Autoionization processes in sym-trifluorobenzene and hexafluorobenzene: Studies involving threshold photoelectrons and ion fluorescence

Autoionization in large polyatomic molecules has been studied using synchrotron radiation as a variable energy VUV source. Threshold photoelectron spectra (TPES) of sym-trifluorobenzene and hexafluorobenzene were measured over the 9–29 eV range. Fluorescence quantum yields φF and lifetimes τ were determined for energy selected levels of the cations using the T-PEFCO technique in which coincidences are counted between threshold photoelectrons and ion fluorescence photons. The relative partial cross sections σX̃(E) and σB̃(E) for threshold ionization to the nonfluorescing X̃ and fluorescing B̃ states of 1,3,5-C6F3H+3 and C6F+6 ions are determined. Peaks corresponding to bands of a Rydberg series converging to the C̃ state of C6F+6 are observed in the TPES of C6F6 and result from threshold autoionization processes forming high vibrational levels of the ion B̃ state. An intense underlying continuum in the TPES of 1,3,5-C6F3H3 and C6F6 is interpreted as being due to autoionization from superexcited neutral states to the X̃ ground states of the corresponding cations. The autoionization processes are also discussed in terms of competition between autoionization and ionic core relaxation of superexcited states.