Transient spectroscopy nanosecond laser flash photolysis

The homolysis product, Phl"+, has been observed directly by laser-flash photolysis probed with transient absorption spectroscopy on both the nanosecond [23,69] and picosecond [22] time scales. Klemm et al. [23] and Hacker and Dektar [70] suggest the possibility of electron transfer between the homolysis fragments leading to products identical with those of heterolysis. DeVoe et al. found no evidence for this process [22], Electron transfer... 

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],... 

Transient intermediates are most commonly observed by their absorption (transient absorption spectroscopy see ref. 185 for a compilation of absorption spectra of transient species). Various other methods for creating detectable amounts of reactive intermediates such as stopped flow, pulse radiolysis, temperature or pressure jump have been invented and novel, more informative, techniques for the detection and identification of reactive intermediates have been added, in particular EPR, IR and Raman spectroscopy (Section 3.8), mass spectrometry, electron microscopy and X-ray diffraction. The technique used for detection need not be fast, provided that the time of signal creation can be determined accurately (see Section 3.7.3). For example, the separation of ions in a mass spectrometer (time of flight) or electrons in an electron microscope may require microseconds or longer. Nevertheless, femtosecond time resolution has been achieved,186 187 because the ions or electrons are formed by a pulse of femtosecond duration (1 fs = 10 15 s). Several reports with recommended procedures for nanosecond flash photolysis,137,188-191 ultrafast electron diffraction and microscopy,192 crystallography193 and pump probe absorption spectroscopy194,195 are available and a general treatise on ultrafast intense laser chemistry is in preparation by IUPAC. 

The detection of short-lived transient species is often achieved by flash photolysis where an extremely short flash of UV/Vis radiation from a laser generates a high concentration of transient species, and a second probe beam monitors any changes that occur after the flash. Traditionally, UVA is spectroscopy has been used as a detection method. However, time-resolved infrared spectroscopy (TRIR), a combination of UV flash photolysis and fast IR detection, also has a long history. There are several different approaches to fast IR spectroscopy and the method of choice depends upon the timescale of the reaction. Measurements on the nanosecond to millisecond timescale are obtained using point-by-point techniques or by step-scan FTIR. In the point-by-point approach, a continuous wave IR laser (GO or diode) or globar is used as the IR source, which is tuned to one particular IR frequency (Figure 3). ... 

The conditions which determine whether flash photolysis can be used to smdy a given chemical system are (i) a precursor of the species of kinetic interest has to absorb light (normally from a pulsed laser) (ii) this species is produced on a timescale that is short relative to its lifetime in the system. Current technical developments make it easy to study timescales of nanoseconds for production and analysis of species, and the use of instrumentation with time resolution of picoseconds is already fairly common. In certain specific cases, as we will see in the last part of this chapter, it is possible to study processes on timescales greater than a few femtoseconds. Once the species of interest has been produced, it is necessary to use an appropriate rapid detection method. The most common technique involves transient optical absorption spectroscopy. In addition, luminescence has been frequently used to detect transients, and other methods such as time-resolved resonance Raman spectroscopy and electrical conductivity have provided valuable information in certain cases. 

Rate constants for interaction of triplet excited states of cyclic enones with alkenes were first reported by Schuster et al. > > using transient absorption spectroscopy (nanosecond flash photolysis). The rate constants were obtained from the relationship (Xx)" = ( o) + (alkene), where Xq is the limiting triplet hfetime of the enone at a given concentration in the absence of alkene. The decay of enone triplet absorption at 280 nm could be conveniently followed upon excitation of the enones (cyclopentenone [CP], 3-methylcyclohexenone [3-MCH], testosterone acetate [TA], and BNEN [4] were aU studied]) in acetonitrile and cyclohexane at 355 nm using the third harmonic of a Nd YAG laser. In aU cases, triplet decays were clearly first order. Quantum efficiencies for capture of enone triplets by alkenes (O,.) are given by fc x (alkene) using the experimentally determined values of and Xq,... 

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