Excited fragments from excited molecules: energy partitioning in the photodissociation of alkyl iodides

Photofragment spectroscopy has been applied to the photodissociation of methyl, ethyl, n-propyl and isopropyl iodide at 266.2 nm, both to study the process of unimolecular break-up of electronically excited molecules and to determine the energy distributions of the resulting excited fragments. By crossing a molecular beam with a powerful pulsed laser beam in high vacuum and monitoring the arrival times of the recoiling photofragments with a mass spectrometer detector, the translational energy distribution of the photodissociation products is measured. From energy balance, the distribution of fragment internal energy is then calculated. Two peaks are seen in these distributions for both methyl and ethyl iodide, corresponding to formation of ground- and excited-state iodine atoms. The fraction of the energy available after exciting the I atom which goes into internal excitation of the alkyl fragments increases from ∼12% for methyl iodide to ∼50% for the propyl iodides. Thus, the “hot” methyl radicals formed in methyl iodide photolysis are predominantly translationally, rather than internally, excited. The experimental results are disscused in relation to the processes involved in alkyl iodide photodissociation lasers. Dynamic models for energy partitioning in molecular photodissociation are compared with the observed data. The simplest statistical models predict much too high excitation in the alkyl radical. The measured results fall in between the predictions of direct impulsive models based on “rigid” and “soft” radicals, and thus a possible picture for the photodissociation of the alkyl iodides is quasi-diatomic excitation of the C—I bond, followed by recoil of the I atom and a partially deformable alkyl radical. An illustrative example is given of how one might use these unimolecular results in modelling the bimolecular reactions of alkali metal atoms with alkyl iodides, following the analogy drawn by Herschbach and co-workers between photodissociation and “harpooning”, i.e., electron transfer, reactions.