Slow time-resolved photodissociation of p-iodotoluene ion

Using time-resolved photodissociation in the ion cyclotron resonance mass spectrometer, the dissociation of p-iodotoluene ion was time resolved at five wavelengths between 532 and 630 nm. This is the slowest dissociation which has been time resolved for ions of well defined internal energies, giving inverse time constants for fragment ion appearance ranging from 2×102 to 1.1×103 s−1. At the longer wavelengths, dissociation is in clear competition with infrared-radiative relaxation of the photoexcited ion, which is calculated by kinetic modeling of the observed curves to give an energy loss rate of 6.2 eV s−1 (50 000 cm−1 s−1). The dissociation kinetics of this ion are in complete accord with a Rice–Ramsberger–Kassel–Marcus (RRKM) model with E0 of 1.9 eV and ΔS1000 K of −4 eu. It is concluded from the high activation energy that the rate limiting step in the dissociation kinetics is a molecular rearrangement preceding dissociation.