Infrared multiple-photon activation and unimolecular dissociation

Infrared multiple-photon activation and unimolecular dissociation 

Indeed, once the molecule reaches the quasicontinuum the normal modes of vibration become strongly mixed by the anharmonic terms in the molecular potential and energy flows rapidly between the modes. Thus, excitation through the quasi-continuum is rapid and, in general, the larger a molecule is, the easier it becomes to reach the quasi-continuum. 

However, it is important to recognize that the first few steps in the absorption process are selective. Owing to the low density of states in this region, only one molecular species (which has a transition resonant with the laser frequency) in a mixmre of other molecules will absorb and thus be selectively excited into the quasicontinuum and on to the dissociation limit. Indeed, it is possible to achieve isotope separation using IR multiple-photon excitation for example, can be selec- 

Useful short reviews of this area, with a more extended discussion of RRKM theory, are given in the books by Levine and Bernstein (1987) and Holbrook etal. (1996). 

Another approach to the study of unimolecular processes is to use UV (i.e. electronic) excitation. 

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