Vibrationally Mediated Photodissociation of C₂H₄⁺

Abstract

We report the vibrationally mediated photodissociation dynamics of C2H4+ excited through the B2Ag state. Vibrational state-selected ions were prepared by two-photon resonant, three-photon ionization of ethylene via (pi, 3s) and (pi, 3p) Rydberg intermediate states in the wavelength range 298-349 nm. Absorption of a fourth photon led to dissociation of the cation, and images of the product ions C2H3+ and C2H2+ were simultaneously recorded using reflectron multimass velocity map imaging. Analysis of the multimass images yielded, with high precision, both the total translational energy distributions for the two dissociation channels and the branching between them as a function of excitation energy. The dissociation of ions that were initially prepared with torsional excitation exceeding the barrier to planarity in the cation ground state consistently gave enhanced branching to the H elimination channel. The results are discussed in terms of the influence of the initial state preparation on the competition between the internal conversion to the ground state and to the first excited state.

Photodissociation and infrared spectra of ethylene cations in solid argon

“…The trend of the branching ratio for the production of C 2 H 3 in the matrix was typically similar to the reported ratio for H elimination from the dissociation of gaseous C 2 H 4 + in the wavelength region of 345-285 nm [17]. The discrepancy in the branching ratios between the matrix and the gas phase might be due to the further dissociation of products in the solid matrix.…”

“…For dissociation from the B 2 A g excited state, they also suggested that the H-elimination channel can adiabatically proceed through isomerization into the CH 3 CH + structure in its electronic ground state. In contrast, the H 2 -elimination channel requires electronic coupling to the A 2 B 3g state through excitation of the torsional vibration ( 4 ) around the conical intersection along the C-H stretching coordinate ( 1 ), which correlates asymptotically to the lowest dissociation pathway (1) [17,18]. In combination, previous experimental and theoretical results suggest that no barriers exist for the elimination of H atoms or H 2 molecules from the ethylene cation.…”

“…An irradiation wavelength less than 459 nm is capable of dissociating C 2 H 4 + to form C 2 H 2 + or C 2 H 3 + . Suits and coworkers used a versatile reflectron velocity map imaging apparatus with a multimass configuration to simultaneously record images of photofragments, C 2 H 3 + and C 2 H 2 + ions, and the velocity distribution of photofragments through the excitation of C 2 H 4 + to its B 2 A g state [17]. They reported the branching ratios for H elimination as a function of excitation energy of C 2 H 4 + and found that the ratio increases from 0.52 to 0.68 with an increase in the excitation energy from 3.55 to 4.3 eV [17].…”

“…Photodissociation of gaseous C 2 H 4 + is a prototype system for the investigation on both electronic relaxation mechanisms and the influence of conical intersections on dissociation dynamics [17][18][19][20][21][22]. It is known that electronically excited C 2 H 4 + undergoes decomposition via the elimination of H and H 2 .…”

Velocity map imaging studies of the photodissociation of H2O+cations

“…Detailed investigations of the photodissociation dynamics of gas phase molecular cations are becoming increasingly commonplace [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15]. Typically, these studies employ resonance enhanced multiphoton ionisation (REMPI) to prepare the cations with a high degree of internal (vibrational and rotational) quantum state selectively.…”