THE BENZYL RADICAL-ETHYLENE MOLECULAR CLUSTER - AN EXAMPLE OF ELECTRONIC-STATE MEDIATION OF CHEMICAL-REACTIVITY

Three types of experimental data are presented for the benzyl radical/ ethylene molecular cluster mass-resolved excitation spectra (MRES), io nization energy threshold determination, and excited-state lifetime me asurements at different excitation energies. MRES of benzyl radical (C 2H4)(1),(2) exhibit broad features in the D-1 <-- D-0 benzyl radical a bsorption region that extend beyond 11 810 cm(-1) of vibrational energ y in D-1. The ionization threshold for the one-to-one cluster is shift ed by -1 160 cm(-1) relative to that of the bare benzyl radical. Clust er excited-state lifetime measurements indicate a shortened lifetime a t higher excitation energy. This collection of benzyl radical/ethylene data differs greatly from that of the previously studied benzyl radic al/ethane cluster system. These latter results consist of well-resolve d spectroscopic structure with low-energy van der Waals modes and mole cular vibrations, a small shift in ionization energy relative to the b are benzyl radical of similar to -50 cm(-1), and vibrational predissoc iation at roughly 700 cm(-1) of vibrational energy in D-1. The anomalo us benzyl radical/ethylene data can be explained if one postulates tha t an excited-state bimolecular addition chemical reaction occurs betwe en benzyl radical and ethylene upon optical excitation of the benzyl r adical (D-2, D-1 <-- D-0) Ab initio calculational results are presente d which support the assertion of an apparent excited-state chemical re action. Finally, unimolecular dissociation rate theories are used to e xtract an excited-state ''binding energy'' for the benzyl radical/ethy lene ''cluster'' from excited-state lifetimes.