C–F and C–H bond cleavage mechanisms of trifluoromethane ions in low-lying electronic states: threshold photoelectron–photoion coincidence imaging and theoretical investigations

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.