The photoredox reaction transients of ferrioxalate in water have been studied by means of time-resolved EXAFS and ultrafast optical transient spectroscopy. The transient spectra and kinetics have been measured from the femtosecond to millisecond range, and the Fe-O bond lengths of the ferrioxalate redox reaction transients have been determined with 2 ps time resolution and 0.04 A accuracy. These data in conjunction with quantum-chemistry DFT and UHF calculations were used to formulate a mechanism for the Fe(III) to Fe(II) redox reaction where dissociation precedes electron transfer. In addition, radical scavenging experiments support the mechanism proposed.

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The mechanism of reverse saturable absorption of meso-substituted tetrabenzoporphyrins was studied by means of picosecond transient spectroscopy. Characteristic 1 (π,π*), 3 (π,π*), and (d,d) transitions have been observed. The kinetics of the excited states vary with metal substitution. The nonlinear transmission of benzoporphyrins was measured, and the excited-state absorption was determined to be the dominant optical limiting mechanism. The results agreed very well with a five-level model.

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Second harmonic optical coherence tomography, which uses coherence gating of second-order nonlinear optical response of biological tissues for imaging, is described and demonstrated. Femtosecond laser pulses were used to excite second harmonic waves from collagen harvested from rat tail tendon and a reference nonlinear crystal. Second harmonic interference fringe signals were detected and used for image construction. Because of the strong dependence of second harmonic generation on molecular and tissue structures, this technique offers contrast and resolution enhancement to conventional optical coherence tomography.

A factor that limits the use of multiphoton microscopy (MPM) in clinical and preclinical studies is the lack of a compact and flexible probe. We report on a miniaturized MPM probe employing a microelectromechanical system (MEMS) scanning mirror and a double-clad photonic crystal fiber (DCPCF). The use of a MEMS mirror and a DCPCF provides many advantages, such as size reduction, rapid and precise scanning, efficient delivery of short pulses, and high collection efficiency of fluorescent signals. The completed probe was 1 cm in outer diameter and 14 cm in length. The developed probe was integrated into an MPM system and used to image fluorescent beads, paper, and biological specimens.

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Picosecond and nanosecond transient structures have been observed directly using time-resolved X-ray diffraction and absorption. These experiments provide insight on the evolution of transient molecular structure on the atomic length scale during the course of a chemical reaction. Recent advances in the generation of short X-ray pulses and detectors have made time-resolved X-ray studies a reality. We discuss a few of the vast number of possible time-resolved structure studies in solids and fluids. Ultrafast relaxation dynamics of crystal lattice structures induced by picosecond and nanosecond laser pulses have been observed by means of time-resolved picosecond and nanosecond X-ray diffraction. Lattice deformation with 10 ps and 10 -3 Å resolution have been performed. The picosecond X-ray system, which we have used, is described, and its application to time-resolved ultrafast X-ray diffraction in crystals and EXAFS in liquids is discussed.