Pulse radiolysis-laser flash photolysis

Pulse Radiolysis and Pulse Radiolysis-Laser Flash Photolysis... 

In this chapter, we summarize recent progress in the photochemistry of shortlived species by use of multibeam excitation, including our recent achievements in this field. Our research group has employed various multibeam irradiation methods to reveal reaction processes of various excited intermediates, including basic molecules and biomolecules. We also achieved direct observation of shortlived species utilizing ultrashort pulse lasers. These results are interesting recent examples of reactions induced by multibeam irradiation. Based on the excitation method, this chapter is divided into the following sections Pulse Radiolysis-Laser Flash Photolysis (Section II), Two-Color Two-Laser Flash Photolysis (Section III), and Three-Color Three-Laser Flash Photolysis (Section IV). Each section is further divided into subsections based on the topics. 

Figure 2.5. Kinetic traces of AO.D.480 during the pulse radiolysis-laser flash photolysis experiment of t-St (5 x 10-3 M) in the absence (a) and presence (b) of ANS (1.0 M) in Ar-saturated 1,2-dichloroethane.

Figure 2.7. Transient absorption spectra observed at Ons, 100 ns, and 1 ps after the 8-ns electron pulse, and transient fluorescence spectrum of TMB + observed at 300 ns after the electron pulse during pulse radiolysis-laser flash photolysis of TMB (lx 10-2M) in Ar-saturated 1,2-dichloroethane. Excitation wavelength, 532 nm. Laser pulse energy, 140 mJ pulse-1.

Many experimental methods may be distinguished by whether and how they achieve time resolution—directly or indirectly. Indirect methods avoid the requirement for fast detection methods, either by detemiining relative rates from product yields or by transfonuing from the time axis to another coordinate, for example the distance or flow rate in flow tubes. Direct methods include (laser-) flash photolysis [27], pulse radiolysis [28]... 

Photolytic methods are used to generate atoms, radicals, or other highly reactive molecules and ions for the purpose of studying their chemical reactivity. Along with pulse radiolysis, described in the next section, laser flash photolysis is capable of generating electronically excited molecules in an instant, although there are of course a few chemical reactions that do so at ordinary rates. To illustrate but a fraction of the capabilities, consider the following photochemical processes ... 

The fates of the G(-H) radicals in DNA are mostly determined by reactions with other substrates. Here, we consider the reactions of the G(-H) radicals with types of free radicals that are generated in vivo under conditions of oxidative stress. One of these radicals is the nitrogen dioxide radical, NO2. This radical can be generated in vivo by the oxidation of nitrite, N02, a process that can be mediated by myeloperoxidase [111, 112] as well as by other cellular oxidants [113, 114]. An alternative pathway of the generation of NO2 is the homolysis of peroxynitrite [102, 115] or nitrosoperoxycarbonate formed by the reaction of peroxynitrite with carbon dioxide [99-101]. The redox potential, E°( NO2/NO2")=1.04 V vs NHE [116] is less than that of guanine, E7[G(-H)7G] = 1.29 V vs NHE [8]. Pulse radiolysis [117] and laser flash photolysis [109] experiments have shown that, in agreement with these redox potentials, N02 radicals do not react with intact DNA. However, N02 radicals can oxidize 8-oxo-dG that has a lower redox potential ( 7=0.74 vs NHE [56]) than any of the normal nucleobases [109]. 

The pulse radiolysis technique is close to the better known laser flash photolysis (for a monograph see Bensasson et al. 1983). There is one essential difference in pulse radiolysis the energy is absorbed by the solvent, e.g., by water in DNA solu-... 

Janata E (1992b) Instrumentation of kinetic spectroscopy. 10. A modular data acquisition system for laser flash photolysis and pulse radiolysis experiments. Radiat Phys Chem 40 437-443 Janata E, Lilie J, Martin M (1993) Instrumentation of kinetic spectroscopy. 11. An apparatus for AC-conductivity measurements in laser flash photolysis and pulse radiolysis experiments. Radiat Phys Chem 43 353-356... 

Since the heroic early mechanistic investigations, there have been two developments of major significance in radical chemistry. The first was the advent of electron spin resonance (ESR) spectroscopy (and the associated technique of chemically induced dynamic nuclear polarisation, CIDNP) [24], which provided structural as well as kinetic information the second is the more recent development of a wide range of synthetically useful radical reactions [20]. Another recent development, the combination of the pulse radiolysis and laser-flash photolysis techniques, is enormously powerful for the study of radicals but beyond the scope of this book. 

Nasr, C. Vinodgopal, K. Hotchandani, S. Kamat, P. V. Excited state and reduced forms of a textile diazo dye, Naphthol blue black. Spectral characterization using laser flash photolysis and pulse radiolysis. Radiat. Phys. Chem. 1997, 49, 159. 

Tanielian C, Duffy K, Jones A. Kinetic and mechanistic aspects of photocatalysis by polyoxotungstates a laser flash photolysis, pulse radiolysis, and continuous photolysis study. J Phys Chem B 1997 101 4276-82. 

Triplet excited states of four derivatives of ubiquinone-6 (256), in which various ring substituents are progressively altered, have been studied by laser flash photolysis (265 nm) and pulse radiolysis (9—12MeV electrons). The triplet absorption spectra, extinction coefficients, lifetimes, energy levels, and quantum efficiencies of formation were determined.117... 

The initial effects of high energy radiation on condensed systems are discussed. Evidence of short lived intermediates, i.e. radical ions and excited states is illustrated by fast pico-second and nanosecond pulse radiolysis A discusion of the nature of early events leading to excited states is discussed at length, and in particular comparisons are made to corresponding experiments at low photon energies, i.e. via laser flash photolysis. 

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