MOLECULAR ROTATION CLOCKING OF THE SUBPICOSECOND ENERGY REDISTRIBUTION IN MOLECULES FALLING APART .2. EXCESS ENERGY-DEPENDENCE OF THE RATESOF ENERGY REDISTRIBUTION IN THE 2 PHOTODISSOCIATION CHANNELS OF IODOBENZENE

For some dissociating molecules, the photodissociation rate becomes co mparable to the energy redistribution rate and molecular rotation freq uency. In this situation, a correlation could be observed between the recoil angular and speed distributions of the fragments. In a previous communication, Hwang and El-Sayed1 were able to use this correlation to clock the energy redistribution process and to determine its rate f or predissociating iodobenzene molecules excited to their pi,pi excit ed surface at 304 nm. In this work we use this technique to examine th e excess energy dependence of this rate. In addition, we use the width of the observed recoil velocity distribution for the rapidly dissocia ting molecules excited to their repulsive, n,sigma surface to examine the excess energy dependence of their rate of energy redistribution. We find that while the rate of energy redistribution increases with ex cess available energy (i.e., with decreasing the wavelength of the pho todissociation laser) for rapidly photodissociating molecules, no defi nite correlation is observed for the predissociating ones. These obser vations are explained by the fact that for rapidly dissociating molecu les the doorway optical state and the dissociating state are the same, which is not the case for the predissociating molecules.