Photofragmentation: understanding the influence of potential surfaces and exit-channel dynamics

Abstract

Photoexcitation is used to prepare species whose subsequent fragmentation can be exploited for the purposes of studying, controlling and manipulating different kinds of molecular processes. First, we show how a form of sub-Doppler resolution spectroscopy can be used to determine centre-ofmass kinetic energy distributions, thereby enabling internal energy distributions to be obtained for elementary processes which occur at a fixed total energy. We present data for H atoms monitored at the Lyman-a wavelength. Secondly, we show how such processes can be used to study binary interactions by preparing van der Waals complexes (e.g. C0,HBr

Kinetics and dynamics in the photodissociation of the allyl radical

“…For both reaction channels, the hydrocarbon fragment would contain a large amount of internal excitation, in agreement with an indirect dissociation process in which the molecule undergoes a large geometry change before entering the exit channel. 25 This geometry change is certainly more pronounced for the formation of the cyclic product, cyclopropene, making it surprising if the formation of cyclopropene were to be accompanied by a translational energy release about twice as high as is typical for similar compounds.…”

Product lambda-doublet ratios as an imprint of chemical reaction mechanism

“…The presence of multiple PESs correlating reactants and products leads to open shell molecules in which the rotational levels may be split into spin–orbit states and, in turn, each of them into two nearly degenerate Λ-doublet levels that can be spectroscopically resolved due to different selection rules. In spite of the tiny energy difference between the Λ-doublet pair of states, a clear preference towards one of them is observed in many chemical processes123456, including inelastic and reactive collisions, and molecular photodissociation, and has even been postulated as the origin of OH astronomical masers78. As pointed out by several authors891011121314151617, the Λ-doublet population acts as a fingerprint to unravel the symmetries of the surfaces involved in the process, such that the propensity for one of the manifolds reflects the competing reactivity on concurrent PESs and addresses the question of where the electrons go when the reaction takes place1517.…”

Excited-state decay of hydrocarbon radicals, investigated by femtosecond time-resolved photoionization: Ethyl, propargyl, and benzyl

“…The resulting electronic excitation is typically converted into vibrational energy by internal conversion ͑IC͒ and subsequently leads to dissociation. The unimolecular dissociation can be monitored either by photofragment Doppler spectroscopy [3][4][5] or by translational energy spectroscopy. 6 The interpretation of such data often assumes a fast redistribution of energy within the molecule, including a fast nonradiative decay of the photoexcited state to the electronic ground state.…”