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We report a femtosecond pump-probe study on the photochemistry of concentrated aqueous solutions of [RuII(bpy)3]2+, as a function of pump power (up to 2 TW/cm2) at 400 nm excitation. The transient absorption spectra in the 345-660 nm range up to 1 ns time delay enable the observation of the following photoproducts: the triplet 3MLCT (metal-to-ligand-charge-transfer) excited state, the reduced form [RuII(bpy)3]+, the oxidized species [RuIII(bpy)3]3+, and the solvated electron e(aq). The 3MLCT state is formed within the excitation pulse and undergoes vibrational relaxation in 3-5 ps, as evidenced by the shift of the ligand-centered (LC) absorption band below 400 nm. Even at the highest pump powers, the majority of e(aq) originates from multiphoton ionization of [RuII(bpy)3]2+ and not from the solvent, generating [RuIII(bpy)3]3+ as a byproduct. At 10 ps time delay, the total concentration of the three product species is balanced by the depleted concentration of [RuII(bpy)3]2+, even at the highest fluences used, indicating that no further reaction products significantly contribute to the overall photochemistry. On the 100 ps time scale, most probably diffusion-controlled reduction of ground-state [RuII(bpy)3]2+ by solvated electrons occurs, next to recombination between e(aq) and [RuIII(bpy)3]3+.

Transient absorption spectroscopy is used to characterize the ultrafast spin-transfer process in single crystal iron(II)-tris(bipyridine)-bis(hexafluorophosphate). Preliminary data analysis shows evidence of the formation of a high spin state and oscillatory signals on multiple time scales.

Several new results and opportunities for ultrafast x-ray absorption spectroscopy (XAS) are demonstrated. It is possible to work with highly dil. solns. in transmission mode without dramatic loss of signal-to-noise ratio. This is very promising as one can envision the study of samples, for which large concns. are impossible to reach. Moreover, it is possible to scan the time delay between the laser pump pulse and the x-ray probe pulse, and therefore follow the evolution of the system from the start. The operation of an optical x-ray cross-correlator is also demonstrated. The time resoln. is not a limiting factor and the expts. are feasible with sources of shorter x-ray pulses, provided the flux is not too low. [on SciFinder (R)]

Beam shaping techniques provide a way to control the shape of the focal volume in laser micromachining applications. This approach has been successfully applied for fs-lasers both in surface and bulk processing of materials [1, 2, 3]. Particularly the use of elliptically shaped beams produced either with cylindrical telescopes [2] or slits [3] has enabled the fabrication of waveguides with nearly circular symmetry by direct fs-laser writing. However, these beam shaping techniques rely on the use of conventional optics and are therefore not flexible enough to be easily adapted to changes of the processing conditions (different processing depths, materials with different refractive indices, or different numerical apertures). Spatial light modulators (SLM's) provide, in turn, a flexible tool for this and other applications like aberration pre-compensation [4].

Nanostructures of transition metal oxides, such as zinc oxide, have attracted considerable interest for solar-energy conversion and photocatalysis. Both applications are sensitive to the transport and trapping of photoexcited charge carriers. The probing of electron trapping has recently become possible using time-resolved element-sensitive methods, such as X-ray spectroscopy. However, valence-band-trapped holes have so far escaped observation. Herein we use X-ray absorption spectroscopy combined with a dispersive X-ray emission spectrometer to probe the charge carrier relaxation and trapping processes in zinc oxide nanoparticles after above band-gap photoexcitation. Our results, supported by simulations, demonstrate that within 80 ps, photoexcited holes are trapped at singly charged oxygen vacancies, which causes an outward displacement by ~15% of the four surrounding zinc atoms away from the doubly charged vacancy. This identification of the hole traps provides insight for future developments of transition metal oxide-based nanodevices., Metal-oxide nanostructures are used in a range of light-driven applications, yet the fundamentals behind their properties are poorly understood. Here the authors probe photoexcited zinc oxide nanoparticles using time-resolved X-ray spectroscopy, identifying photocatalytically-active hole traps as oxygen vacancies in the lattice.

A review is presented concerning the exploitation of spectral information carried by probe pulse. The method reviewed allow observing changes and dynamics of the solute mol. structure in polar solvents.

Understanding the initial steps during ultrafast molecular reactions involving large spin state changes is a vital goal in structural dynamics research. These involve knowledge of the geometric structure of the system. Ultrafast X-ray absorption spectroscopy with 50-100 picosecond time resolution establishes the geometric structure of the short-lived (tau = 0.6 ns) high spin state. Here we focus on time-resolved X-ray absorption studies with 160-200 fs temporal resolution to monitor the structural evolution in this spin-conversion process.

Light-harvesting materials, such as the N719 dye, have been introduced as one of the next-generation energy sources. Photophysical and electrochemical studies on such systems have been carried out to understand the light-harvesting phenomena with a particular interest in noble-metal free photosensitizers and photocatalysts. The realization of ultrashort optical and X-ray pulses together with their spectroscopic applications enabled to finely trace the corresponding ultrafast changes in the photosensitizers and photocatalysts, both globally and locally. This study investigates the ultrafast dynamics of energy conversion in model complexes by means of ultrafast optical and X-ray methods to obtain more insights into their steady state performances. Differences in the optical absorption spectrum with respect to the arrival time changes between two pulses revealed a global interaction in a bimetallic photocatalyst such as Ir(III)-Co(III) triads, Ru(II)-Mo(VI) dyads, and Fe(II)-Co(III) dyads. Complementary to this, time-resolved X-ray absorption and emission spectroscopy visualized the electronic and geometric structure changes of the target element that inherently trigger the relevant chemical events in photoactive molecules like Ru(II), Cu(I), and Fe(II) photosensitizers. This study finds that the 3d-chelates undergo a femtosecond symmetry breaking after the charge separation that often recombines non-radiatively, whereas the 4d- and 5d-chelates with their chemical potential in the charge separation state can perturb the nearby electron densities such that either a chemical energy or an electron gets transferred on the picosecond timescale.