Nanosecond laser flash photolysis

Nanosecond laser Flash Photolysis experiments were performed with 355 and 532 nm laser pulses from a Brilland-Quantel Nd YAG system (5 ns pulse width) in a front face (VIS) and side face (NIR) geometry using a pulsed 450 W XBO lamp as white light source. Similarly to the femtosecond transient absorption setup, a two beam arrangement was used. However, the pump and probe pulses were generated separately, namely the pump pulse stemming from the Nd YAG laser and the probe from the XBO lamp. A schematic representation of the setup is given below in Fig. 7.3. 0.5 cm quartz cuvettes were used for all measurements. 

With the photographic flash lamp the light pulse has a duration of several microseconds at best. The Q-switched pulsed laser provides pulses some thousand times faster, and the kinetic detection technique remains similar since photomultiplier tubes and oscilloscopes operate adequately on this time-scale. The situation is different with the spectrographic technique electronic delay units must be replaced by optical delay lines, a technique used mostly in picosecond spectroscopy. This is discussed in Chapter 8. 

One important difference in the design of a is conventional flash photolysis apparatus and the ns laser flash photolysis system is the size of the sample. The energy of laser pulses is usually very much lower than that of photographic flashes, typically 0.1 J as against 103 J. For this reason the laser light must be focussed on very small samples (0.1 ml for example). 

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Much attention has been devoted to the development of methods to generate quinone methides photochemically,1,19-20 since this provides temporal and spatial control over their formation (and subsequent reaction). In addition, the ability to photogenerate quinone methides enables their study using time-resolved absorption techniques (such as nanosecond laser flash photolysis (LFP)).21 This chapter covers the most important methods for the photogeneration of ortho-, meta-, and para-quinone methides. In addition, spectral and reactivity data are discussed for quinone methides that are characterized by LFP. 

For a detailed discussion of nanosecond laser flash photolysis, see Scaiano, J. C. Nanosecond laser flash photolysis a tool for physical organic chemistry. In Reactive Intermediate Chemistry Moss, R. A. Platz, M. S. Jones, M., Jr., Eds. Wiley New York, 2005. 

Using nanosecond laser flash photolysis techniques, Leigh80 observed transient absorption spectra which he attributed to the silenes derived from photolysis of various methylphenyldisilylbenzenes. Thus the silenes 52,53, and 54 were found to absorb at 425,460, and 490 nm, respectively, in isooctane, and 55 was also found to absorb at 490 nm.75 In other studies, the silene Ph2Si=CH2 derived by laser flash photolysis was found to absorb at 323 nm.111... 

The results, displayed in Figure 2.28, show a good agreement between the three methods within their range of applicability, noting that nanosecond laser flash photolysis and redox catalysis have similar capabilities, with a slight advantage to the former method. 

Nanosecond laser flash photolysis was applied to study excited-state 2-nitrothiophene in polar and non-polar solvents76 the transient absorption at 545 5 nm was assigned to its lowest triplet state. The rate constants of the interaction of this triplet excited state, with a number of substrates such as cyanide and hydroxide ions, have been determined77. Similarly, the transient absorption at 490 5 nm was assigned to the lowest triplet excited state of 5-nitro-2-furoic acid78, and that at 500 5 nm to that of 7V-(n-butyl)-5-nitro-2-furamide79. 

The photodecarboxylation of p-(nitrophenyl) glyoxylic acid 156, which was studied by time-resolved and steady-state methods at room temperature93, leads to p-nitrosobenzoic acid and carbon dioxide in good yields with = 0.28 in aqueous solution at pH 2-12 and excitation at 313, 280 or 254 nm (equation 76). An intermediate (Xmax = 350, r 2 xs) observed by nanosecond laser flash photolysis is assigned to the aci-form of the nitroketene... 

Nanosecond Laser Flash Photolysis A Tool for Physical Organic Chemistry... 

Nitrogen and ruby lasers played a key role in the early development of nanosecond laser flash photolysis (nLFP). Their role has gradually been taken over by the more convenient excimer and NdA AG lasers. Table 18.1 gives typical wavelengths... 

EXPANDING THE CAPABILITIES OF NANOSECOND LASER FLASH PHOTOLYSIS... 

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