The use of threshold photoelectron - fluorescence photon coincidence spectroscopy for the measurement of the radiative lifetimes of emitting states of CF3X+ (X = F, H, Cl, Br) ions

Two different experimental techniques are used to measure the lifetime of excited electronic states of CF3X+ (X = F, H, Cl, Br) in the gas phase with excitation energies in the range 20–23 eV. Using the single-bunch mode of a synchroton source, the first technique utilises pulsed photon excitation to excite the parent neutral molecule, and the decay of the fluorescence is observed in real time. This technique detects time-resolved fluorescence from all emitters, both neutrals and ions, excited at the proton energy, and is therefore a poor method to measure the lifetime of a parent ion if the ionisation quantum yield at the incident energy is less than unity. Also using tunable vacuum-UV radiation from a synchrotron source but now in the multi-bunch, quasi-continuous mode, the second technique detects coincidences between threshold photoelectrons and fluorescent photons. Because this experiment only detects coincidences in ions and not in neutrals, the decay of the fluorescence in real time is only due to the ionic component of the emission. In this way lifetimes of emitting states of molecular ions, absent of contributions from neutrals, can be measured. With sufficient resolution in both photon source and threshold electron analyser, vibrationally state-selected lifetimes can be measured. We use this coincidence technique to measure the lifetimes of the C̃2T2 state of CF4+ (v = 6), the D̃ 2A1 state of CF3H+ and the Ẽ2A1 state of CF3Cl+ to be 8.4, 12.6 and 9.4 ns, respectively. The fluorescence quantum yield of the Ẽ2A1 state of CF3Br+ is too small for signal to be observed in the coincidence experiment. These results are compared with those obtained from pulsed photoexcitation of CF3X, and some conclusions are drawn about the nature of the emitters when CF3X is excited at the appropriate energy. In the cases of CF4 and CF3H emission is solely in the parent ion. With CF3Cl parent ion emission is a minor component and the main emitters are excited states of the CF radical. With CF3Br emissions are only due to neutrals with, again, excited states of CF being the main contributor.