Use of threshold electron and fluorescence coincidence techniques to probe the decay dynamics of the valence states of CF+4, SiF+4, SiCl+4, and GeCl+4
Authors: Smith D. M. , Tuckett R. P. , Yoxall K. R. , Codling K. , Hatherly P. A. , Aarts J. F. M. , Stankiewicz M. 
Published: July 2002

Threshold photoelectron–photoion coincidence (TPEPICO), photoion–fluorescence coincidence (PIFCO), and threshold photoelectron–fluorescence coincidence (TPEFCO) spectroscopies have been used to measure, state selectively, the decay pathways of all the valence states of four gas-phase tetrahedral ion CF+4, SiF+4, SiCl+4, and GeCl+4 in the range 11–26 eV. Vacuum UV radiation from a synchrotron source dispersed by a 5 m normal-incidence McPherson monochromator ionizes the parent molecule, and electrons and ions are detected by threshold electron analysis and time-of-flight mass spectrometry, respectively. Undispersed fluorescence from the interaction region can also be detected, allowing the three different types of coincidence experiment to be performed. The optimum resolution of the monochromator is matched to that of the threshold analyzer, and this work improves on preliminary results using a 1 m Seya monochromator [Chem. Phys. 174, 441 and 453 (1993)] where the resolution of the spectra was limited by that of the optical source. TPEPICO spectra are recorded continuously as a function of photon energy, allowing both threshold photoelectron spectra and yields of all the fragment ions to be obtained. Kinetic energy releases can also be measured at fixed photon energies with good time resolution. PIFCO and TPEFCO spectra are recorded at fixed photon energies. The former experiment can yield the fate of the lower electronic state of the parent ion to which fluorescence occurs.

The latter experiment yields the lifetime of the fluorescing state; with sufficient resolution of the photoionizing radiation, the lifetime is specific to one vibrational level of the emitting electronic state. For CF+4 and SiF+4 work has concentrated on the third and fourth excited states, C̃ 2T2 and D̃ 2A1, of which only the C̃ state of SiF+4 does not decay radiatively. Vibrationally state-selected fluorescence quantum yields and lifetimes have been measured for four levels of the C̃ state of CF+4, and absolute values of radiative and nonradiative decay rates have been evaluated for these levels. Jahn–Teller distortion of the C̃ state of SiF+4 from Td to C3v geometry assists internal conversion of the C̃ 2T2 state into high vibrational levels of the B̃ 2E state, and is an efficient route for nonradiative decay. A non-Franck–Condon distribution of intensities is observed in the threshold photoelectron spectrum of the D̃ 2A1 state of CF+4, due to autoionization from a high-lying Rydberg state of neutral CF4. For the two chloride molecules, SiCl4 and GeCl4, fragmentation of the ground (X̃) and the first four excited states (Ã–D̃) of the parent ion have been studied at slightly lower resolution. For SiCl+4 an important result is confirmation of the stability of its electronic ground state with respect to dissociation to SiCl+3+Cl. By contrast, a substantial part of the Franck–Condon zone of the ground state of GeCl+4 is energetically unstable with respect to GeCl+3+Cl. Radiative decay from the C̃ 2T2 state of both ions is an important process. The decay dynamics of all the valence states of this family of tetrahedral ions are reviewed. Dynamical, rather than statistical, processes generally dominate, and reasons for these surprising phenomena are discussed.