Imaging the elusive C–C bond dissociation channel of photoexcited ethyl radical

Abstract

The C-C bond dissociation channel of the ethyl (CH3-CH2) radical is investigated at 197.4, 199.98 and 201 nm by velocity map imaging and resonance enhanced multiphoton ionization (REMPI) detection of the produced CH3 fragments. On the light of high level ab initio calculations performed in this work, up to four dissociation pathways are identified leading, respectively, to ground state CH3 radicals in correlation with CH2 fragments in the first four electronic states. The major pathway is associated to threshold dissociation over an exit barrier located at the point of a conical intersection between the initially populated 3p A state and a dissociative valence state. As discussed in the text, the concomitance of both features opens the narrow window in which C-C bond dissociation occurs for ethyl and, presumably, for other similar alkyl radicals.

An ab initio study of the photodissociation of the vinyl radical

“…The calculated ab initio PECs of the ground and excited electronic states, and the couplings between them for the different dissociation pathways helped rationalize the experimental results and the photodissociation mechanisms. 27,50–52 The PECs of the different electronic states reported here for pathways (1)–(3) of VR also intend to provide a guide to interpret currently available and future photodissociation experiments.…”

“…Alkyl radical molecules like vinyl typically present different fragmentation pathways upon photoexcitation depending on the specific bond of the molecule which is broken. The ethyl radical, C β H 3 –C α H 2 , can be considered the saturated counterpart of VR, and recently dissociation pathways similar to (1)–(3), namely H-atom elimination from the C β H 3 and C α H 2 groups 27,50,51 and C–C bond cleavage, 52 have been investigated both experimentally and theoretically. The calculated ab initio PECs of the ground and excited electronic states, and the couplings between them for the different dissociation pathways helped rationalize the experimental results and the photodissociation mechanisms.…”

Imaging the photodissociation dynamics of internally excited ethyl radicals from high Rydberg states

“…The non-adiabatic coupling matrix elements (NACMEs) between the different states (not shown here) were obtained as the derivative of the corresponding mixing angle as a function of the chosen coordinate, in this case, the C-H bond. They were published in previous works 4,8 allowing us to identify conical intersections and avoid crossings characterizing the reaction pathways.…”

“…The photodissociation dynamics of the methyl radicals were studied by our group and published in 2016 [1][2][3] as a first step of a wider project centered in the characterization of the photodissociation dynamics of small alkyl radicals using nanosecond pulsed laser excitation and velocity map or slice ion imaging detection. [4][5][6][7][8] The reported translational energy and angular distributions of the H( 2 S) photofragments produced in the photodissociation of the methyl radical from the 3s and 3p z Rydberg states highlighted different dissociation mechanisms. 2 A fast predissociation mechanism, characterized by the coupling between the 3s Rydberg state and a valence repulsive state, rationalizes the observed narrow translational energy distribution for photodissociation via the 3s Rydberg state.…”

“…The photodissociation dynamics of the methyl radicals were studied by our group and published in 2016 1–3 as a first step of a wider project centered in the characterization of the photodissociation dynamics of small alkyl radicals using nanosecond pulsed laser excitation and velocity map or slice ion imaging detection. 4–8…”