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 fragments formed from photodissociation of methyl iodide in the A-band region is measured using velocity mapping. State-selective detection combined with the very good image quality afforded by the two-dimensional imaging technique allow a detailed analysis of the kinetic energy and angular distributions. Product vibrational energy is, as previously known, mainly partitioned into n2 , the umbrella mode of the methyl fragment, but a substantial fraction of molecules is also excited with one quantum of n1 , the symmetric C–H stretch, especially at higher dissociation energies. Preliminary evidence is also presented for excitation of several quanta of n4 , the asymmetric deformation mode. Rotational energy partitioning is similar for CH3 products formed in both the ground-state I(P3/2) and the spin–orbit excited I*( P1/2) channel for photodissociation across the full A-band spectrum. Dissociation of vibrationally excited molecules plays an increasingly important role at longer dissociation wavelengths. Two CH3I modes remain populated in the pulsed beam expansion, n2(a1), the C–I stretch, and n6(e), the methyl rock. Each reactant vibrational mode couples in a very specific manner into the I and I* dissociation channels. Trends in vibrational and rotational energy disposal are compared with recent theoretical predictions. Readjustment of many aspects of the ab initio multidimensional potential energy surfaces which have recently been calculated for CH3I appears to be necessary. The improved resolution offered by velocity mapping also allows a more accurate determination of the C–I bond energy. A dissociation energy of 2.41 60.02 eV is found. © 1999 American Institute of Physics. @S0021-9606~99!01102-2#