Molecule-specific laser chemistry

This reaction has been carried out with a carbon dioxide laser line tuned to the wavelength of 10.61 p.m, which corresponds to the spacing of the lowest few states of the SF ladder. The laser is a high power TEA laser with pulse duration around 100 ns, so that there is no time for energy transfer by coUisions. This example shows the potential for breakup of individual molecules by a tuned laser. As with other laser chemistry, there is interest in driving the dissociation reaction in selected directions, to produce breakup in specific controllable reaction channels. 

One of tbe central properties of lasers is the ability to furnish large numbers of photons at very specific energies. This ability has caused many investigators to hope that laser chemistry might be possible, that is that the energy from the laser might be deposited in molecules in very specific ways in order to initiate very selective, interesting, or remunerative chemistry. 

Bond- or Mode-selective Laser Chemistry. Suppose we wish to break a specific bond in a molecule or cause a molecule to rearrange in a specific way, and the desired transformation is not the one which will occur if the molecule is simply heated (i.e., it is not the weakest coordinate in the molecule). Can we, by selectively exciting with a laser the bond or motion in question, cause the desired transformation to occur in greater than thermal yield ... 

Photodissociation of small polyatomic molecules is an ideal field for investigating molecular dynamics at a high level of precision. The last decade has seen an explosion of many new experimental methods which permit the study of bond fission on the basis of single quantum states. Experiments with three lasers — one to prepare the parent molecule in a particular vibrational-rotational state in the electronic ground state, one to excite the molecule into the continuum, and finally a third laser to probe the products — are quite usual today. State-specific chemistry finally has become reality. The understanding of such highly resolved measurements demands theoretical descriptions which go far beyond simple models. 

Laser-based spectroscopic probes promise a wealth of detailed data--concentrations and temperatures of specific individual molecules under high spatial resolution--necessary to understand the chemistry of combustion. Of the probe techniques, the methods of spontaneous and coherent Raman scattering for major species, and laser-induced fluorescence for minor species, form attractive complements. Computational developments now permit realistic and detailed simulation models of combustion systems advances in combustion will result from a combination of these laser probes and computer models. Finally, the close coupling between current research in other areas of physical chemistry and the development of laser diagnostics is illustrated by recent LIF experiments on OH in flames. 

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