PHOTODISSOCIATION OF IODOBENZENE MOLECULAR ION - INVESTIGATION OF ENTROPY BOTTLENECK IN IONIC SYSTEMS

Photodissociation kinetics of iodobenzene molecular ion has been investigated by mass-analyzed ion kinetic energy spectrometry (MIKES). A method has been devised to correct for the influence of collisional relaxation of the molecular ion occurring in the ion source on the rate constant. Theoretical analysis of the present rate-energy data on a nanosecond time scale together with previous microsecond data has found no firm evidence to support the transition state switching from an orbiting transition state to a tight transition state, especially at internal energy well above the threshold. At internal energy near the threshold, the transition-state switching has been found possible when the best literature value of 2.38 eV was used as the energy barrier. If the energy barrier is adjusted to a slightly higher value (2.46 eV), however, such a possibility can be ruled out. More accurate determination of the energy barrier and the measurement of the rate constant on a longer time scale are needed to clear the situation near the threshold.