Imaging the molecular channel in acetaldehyde photodissociation: roaming and transition state mechanisms

Abstract

The roaming dynamics in the photodissociation of acetaldehyde is studied through the first absorption band, in the wavelength interval ranging from 230 nm to 325 nm. Using a combination of the velocity-map imaging technique and rotational resonance enhanced multiphoton ionization (REMPI) spectroscopy of the CO fragment, the branching ratio between the canonical transition state and roaming dissociation mechanisms is obtained at each of the photolysis wavelengths studied. Upon one photon absorption, the molecule is excited to the first singlet excited S(1) state, which, depending on the excitation wavelength, either converts back to highly vibrationally excited ground S(0) state or undergoes intersystem crossing to the first excited triplet T(1) state, from where the molecule can dissociate over two main channels: the radical (CH(3) + HCO) and the molecular (CO + CH(4)) channels. Three dynamical regions are characterized: in the red edge of the absorption band, at excitation energies below the T(1) barrier, the ratio of the roaming dissociation channel increases, largely surpassing the transition state contribution. As the excitation wavelength is increased, the roaming propensity decreases reaching a minimum at wavelengths ∼308 nm. Towards the blue edge, at 230 nm, an upper limit of ∼50% has been estimated for the contribution of the roaming channel. The experimental results are interpreted in terms of the interaction between the different potential energy surfaces involved by means of ab initio stationary points and intrinsic reaction coordinate paths calculations.

Communication: Photodissociation of CH3CHO at 308 nm: Observation of H-roaming, CH3-roaming, and transition state pathways together along the ground state surface

“…The theoretical fit was used to infer that the major route via roaming pathway accounts for 84% ± 10% of the total CH 4 +CO products, 5 showing a substantial deviation from the previous values. 6,8 Until a recent study, 4 Kable and co-workers reconciled the inconsistent results by probing CO with (2 + 1) REMPI ion-imaging in the wavelength range of 308-328 nm. They demonstrated that the low-J, a low speed component of the CO bimodality, results from roaming of a H-atom about a CH 3 CO core, while the high-J, a fast speed component, arises from a second roaming of CH 3 around CHO.…”

“…At the same wavelength, Bañares and co-workers analyzed the kinetic energy distribution of the CO fragment acquired by resonance-enhanced multiphoton ionization spectroscopy (REMPI) coupled with ion-imaging and obtained a roaming branching to be 20%. 8 In a combined experimental and theoretical study, Heazlewood et al obtained the CH 4 product with highly vibrationally excited states, which was predicted consistently by quasi-classical trajectory (QCT) calculations. The theoretical fit was used to infer that the major route via roaming pathway accounts for 84% ± 10% of the total CH 4 +CO products, 5 showing a substantial deviation from the previous values.…”

“…The branching to each pathway obtained herein is averaged over all the J levels, similar to that by Lee et al based on integration of all rotational states, 4 but different from most J-dependence results by using ion imaging method. 2,7,8,14 Note that the low-J component for CH 3 CHO photolyzed at 308 nm is free from interference by triple fragmentation (CH 3 + CO + H). 15 The HCO obtained even at 266 nm may not undergo secondary decomposition.…”

“…4 Prior studies on photodissociation of CH 3 CHO at 308 nm gave rise to very diverse results for roaming/TS branching fraction. 5,6,8 It arises from the assignment of CO rotational bimodality that was anticipated to result from CH 3 -roaming and TS contribution accounting for the total CH 4 + CO products. 6,8 Kable and coworkers recently assigned the low-J, slow speed component of CO fragment, to the H-roaming.…”

“…In addition, an alternative roaming pathway is initiated from the barrierless radical products (HCO + CH 3 ) on the ground state surface. [4][5][6][7][8] The incipient radicals meander with slow motion around varied configuration spaces in the weak attractive field of the moieties and ultimately undergo intramolecular abstraction to form the same molecular products.…”