C–F and C–H bond cleavage mechanisms of trifluoromethane ions in low-lying electronic states: threshold photoelectron–photoion coincidence imaging and theoretical investigations
Yan Chen, Tongpo Yu, Xiangkun Wu, Xiaoguo Zhou, Shilin Liu, Fuyi Liu, Xinhua Dai
Abstract
Dissociative ionization of trifluoromethane (CHF3) is investigated in the 13.9-18.0 eV energy range using the threshold photoelectron-photoion coincidence (TPEPICO) technique coupled to a vacuum ultraviolet synchrotron radiation source. Four electronic states of CHF3+, i.e., the X2A1, A2A2, B2E, and C2E states, are populated upon ionization. In this energy range, the parent CHF3+ ions fully dissociate. For the CHF3+ ions in the ground state, the analysis of the time-of-flight profile of the unique CF3+ fragment ions suggests statistical dissociation. For the electronically excited CHF3+ ions, the C-F bond cleavage preferentially occurs to predominantly produce CHF2+ + F. Moreover, all TPEPICO images of the CHF2+ ions exhibit identical patterns, with a weak central spot revealing a previously unobserved statistical decomposition pathway, and the predominant ring in the images documents a fast nonstatistical dissociation channel. The unimolecular decomposition mechanisms of the CHF3+ ions are illuminated with the aid of the one-dimensional potential energy curves along the C-H and C-F coordinates calculated using the time-dependent density-functional theory. Moreover, a comparison of the dissociation dynamics of CHF3+ in these low-lying states with those of CF3Cl+ strongly suggests a substituent effect of chlorine atoms on the binding structure.