Photofragment imaging applied to atmospheric photochemistry

Summary form only given. The author reports his progress in developing a photofragment imaging technique to record state-resolved angle-velocity distributions of photofragments from excitation of molecules and clusters. The technique is similar in spirit to the landmark method of photofragment imaging developed by Chandler and Houston (1987). These investigators record 2D projections of 3D angle-velocity distributions on an intensified CCD camera, which they invert by Abel transformation to obtain the angle-velocity resolved differential cross sections. The author´s technique is based on rastering a 3D ion packet across a detection aperture and measuring 1D TOF spectra at each position to define a 2D image. In so doing, a sectional or planar image is collected rather than a projection. This eliminates the need to carry out an inversion to obtain the angle-velocity resolved differential cross sections. Measurements were made of O(/sup 3/P/sub 1/) recoil from photodissociation of the molecule O/sub 3/ and the van der Waals complex O/sub 3/-H/sub 2/O. The spin allowed reaction channel for gas-phase O/sub 3/ photodissociation is O/sub 3//spl rarr/ O(/sup 3/P/sub 1/)+O/sub 2/(X/sup 3//spl Sigma//sub g//sup -/). A 2D image for 226 nm photolysis is presented. In these results the pump and probe excitation are from the same 5-ns laser pulse. O(/sup 3/P/sub 0,1,2/) fragments were ionized by 2+1 resonance-enhanced multiphoton ionization through a two-photon transition. An anisotropic bimodal velocity distribution is clearly evident. The two peaks in the velocity distribution correspond to vibrational excitation in O/sub 2/(X) peaking at v=15 and v=27. The latter result is important for stratospheric O/sub 3/ chemistry.