Pulse excitation excited-state photophysics

Knowledge of the dynamics of excited states is of major importance in understanding photophysical, photochemical and photobiological processes. Two time-resolved techniques, pulse fluorometry and phase-modulation fluorometry, are commonly used to recover the lifetimes, or more generally the parameters characterizing the S-pulse response of a fluorescent sample (i.e. the response to an infinitely short pulse of light expressed as the Dirac function S). 

The lack of many exceptions to Kasha s rule subsequently led to the erroneous inference that no other upper excited state process, be it photophysical or photochemical, intra- or intermolecular, could efficiently compete with internal conversion. However, with the increasingly common use of pulsed lasers in photochemical studies over the past 20 years, it has been possible to show that the upper excited states of many systems have a rich and varied chemistry. Up to this... 

Therefore, information about the efficiency and excited state intermediates for a phototoxic reaction in the eye obtained by using photophysical techniques (fluorescence, flash photolysis, pulse radiolysis, ESR) can be predictive of phototoxicity in vivo. It has been confirmed that photophysical studies collate well with in vivo data (Roberts et al., 1991a). For instance, tetrasulphonatophenylporphyrin (TPPS), which binds to lens proteins, shows a long-lived triplet in the intact calf and human... 

Upon absorption of light, any drug molecule gets excited to a higher energy state from which a number of photochemical and photophysical reactions may occur. The products of these reactions may be permanent, in which case they can be analyzed and quantified by the methods discussed in Section 12.2.1. However, these permanent products can arrive via short-lived free radicals. In order to have a comprehensive understanding of the photoreactions, it is necessary to study the characteristics and the reactivities of these short-lived free radicals and the excited states. Such studies can be conveniently carried out using flash photolysis or pulse radiolysis. 

Photophysical models Thermal and nonthermal features are important. The models consider two independent channels of bond breaking [81, 82], or imply different bond breaking energies for ground state and electronically excited chromophores [83, 84], These models are mainly important for short pulse (ps and fs) lasers [85],... 

Different parameters are required to characterize the resonant and the nonresonant optical nonlinearity. This has often been a source of confusion in the literature, even today. For nonresonant processes, the magnitude of the nonlinearity is measured by either x(3) or n2. However, for resonant processes, x(3)> a2> or ni alone cannot measure the magnitude of the nonlinearity. For example, a different x(3) value can result from the same material when lasers with different pulse widths are used for the measurement. A complete characterization of the nonlinearity requires a set of parameters, including % 3), the ground state absorption coefficient, the laser pulse width, and the excited state relaxation time. In a simple two- or three-level system, once all these factors are properly taken into account, the best parameter for measuring the resonant nonlinearity is simply the ground state absorption cross section of the material. In the following section I focus on the resonant nonlinearity only as this is closely related to the photophysical properties. The discussion of nonresonant nonlinearity of semiconductor nanoclusters can be found elsewhere [17, 84-86],... 

Biological Aspects.—The lowest excited triplet states of all-rran.s- -carotene produced by pulse radiolysis has been studied by time-resolved resonance Raman spectroscopy.Six transient Raman bands at 965, 1009, 1125, 1188, 1236, and 1496cm were observed and assigned to the triplet state of ) -carotene. The authors conclude that the molecule may be substantially twisted, presumably at the 15,15 band in the triplet state. Further work has also been carried out by the same workers on the triplet state of all-rran -retinal. The results indicate increased 7r-electron delocalization in the triplet state and propose that the relaxed excited triplet-state exists in either sAX-trans or 9-cis conformation. Das and Becker" have also employed pulse radiolysis and laser flash photolysis to study several photophysical properties of the triplet states of the series of polyenals (29)—(33) related to retinal (31) as homologues (Table 35). Results are presented... 

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