Energy partitioning following photodissociation of methyl iodide in the A band: A velocity mapping study

Translational and internal energy partitioning in the methyl and iodine fra gments formed from photodissociation of methyl iodide in the A-band region is measured using velocity mapping. Stare-selective detection combined with the very good image quality afforded by the two-dimensional imaging techni que allow a detailed analysis of the kinetic energy and angular distributio ns. Product vibrational energy is, as previously known, mainly partitioned into nu(2), the umbrella mode of the methyl fragment,but a substantial frac tion of molecules is also excited with one quantum of nu(1), the symmetric C-H stretch, especially at higher dissociation energies. Preliminary eviden ce is also presented for excitation of several quanta of nu(4), the asymmet ric deformation mode. Rotational energy partitioning is similar for CH3 pro ducts formed in both the ground-state I(P-2(3/2)) and the spin-orbit excite d I*(P-2(1/2)) channel for photodissociation across the full A-band spectru m. Dissociation of vibrationally excited molecules plays an increasingly im portant role at longer dissociation wavelengths. Two CH3I modes remain popu lated in the pulsed beam expansion, nu(2)(a(1)), the C-I stretch, and nu(6) (e), the methyl rock. Each reactant vibrational mode couples in a very spec ific manner into the I and I* dissociation channels. Trends in vibrational and rotational energy disposal are comparedwith recent theoretical predicti ons. Readjustment of many aspects of the ab initio multidimensional potenti al energy surfaces which have recently been calculated for CH3I appears to be necessary. The improved resolution offered by velocity mapping also allo ws a more accurate determination of the C-I bond energy. A dissociation ene rgy of 2.41 +/- 0.02 eV is found. (C) 1999 American Institute of Physics. [ S0021-9606(99)01102-2].