High-pressure laser-induced reactions

Table II lists a number of the high-pressure laser-induced reactions that have been studied. (References 29 to 30 contain more extensive tables of similar reactions not included here.) These reactions are carried out either directly or with a second component added as a sensitizer. Reaction (21) proceeds only in the presence of a sensitizer such as SiF4, since...

In this work, we have demonstrated that the CH radical can be generated with sufficiently high concentrations by means of the multiphoton dissociation of CHBr at 193 nm for kinetic measurements. The formation and decay of the CH radical was monitored by the laser-induced fluorescence technique using the (A2 b — X2ir) transition at 430 nm. Several rate constants for the reactions relevant to high temperature hydrocarbon combustion have been measured at room temperature. One of the key reactions, CH + N2, has been shown to be pressure-dependent, presumably due to the production of the CHN2 radical at room temperature. 

The thermal, unsensitized infrared laser, and SiF4 laser sensitized decomposition of 1,2-dichloropropane were described by Tsang and coworkers96. The dichloro substrate yielded via four reaction channels 3-chloropropene, cw-l-chloropropene, trans- 1-chloro-propene and 2-chloropropene. These products have also been observed in thermal and laser-induced processes. Comparative data of the dichloropropane decomposition between the laser-induced experiment and the other pyrolytic methods indicated the former to be associated with complex phenomena with infrared multiphoton-induced decomposition. Consequently, a wide range of processes appeared to occur and the unimolecular rate parameters can be unambigously interpreted only at definite high pressure. 

The time to measure spectra of this quality under high-pressure conditions has been about I min. The absolute time scale of the experiment depends on the method of initiation. In thermally initiated (spontaneous) polymerizations reaction time can be several hours or even days. In contrast, in excimer laser-initiated free radical polymerizations application of a few laser pulses each of about 20 ns duration can induce changes between subsequent spectra as on this figure. 

Differences and similarities in the behaviour of reactions induced thermally and by CO2 laser radiation at relatively high pressures have been described in several cases. Decomposition of CH3CF2CI with a CW laser source takes place with a rate constant equal to the thermal value at 200 Torr, but vibrational and translational degrees of freedom appear not to be completely equilibrated at lower pressures. 

Further information on the rate coefficients for the reactions in Eqs. 2a and 2b is given in some recent papers. Gutman and Nelson (1983) used laser-induced fluorescence to study the addition reactions of the (32H3O radical with O2 and NO. The bimolecular rate coefficients for the reaction with NO were observed over the major portion of the transition region from the low- to high-pressure limits and give at room temperature hP = (2.36 0.31) X 10 ilf -sec with N2 as a chaperone and k" = (1.51 0.18) x 10 ° A/ -sec . However the C2H3O radical has two resonance forms. 

Detailed studies on high-pressure polymerization kinetics require the measurement of concentrations under reaction conditions. Particularly, valuable insight comes from highly time-resolved measurements of monomer concentration after instantaneous laser-induced photoinitiator decomposition, which produces an intense birtst of free radicals. 

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