THE USE OF THRESHOLD PHOTOELECTRON-FLUORESCENCE PHOTON COINCIDENCE SPECTROSCOPY FOR THE MEASUREMENT OF THE RADIATIVE LIFETIMES OF EMITTING STATES OF CF(3)X(+) (X=F, H, CL, BR) IONS

Two different experimental techniques are used to measure the lifetime of excited electronic states of CF(3)X(+) (X = F, H, Cl, Br) in the g as phase with excitation energies in the range 20-23 eV. Using the sin gle-bunch mode of a synchrotron source, the first technique utilises p ulsed photon excitation to excite the parent neutral molecule, and the decay of the fluorescence is observed in real time. This technique de tects time-resolved fluorescence from all emitters, both neutrals and ions, excited at the photon energy, and is therefore a poor method to measure the lifetime of a parent ion if the ionisation quantum yield a t 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 th reshold photoelectrons and fluorescent photons. Because this experimen t 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 th e emission. In this way lifetimes of emitting states of molecular ions , absent of contributions from neutrals, can be measured. With suffici ent resolution in both photon source and threshold electron analyser, vibrationally state-selected lifetimes can be measured. We use this co incidence technique to measure the lifetimes of the (C) over tilde(2)T (2) state of CF4+ (nu(1) = 6), the (D) over tilde(2)A(1) state of CF3H + and the (E) over tilde(2)A(1) state of CF3Cl+ to be 8.4, 12.6 and 9. 4 ns, respectively. The fluorescence quantum yield of the (E) over til de(2)A(1) state of CF3Br+ is too small for signal to be observed in th e coincidence experiment. These results are compared with those obtain ed from pulsed photoexcitation of CF(3)X, and some conclusions are dra wn about the nature of the emitters when CF(3)X is excited at the appr opriate 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 t he main emitters are excited states of the CF radical. With CF3Br emis sions are only due to neutrals with, again, excited states of CF being the main contributor.