FEMTOSECOND DYNAMICS OF UNIMOLECULAR AND UNRESTRICTED BIMOLECULAR REACTIONS

photoinduced detachment of It from methylene iodide is studied. The pr ogress of the reaction is monitored by selective detection of fluoresc ence from the iodine product; molecular dynamics are probed by depleti on of the fluorescent state. Pump-probe spectroscopy of the halogen mo iety following high-energy (12 eV) excitation reveals that the reactio n proceeds by a concerted asynchronous mechanism. The second experimen t is a real-time study of an unrestricted bimolecular reaction. In thi s process, gas phase mercury atoms are photoassociated to an electroni cally excited state using a femtosecond pulse; the real-time dynamics of the resulting excimers are probed by fluorescence depletion using a second pulse. Analysis of the rotational anisotropy in the nascent di mers reveals the degree of rotational excitation in the excited state and indicates the impact parameter selectivity of the photoassociation process.