Dissociative photoionization of CF3Cl via the C2E and D2E states: competition of the C–F and C–Cl bond cleavages

Abstract

The dissociative photoionization of CF₃Cl was investigated using threshold photoelectron photoion coincidence (TPEPICO) imaging in the energy range of 12.30–18.50 eV. The coincident time-of-flight mass spectra and three-dimensional time-sliced images of the CF₂Cl⁺ fragment were recorded at a few specific photon energies. Two fragmentation pathways were observed that led to the breakage of the C–Cl and C–F bonds, while the branching ratio elicited an energy-dependent relationship. The CF₃⁺ fragment was dominant in the dissociation of the X²E, A²A′ and B²A′′ states, while CF₂Cl⁺ became the predominant fragment and its branching ratio remained constant in the energy range associated with the C²E and D²E states. Based on the inflection point in the energy-dependent curve of the fragment branching ratios, the adiabatic ionization energy (IE_a) of C²E is suggested to be 15.46 eV. Although the excess energy increased considerably from C²E to D²E, the kinetic energy release distributions (KERDs) of CF₂Cl⁺ were similar. Moreover, the anisotropy parameters β for the F-loss channel were positive and larger than those for the Cl-loss channel. The calculated F-loss potential energy curves of CF₃Cl⁺ suggested that for the C²E and D²E states, the C–F bond rupture occurred via the internal conversion to the A²A′ state followed by the dissociation attributed to the crossing of the barrier along the C–F coordinate. Based on the experimental and theoretical conclusions, the internal conversion is the rate-determining step in the dissociative photoionization of CF₃Cl via the C²E and D²E ionic states, irrespective of whether the C–F and C–Cl bonds rupture.