Photodissociation dynamics of the allyl radical

The photochemistry and photodissociation dynamics of the allyl radical upon ultraviolet (UV) excitation is investigated in a molecular beam by using t ime- and frequency-resolved photoionization of hydrogen atoms with Lyman-al pha-radiation. The UV states of allyl decay by internal conversion to the g round state, forming vibrationally hot radicals that lose hydrogen atoms on a nanosecond time scale. Two channels are identified, formation of allene directly from allyl, and isomerization from allyl to 2-propenyl, with a sub sequent hydrogen loss, resulting in both allene and propyne formation. The branching ratio is between 2:1 and 3:1, with direct formation of allene bei ng the dominant reaction channel. This channel is associated with site-sele ctive loss of hydrogen from the central carbon atom, as observed in experim ents on isotopically labeled radicals. Ab initio calculations of the reacti on pathways and Rice-Ramsperger-Kassel-Marcus (RRKM) calculations of the ra tes are in agreement with the mechanism and branching ratios. From the meas ured Doppler profiles a translational energy release of 14+/-1 kcal/mol is calculated. The calculated value of 66 kcal/mol for the barrier to the 1,2 hydrogen shift from allyl radical to 2-propenyl is confirmed by the experim ental data. (C) 1999 American Institute of Physics. [S0021-9606(99)01003-X] .