Can Lasers Be Used To Break Chemical Bonds Selectively?

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Abstract
We discuss the possibility of selective bond breaking induced by the multiple photon absorption of infrared (IR) radiation. The rate at which a reactant molecule's internal vibrational energy distribution relaxes to a random distribution (intramolecular vibrational relaxation [IVR] rate) plays the central role in this discussion. It is pointed out that statistical theories of unimolecular decay, like the Rice Rampsburger Kassel (RRK) and Rice Rampsburger Kassel Marcus (RRKM) theories, preclude any possibility of selectivity because of their assumption of very rapid IVR rates. A new model of IVR and unimolecular decay which does not assume very rapid IVR rates, the restricted IVR model, is presented. Within the framework of this model the conditions necessary for selectivity are discussed quantitatively. A review of the available evidence concerning IVR rates suggests that selectivity is possible with sufficiently short and intense laser pulses. In the light of current knowledge about IVR rates we see no reason to doubt that Hall and Kaldor's experiment using separately two laser pulses of widely differing frequencies to induce the simultaneous isomerization and fragmentation of cyclopropane is a genuine example of selective bond breaking.

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