Radical production, laser-initiated

Laser-initiated Radical Production. Although there are different physical mechanisms involved in laser chemistry, we are concerned here with the photodissociation, i.e., the breaking of molecular bonds directly by UV photons. The laser emission is used to produce electronically excited molecules which split into reactive radicals, with the highest possible quantum yield. Since the substrate usually behaves as a poor photoinitiator, an additional molecule must be introduced in order to enhance the radical production, much in the same way as in conventional photoinitiated reactions. In this work,... 

In recent high-resolution, pulsed-field ionization-photoelectron (PFI-PE) spectroscopic studies of laser-initiated radicals, such as SH [61] and CHjS [54] formed in the laser photodissociation of H2S and CH3SH and CH3SSCH3, respectively, we have demonstrated that the PFI-PE spectroscopic technique is potentially useful for probing nascent state distributions of primary products formed in photochemical or reactive processes. 

The five fold increase of p observed when BZP was replaced by DPB must therefore result from a much more effective production of initiating radicals with this novel photoinitiator. The reasons for such a large effect of the phenoxy substituents on the photolysis mechanism of benzophenone are now being investigated by laser spectroscopy. 

Free-radical polymerization in aqueous solution is of significant industrial importance. To model polymerization processes and product properties, reliable rate coefficients for the individual reaction steps are required. The propagation rate coefficient, kp, may be precisely obtained by the PLP-SEC method, which combines pulsed-laser initiated polymerization with subsequent... 

Figure 12. An initial distribution of HClfi = 1-3) was obtained, and is believed to originate from the reaction of Cl atoms with the ethyl radical, although several reactions can generate vibrationally excited HC1. Very recently 193-nm ArF excimer laser photolysis of Mel or acetone and S02 mixtures was employed to generate O atoms and CH3 radicals, and their reaction was monitored using the product CO(t/) chemiluminescence [51].

Absorption of radiation excites the reactant to excited states, from which the molecule can be dismpted into various radical fragments. Conventional sources produce steady state concentrations. Flash and laser sources produce much higher concentrations, enabling more accurate concentration determination, and allowing monitoring of production and removal by reaction of these radicals this is something which is not possible with either thermal initiation or conventional photochemical initiation. 

The CO laser resonance absorption technique is a useful tool for studying the dynamics of chemical reactions that involve the initial production of vibrationally excited CO molecules. We have recently applied this technique to study various atomic and free radical reactions related to combustion and electronic-to-vibrational energy transfer processes U—6). In this brief account, we discuss mainly the dynamics of 0(3P) + 1-alkynes and associated free radical reactions. 

After the application of the microwave pulse at r = to. Curve a of Fig. 14-10 was found to be changed to Curve b of Fig. 14-10. The b-a difference is illustrated by Curve d in Fig. 14-12(a). This curve shows that a-b value gradually increases immediately after the microwave pulse irradiation and that the value slowly decreases after its maximum at t=/Mi- The initial increase corresponds to the disappearance of [M] with its rate constant (kp) given by Eq. (14-8a). In principle, this rate constant for radical pairs generated by the photoreduction of carbonyl and quinone molecules in micelles can be obtained from their A(t) profiles with ns-laser photolysis measurements. In the actual measurements, however, this rate constant has never been obtained from their A(t) profiles because there are many components due to other species such as triplet precursors and reaction products in the profiles. From the present ODESR measurement, Sakaguchi et al. could purely generate the mixed M state with the microwave pulse and succeeded in direct measurement of the kp value for the first time. From the decay of the a-b value, the kj value can be obtained. They, therefore, could determine the kp value for the first time from their ODESR method. 

---