Picosecond laser flash photolysis

Gratzel and Serpone and co-workers recently reported on a picosecond laser flash photolysis study of TiO. They observed the absorption spectrum immediately after the 30 ps flash and attributed it to electrons trapped on Ti" " ions at the surface of the colloidal particles. The absorption decayed within nanoseconds, the rate being faster as the number of photons absorbed per colloidal particle increased. This decay was attributed to the recombination of the trapped electrons with holes. 

The direct detection of the S <- Sj absorption in organic compounds has so far been achieved by a nanosecond or picosecond laser flash photolysis method. The general features of transient absorption spectra of metalloporphyrins actually suggest the presence of strong absorption bands in visible or ultraviolet region (38-40). However, as the transient absorption of the state often overlaps with that of ground state depletion, it is usually difficult to evaluate the absolute absorption cross sections for the transition by... 

In the first place, we shall take a look at the recent advances in fast reaction photochemical kinetics and spectroscopy, in particular at picosecond laser flash photolysis and femtosecond observations. Next, photophysics and photochemistry in molecular beams will be considered. Here observations are made under single molecule-single photon conditions, and these experiments provide insight into the most fundamental unimolecular gas phase reactions. 

A picosecond laser flash photolysis study appeared on 268a-b (94CPL (230)249). After excitation at 355 nm of 268a, the absorption maximum at ca. 560 nm is ascribed to the closed form and the depletion below 450 nm is due to the bleaching of the open-form, respectively. 

Kamat, P. V. Ebbesen, T. W. Dimitrijevic, N. M. Nozik, A. J. Photoelectrochemistry in semiconductor particulate systems. Part 12. Primary photochemical events in CdS semiconductor colloids as probed by picosecond laser flash photolysis, transient bleaching, Chem. Phys. Lett. 1989, 157, 384. 

Photophysical characteristics of Pis (Scheme 12.1), especially the quantum yields of their dissociation O iss, are very important. Most of the photophysical data were measured by nanoseconds or picoseconds laser flash photolysis (LFP) or phosphorescence at low temperature. The properties of representative Pis are given in Table 12.1. 

When primary and secondary co-arylalkylamines are excited, exciplex fluorescence is not observed, but fluorescence quenching does take place (Shizuka et al., 1979). Presumably chemical reaction competes effectively with exciplex formation. However, by means of picosecond laser flash photolysis, the time-resolved absorption spectra of the species produced on reaction of pyrene with diphenylamine have been obtained (Okada et al., 1980b). It was shown that the reaction leads to neutral radicals via an exciplex. Although the formation of neutral radicals in such systems had been previously identified (Okada et al, 1976a) the role of exciplexes had been purely speculative. 

Nanosecond and picosecond laser-flash photolysis techniques have been used by different groups to elucidate the various intermediate stages involved in the photo-induced reactions of amines. The overall mechanism involving the electron-transfer process in a fluid medium is illustrated in Scheme 13. The dynamics of the process involve the formation of an encoimter complex between the excited-state molecule and the ground-state molecule [117, 140, 141]. The encounter complex can be described as an intermoleculai ensemble of excited- and ground-state molecules, separated by a small distance (ca 7 A) and surroimded by solvent molecules. During... 

Direct evidence for the formation of radical ion intermediates in the benzophenone-amine system was also obtained by Peters et al. [153-156]. Picosecond laser-flash photolysis studies have indicated the formation of ketyl radical anions concomitant with the decay of the benzophenone triplet. For 1.0 M dime-thylaniline and diethylaniline the rate of electron transfer to the benzophenone triplet was 3.6 x 10 and 4.2 x 10 m s , respectively. On the basis of their studies Peters et al. proposed a mechanism in which a solvent-separated ion pair... 

Conclusive evidence for the cation intermediate was obtained by detection of the absorption spectra of several diarylmethyl cations following nanosecond or picosecond laser flash photolysis of their respective diazo, diphenylazi-ridinylimine, or 3H-indazole precursors in acidic media [107-112], Photochemically generated vinyl carbenes were recently shown to similarly protonate by deuterium labeling experiments to give allylic cations that were detected by transient absorption spectroscopy [113],... 

Flash Photolysis Spectrometers. Picosecond timescale measurements were carried out at 355 nm (3rd Harmonic, Nd YAG) with 20 ps half width pulses from the passively mode locked laser system at the Canadian Center for Picosecond Laser Flash Photolysis (11.). The pulse energy was 3 mj and the wavelength range of the probe pulse used for observation of spectra extended from 400 to 700 nm. 

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