Nonradiative decay pathways of electronic states of group IV tetrafluoro and tetrachloro molecular ions studied with synchrotron radiation

Abstract

The nonradiative decay channels of the valence electronic states of the gas-phase tetrahedral ions CF+4, SiF+4, CCl+4, SiCl+4, and GeCl+4 have been studied in the range 35–100 nm by a novel form of photoionization mass spectrometry. Tunable vacuum UV radiation from a synchrotron source ionizes the parent neutral molecule, and electrons and ions are detected by the photoelectron–photoion coincidence technique. The experiment is repeated continuously as a function of photon energy, and a three-dimensional histogram of photon energy versus ion time of flight versus coincidence count rate is produced. By taking cuts through this histogram, photoionization curves for the different fragment ions can be extracted. The appearance energies of the fragment ions (e.g., CF+2 from CF4, CCl+ from CCl4) occur at the adiabatic ionization potential of an electronic state of the parent ion, and not at the thermodynamic appearance energy of that ion. Attempts to measure the kinetic-energy releases in the fragmentation pathways have only been partially successful. The results are complementary to those of recent experiments to probe the radiative decay of these electronic states of MX+4 [M=C, Si, Ge; X=F, Cl] [J. Chem. Phys. 89, 2675 (1988); 89, 2683 (1988)], where the C̃ 2T2 and D̃ 2A1 third and fourth excited electronic states can show radiative decay to a surprising degree. The decay dynamics of the C̃ and D̃ states of MX+4 are reviewed. In general, the fluorides show different behavior to the analogous chlorides, and the carbon species behaves differently to the corresponding silicon or germanium species.