection retardation remains controversial. Here we present a complementary time-resolved spectroscopy study on the electron injection dynamics from organic dyes into ZnO using model perylene derivatives with systematically lengthened bridge units to clarify the influence of the positively charged cation on the escape of the injected electron. The combination of transient absorption, opticalpump terahertz-probe, and time-resolved two-photon photoemission spectroscopy reveals that the delayed release of charges into ZnO is independent of Coulomb attraction between the dye cation and the injected electron. Rather, following dye photoexcitation the primary acceptor states of electron transfer into ZnO are interface states formed between the dye and the ZnO surface, which retard the formation of free charges.

We report a good agreement between the shapes of tailored pulses obtained theoretically and experimentally by using the optimal-control theory and the closed-loop learning technique to maximize the ionization yield in NaK. The theoretical pulse shapes are found to be robust regarding the choice of the initial guess. We assign the leading features of the pulse shapes to processes underlying the optimal control and reveal the mechanism which involves an electronic transition followed by a direct two-photon process and sequential one-photon processes at later times. We show that the optimal control not only serves for maximizing the desired yield but also as a tool for the analysis and the identification of the responsible processes.