General expressions are derived for calculating Franck—Condon factors for most transitions of polyatomic molecules (excluding transitions between linear and bent configurations) in the harmonic oscillator approximation. The derivation employs the method of generating functions and also linear transformation of normal mode coordinates between initial and final states. Input data required in the general case are geometries, frequencies, and vibrational force fields for the initial and final states. Explicit algebraic expressions are presented for the transition between linear, symmetric, triatomic initial and final states. The results are used to show that fragmentation cannot result from direct vertical ionization to the ground electronic states of CS2+ and CO2+ or to the excited 2IIu state of CO2+. An attempt was made to fit the experimental photoionization data for C2H2 and C2D2. This resulted in an estimated increase of 0.05 Å in the equilibrium C–C bond distance and no change in the C–H bond distance in C2H2+ as compared to C2H2.

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The technique of variable time photoelectron–photoion coincidence mass spectrometry has been applied to the fragmentation of bromobenzene ion producing a phenyl ion. A detailed analysis of the variation of the breakdown curve with parent ion residence time was performed. The results lead to ΔH °f0 (phenyl ion)=270 kcal/mole in close agreement with recalculated results from an earlier study on chlorobenzene. This, combined with other photoionization results leads to ΔH °f0 (phenyl radical)=83±3 kcal/mole, slightly higher than the value 80.9±2 kcal/mole obtained from neutral kinetics. The analysis leads to a rate-energy dependence for the fragmentation process and an equivalent 1000 K Arrhenius pre-exponential factor of ∼9.4×1014 sec−1, which may be compared to the value 2×1015 sec−1 for the analogous neutral process. The possible contribution of spin orbit splitting is discussed.