NEGATIVE-ION MASS-SPECTROMETRIC STUDY OF ION-PAIR FORMATION IN THE VACUUM ULTRAVIOLET .5. CF4-]F-+CF3+

Ion-pair formation from photoexcitation of CF4 has been studied by negative-ion mass spectrometry using synchrotron radiation in the 11-31 eV photon energy range. Negative F-ions have been observed. The appearance energy in the F- photodissociation efficiency curve is about 2 eV higher than the thermochemical threshold for the formation of the ground-state ions F- (1S(g) and CF3+ (X approximately 1A1'). The peak features observed in the spectrum are interpreted as resulting from transitions to Rydberg states with the 1T2 symmetry. Assignments of the peaks have been made on the basis of the previous work on photoabsorption cross-section curves of CF4. Repulsive Rydberg states converging to CF4+ (X approximately 2T1, A approximately 2T2) give the strong and broad peaks in the F- efficiency curve. Conversion to the ion-pair state is considered to occur through the avoided potential surface crossings along the dissociation coordinate of the CF3-F bond. In contrast, the npt2 Rydberg states (n greater-than-or-equal-to 4) converging to CF4+ (C approximately 2T2) show long vibrational progressions. The radiative and nonradiative decay pathways for these Rydberg states are discussed.