Photodissociation of OClO and Ar/OClO and H_2O/OClO clusters studied by the resonance enhanced multiphoton ionization-time of flight method

摘要

The photodissociation of jet-cooled OClO following excitation into the A-tilde ~2A_2 state at around 350 nm was investigated in homogeneous OClO and large heterogeneous Ar/OClO and H_2O/OClO clusters (estimated cluster size n-bar approx 800-2600) by probing the O (~3P) and ClO (X-tilde ~2#PI#) photofragments using the resonance enhanced multiphoton ionization-time of flight technique. Action spectra, photofragment excitation spectra and photofragment speed distributions were recorded and compared to those for monomer dissociation. OClO was found to occupy both surface and interior sites in the heterogeneous clusters with the percentage of surface and interior dissociation processes being approx 50% for large cluster sizes. Both O and ClO photofragments generated in the cluster interior are translationally thermalized with T approx 300 K and the ClO fragments are strongly rotationally and vibrationally relaxed. This is most important for vibration as monomer dissociation yields ClO containing up to 8 vibrational quanta at this photolysis wavelength. Photodissociation on the cluster surface is found to proceed with little interaction with the cluster host. The distribution of counterfragment masses leads to a broadening of the speed distributions compared with monomer dissociation. In addition, cluster chemistry was found to occur in OClO-rich heterogeneous clusters as manifested by detection of O photofragments with velocities exceeding the highest thermodynamically possible value. This result, consistent with that from homogeneous OClO cluster dissociation, indicates the presence of small OClO aggregates on the surface and within heterogeneous clusters. From a standpoint of atmospheric chemistry, H_2O/OClO clusters yield a substantial fraction of thermalized primary photofragments, in contrast to OClO monomer dissociation.