Nonstatistical unimolecular dissociation over a barrier

Abstract

A general formulation is presented to model photodissociation processes in which internal conversion is followed by unimolecular dissociation over an exit barrier; this classification of dissociation mechanism results in a nonstatistical product state distribution. The energy available to products is divided into independent statistical and impulsive energy reservoirs. The statistical reservoir considers direct projections of a vibrational microcanonical ensemble at the transition state ͑TS͒ onto product quantum states, conserving vibrational adiabaticity and angular momentum. The impulsive reservoir represents the energy released in passing from the TS to products; this reservoir is treated assuming sudden dissociation of the zero-point TS wave function using a combination of Franck-Condon and impulsive models. We derive the statistical adiabatic impulsive model, which convolutes these two energy reservoirs, to predict the product translational energy distribution for nonstatistical dissociation over a barrier. Two test cases are modeled and compared with experimental data: unimolecular dissociation of acetyl radicals and photodissociation of vinoxy radicals via the B 2 AЉ-X 2 AЉ band.

Ultrafast Photodissociation Dynamics of Acetone at 195 nm: I. Initial-state, Intermediate, and Product Temporal Evolutions by Femtosecond Mass-Selected Multiphoton Ionization Spectroscopy

“…The acetyl-vinoxy isomerization has been reported to have a high barrier of ∼170 kJ/mol on the vinoxy side and ∼200 kJ/mol on the acetyl side. 56,57 However, given that the average vibrational energy is only about 138 kJ/mol above the vinoxy zero-point level (see below), the vinoxy-acetyl isomerization is probably too slow to account for the rise of few tens of ps observed here.…”

Photodissociation Dynamics of the Ethoxy Radical Investigated by Photofragment Coincidence Imaging

“…Under these circumstances, the P(E T ) distribution for a channel with little or no exit barrier peaks at or close to E T ) 0, 41 while channels involving passage over an exit barrier can be described by a model in which the available energy in excess of the barrier is distributed statistically among the available modes, with the P(E T ) distribution peaking at some fraction of the barrier height. 42,43 The P(E T ) distribution for the CH 3 + CH 2 O channel peaks close to 1.5 eV, a value almost five times the size of the exit barrier for this channel, and considerably larger than one would expect if statistical dynamics were operative. Similarly, the H + CH 3 CHO channel has a P(E T ) distribution peaking at about 1 eV, again larger than the exit barrier height for this channel.…”

Nonadiabatic effects in C–Br bond scission in the photodissociation of bromoacetyl chloride

“…136 In general, for negligible angular momentum in the parent, the rotational products, where at least 60% of the potential energy from the top of the barrier is converted into relative product translation. 133,135 Since about 60% of the 1.25 eV exit channel barrier would go into product relative translation if the C α -Br photodissociation proceeded adiabatically, the peak in the P(E T ) distribution should be at 0.75 eV or higher, instead of 0.2 eV as observed experimentally. Another comparison can be made to a system that presents similar characteristics to BrC α H 2 C(O)Cl, e.g., photodissociation of methyl vinyl ether at 193 nm to yield vinoxy (CH 2 CHO) and methyl (CH 3 ) radicals.…”

“…[128][129][130][131] In particular, one usually expects that most of the potential energy released along the reaction coordinate after passing the barrier is converted to relative translational energy of the photofragments, and that the translational energy distribution is rather insensitive to the total available energy. 108,[132][133][134][135] In contrast, reactions without an exit channel barrier typically have an asymmetric bell-shaped translational energy distribution with its maximum somewhat away from zero translational energy. 128,130 It can be seen from Also, from the 6S-MC-QDPT energies in Fig.…”