Photodissociation of OClO and Ar/OClO and H2O/OClO clusters studied by theresonance enhanced multiphoton ionization time of flight method

The photodissociation of jet-cooled OClO following excitation into the (A) over tilde(2)A(2) state at around 350 nm was investigated in homogeneous OC lO and large heterogeneous Ar/OClO and H2O/OClO clusters (estimated cluster size (n) over bar similar to 800-2600) by probing the O (P-3) and ClO ((X) over tilde (2)Pi) photofragments using the resonance enhanced multiphoton ionization-time of flight technique. Action spectra, photofragment excitati on spectra and photofragment speed distributions were recorded and compared to those for monomer dissociation. OClO was found to occupy both surface a nd interior sites in the heterogeneous clusters with the percentage of surf ace and interior dissociation processes being similar to 50% for large clus ter sizes. Both O and ClO photofragments generated in the cluster interior are translationally thermalized with T similar to 300 K and the ClO fragmen ts are strongly rotationally and vibrationally relaxed. This is most import ant for vibration as monomer dissociation yields ClO containing up to 8 vib rational quanta at this photolysis wavelength. Photodissociation on the clu ster surface is found to proceed with little interaction with the cluster h ost. The distribution of counterfragment masses leads to a broadening of th e speed distributions compared with monomer dissociation. In addition, clus ter chemistry was found to occur in OClO-rich heterogeneous clusters as man ifested by detection of O photofragments with velocities exceeding the high est thermodynamically possible value. This result, consistent with that fro m homogeneous OClO cluster dissociation, indicates the presence of small OC lO aggregates on the surface and within heterogeneous clusters. From a stan dpoint of atmospheric chemistry, H2O/OClO clusters yield a substantial frac tion of thermalized primary photofragments, in contrast to OClO monomer dis sociation. (C) 1999 American Institute of Physics. [S0021-9606(99)01307-0].