Fragmentation dynamics of the ethyl bromide and ethyl iodide cations: a velocity-map imaging study

Abstract

The photodissociation dynamics of ethyl bromide and ethyl iodide cations (C 2 H 5 Br + and C 2 H 5 I + ) have been studied. Ethyl halide cations were formed through vacuum ultraviolet (VUV) photoionization of the respective neutral parent molecules at 118.2 nm, and were photolysed at a number of ultraviolet (UV) photolysis wavelengths, including 355 nm and wavelengths in the range from 236 to 266 nm. Time-offlight mass spectra and velocity-map images have been acquired for all fragment ions and for ground (Br) and spin-orbit excited (Br*) bromine atom products, allowing multiple fragmentation pathways to be investigated. The experimental studies are complemented by spin-orbit resolved ab initio calculations of cuts through the potential energy surfaces (along the R C-Br/I stretch coordinate) for the ground and first few excited states of the respective cations. Analysis of the velocity-map images indicates that photoexcited C 2 H 5 Br + cations undergo prompt C-Br bond fission to form predominantly C 2 H 5 + + Br* products with a near-limiting 'parallel' recoil velocity distribution. The observed C 2 H 3 + + H 2 + Br product channel is thought to arise via unimolecular decay of highly internally excited C 2 H 5 + products formed following radiationless transfer from the initial excited state populated by photon absorption. Broadly similar behaviour is observed in the case of C 2 H 5 I + , along with an additional energetically accessible C-I bond fission channel to form C 2 H 5 + I + products. HX (X = Br, I) elimination from the highly internally excited C 2 H 5 X + cation is deemed the most probable route to forming the C 2 H 4 + fragment ions observed from both cations. Finally, both ethyl halide cations also show evidence of a minor C-C bond fission process to form CH 2 X + + CH 3 products.dissociation limit involves CH 3 + + Br* products, whereas for CH 3 I + , the larger spin-orbit splitting in atomic iodine relative

Photofragmentation dynamics of N,N-dimethylformamide following excitation at 193 nm

“…The velocity-map imaging experimental setup has been described in detail elsewhere. 11,12 The following is a brief description of the experimental conditions. A seeded beam (seeding ratio ∼0.2%) of ∼4 mbar N,N-dimethylformamide (Sigma-Aldrich, >99.8%) in 2 bars of He (BOC, >99.9%) undergoes a supersonic expansion through a pulsed solenoid valve (Parker Hannifin, Series 9) into vacuum to generate the molecular beam.…”

UV photofragmentation dynamics of acetaldehyde cations prepared by single-photon VUV ionization

“…Most studies have used resonance-enhanced multiphoton ionization (REMPI) to stateselectively prepare cations, with absorption of an additional photon of the same wavelength inducing fragmentation. Ashfold and co-workers have used this approach to examine the photochemistry of the cations of diatomic halogens and interhalogens, [10][11][12] alkyl halides, 13 and first-row hydrides. 14,15 Suits and co-workers have examined the photochemistry of several aldehyde, [16][17][18] amine, 19,20 and alkene cations; 21,22 their work on cation photofragmentation using imaging techniques has been summarized in a mini-review.…”

“…25,26 Examples demonstrating the involvement of excited states in the fragmentation of molecular cations are few. 13,15,16,19 Information about the electronic states and energy-dependent fragmentation of CH3CHO + comes largely from photoelectron spectroscopy and photoelectron/photoion coincidence (PEPICO) measurements. He I and threshold photoelectron spectra of acetaldehyde show an intense, sharp 0-0 band near 10.29 eV, [27][28][29][30][31] indicative of ionization from the non-bonding nO (10a′) orbital.…”

Evidence for concerted ring opening and C–Br bond breaking in UV-excited bromocyclopropane

“…4 Numerous studies have examined production of Br atoms from UV photodissociation of straight-chain and branched alkyl bromides. [5][6][7][8][9][10] These bromoalkanes absorb UV light at wavelengths below 250 nm; the observed broad UV absorption feature is referred to as the A-band, and corresponds to a σ*← Br (n) electronic excitation. The anti-bonding * character in the C-Br bond encourages rapid dissociation into a bromine atom and an alkyl radical.…”

“…The anti-bonding * character in the C-Br bond encourages rapid dissociation into a bromine atom and an alkyl radical. [5][6][7][8][9][10] The A band of methyl bromide, a molecule with C3v symmetry, can be decomposed into absorptions to three repulsive electronic states, 5 denoted by Mulliken 11 as 3 Q1, 1 Q1 and 3 Q0, and listed in order of increasing energy in the Franck-Condon region. In alkyl bromides with Cs symmetry, the 1 Q1 and 3 Q1 states each split into a pair of A and A states.…”