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The advancement of crossed molecular beam methods, modern spectroscopy and laser technology allows us to observe chemical reactions on atomic and molecular levels in great detail. After a brief history of crossed molecular beams studies, the author describes and discusses the universal molecular beam apparatus and gives examples of crossed molecular beam studies. The crossed beam technique is compared to other techniques used to provide microscopic information on reaction dynamics. Application of crossed laser and molecular beam studies to the problem of IR multiphoton dissociation of polyatomic molecules is discussed. Study of vibrational predissociation of hydrogen-bonded and van der Waals molecular clusters are discussed. Future cases that the author considers worth pursuing that could benefit from the collisionless environment of molecular beams are enumerated.
Multiphoton dissociation (MPD) of ethyl chloride was studied using a tunable 3.3 .mu.m laser to excite CH stretches. The absorbed energy increases almost linearly with fluence, while for 10 .mu.m excitation there is substantial saturation. Much higher dissociation yields were observed for 3.3 .mu.m excitation than for 10 .mu.m excitation, reflecting bottlenecking in the discrete region of 10 .mu.m excitation. The resonant nature of the excitation allows the rate equations description for transitions in the quasicontinuum and continuum to be extended to the discrete levels. Absorption cross sections are estimated from ordinary ir spectra. A set of cross sections which is constant or slowly decreasing with increasing vibrational excitation gives good fits to both absorption and dissociation yield data. The rate equations model was also used to quantitatively calculate the pressure dependence of the MPD yield of SF6 caused by vibrational self-quenching. Between 1000-3000 cm−1 of energy is removed from SF6 excited to approx.> 60 kcal/mole by collision with a cold SF6 molecule at gas kinetic rate. Calculation showed the fluence dependence of dissociation varies strongly with the gas pressure. Infrared multiphoton excitation was applied to study thermal unimolecular reactions. With SiF4 as absorbing gas for the CO2 laser pulse, transient high temperature pulses were generated in a gas mixture. IR fluorescence from the medium reflected the decay of the temperature. The activation energy and the preexponential factor of the reactant dissociation were obtained from a phenomenological model calculation. Results are presented in detail. (WHK).
The laser as a radiation source with temporal and spatial coherence has made a tremendous impact in the different fields of science. As a result, new and exciting research has been developing allover the world. Laser spectro scopy shares a large fraction of this research, and in the last decade nu merous books and monographs have been published on this subject. Most of these books and monographs contain the work done in the physics community. Very few books represent the advances made in laser chemistry, a field that is flourishing and whose future is indeed very exciting. It was felt that a meeting that focused on the important questions being asked in the chemistry community, and on new and possible directions in laser chemistry, was needed. This three-day conference, held at the California Institute of Technology, Pasadena, California, on March 20-22, 1978, covered five important areas in laser chemistry: Laser-induced chemistry, picosecond processes and techniques, nonlinear optical spectroscopy and dephasing processes, multiphoton exci tation in molecules, and molecular dynamics by molecular beams.