Cj. Kreher et al., Photodissociation of OClO and Ar/OClO and H2O/OClO clusters studied by theresonance enhanced multiphoton ionization time of flight method, J CHEM PHYS, 110(7), 1999, pp. 3309-3319
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].