Kinetics and dynamics in the photodissociation of the allyl radical

Abstract

The direct observation of the products, kinetics and translational energy release from the photodissociation of the allyl radical, C3H5, upon excitation in the near-uv is reported. A statistical analysis of the data shows that they are in agreement with allene formation being the dominant H-loss reaction channel.

Time-resolved photoelectron spectroscopy of the allyl radical: The lifetimes of the ultraviolet bands

“…49 However, there are several strong arguments against an excited state isomerization in the UV bands. First, the statistical analysis of the nanosecond data 21 is in agreement with a reaction from a hot ground state. Second, two different pathways for the C 3 H 5 →C 3 H 4 ϩH reaction with different rates on the ns-time scale were identified in the nanosecond experiments, the faster one being site-specific allene formation, associated with the loss of the central hydrogen atom.…”

“…It might be possible, for example, that B-state contributions lead to a lengthening of the lifetime of formally C-state levels, or that additional interactions with the A-state modulate the electronic matrix elements ␤ IC for the decay into the ground state. We note, however, that our nanosecond experiments 21,49 indicate no change in the photodissociation dynamics when different UV bands are initially excited.…”

“…1; ͑b͒ dissociation into C 3 H 4 plus a hydrogen atom does occur, but on a nanosecond time scale, as shown recently. 21 Thus the decays reported here cannot be due to direct or predissociative hydrogen loss from the UV states. ͑c͒ A third possibility would be excited state isomerization, leading to the formation of either cyclopropyl or methylvinyl radical.…”

“…4 At the excitation energies present in our experiment, the cyclopropyl radical would lose a hydrogen atom and form the closed-shell molecule cyclopropene. In our recent nanosecond experiments, 21 investigating the Hatom product of the unimolecular dissociation of allyl radicals, no indication for cyclopropene formation was found. Thus we conclude that under our conditions photocyclization does not constitute an important decay channel for the UV bands of allyl.…”

“…On the one hand, we applied picosecond time-resolved photoelectron spectroscopy to follow the initial decay of low-lying vibrational levels of the B-state in real time, 20 obtaining lifetimes around 20 ps due to internal conversion of the B-state. In a different experiment we detected the loss of hydrogen atoms from allyl radicals upon UV excitation 21 by time-resolved ionization of H-atoms with Lyman-␣ radiation with a rate of 4 ϫ10 7 s Ϫ1 , i.e., on a ns time scale. A statistical analysis of the data revealed that allene formation being the dominant reaction channel is in better agreement with the data than cyclopropene formation.…”