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.
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.
The role of orbital magnetism in the laser-induced demagnetization of Fe/Gd multilayers was investigated using time-resolved X-ray magnetic circular dichroism at 2-ps time resolution given by an xray streak camera. An ultrafast transfer of angular momentum from the spin via the orbital momentum to the lattice was observed which was characterized by rapidly thermalizing spin and orbital momenta.
Strong interlayer exchange coupling between Fe and Gd led to a simultaneous demagnetization of both layers.1 Author to whom correspondence should be addressed; electronic mail: afbartelt@lbl.gov. 2 Author to whom correspondence should be addressed; electronic mail: a_scholl@lbl.gov.
2Ultrafast magnetic storage and processing is founded on our ability to control magnetism on picosecond and femtosecond time scales. Magnetic phase transitions conserve the total angular momentum and usually involve the crystal lattice as a quasi-infinite reservoir of angular momentum. A prototypical ultrafast magnetic phenomenon is the demagnetization after excitation by an intense laser pulse [1][2][3][4][5]. Here, the orbital momentum is crucial as it links the electron spin, which carries most of the magnetic moment, to the lattice via the spinorbit interaction. In this letter, we investigate the orbital momentum dynamics during an ultrafast demagnetization in the model system Fe/Gd using X-ray magnetic circular dichroism (XMCD) [6].The Fe/Gd multilayer consists of two metals of very different electronic structure. Fe has exchange-split 3d spin bands which intersect the Fermi surface, allowing both low-energy spin-flip (Stoner) and spin wave excitations (magnons). The spin momentum dominates the total angular momentum while the orbital momentum is quenched by the strong ligand field and only partially restored by the spin-orbit interaction. The coupling of the orbital momentum to the anisotropic ligand field enables the flow of angular momentum from the spin system to the lattice during the demagnetization. A direct photon-driven exchange of spin and orbital momentum as proposed by Hübner [7] would, for example, appear as a temporary accumulation of orbital and concomitant reduction of spin momentum. In contrast, a bottleneck caused by the spin-orbit interaction would be visible as a reduced orbital to spin momentum ratio. The second component of the multilayer, Gd, is best described as a Heisenberg ferromagnet with localized 4f electrons. Gd does not exhibit an orbital momentum in the 4f shell, which is half full. A large exchange energy of about 11 eV separates the majority and minority 4f states, inhibiting low-energy spin-flip excitations. Magnetic long range order in Gd is established via 4f-5d exchange with the Gd 5d valence states and their exchange interaction with Gd 5d orbitals of nearest neighbors [8]. Therefore, the Gd 4f demagnetization occurs indirectly via 4f-5d exchange and subsequent 5d electronphonon scattering while the Fe demagnetization occurs directly via 3d electron-phonon scattering. The Gd orbital momentum should t...
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