Product imaging studies of photodissociation and bimolecular reaction dynamics

This paper briefly reviews recent progress in product imaging studies of ph otodissociation and bimolecular reaction dynamics. The SO2 + hv --> SO((3)Sigma (-)) + O(P-3(2)) channel in the ultraviolet ph oto dissociation of sulfur dioxide at photolysis wavelengths between 202 an d 207 mn has been studied using resonance-enhanced multiphoton ionization w ith time-of-flight product imaging. These imaging experiments allowed the d etermination of the vibrational populations of the SO((3)Sigma (-)) fragmen t at several wavelengths. A change in the vibrational populations occurs fr om a distribution where most of the population is in v = 0 for wavelengths shorter than 203.0 nm to one where the population is more evenly distribute d for longer wavelength dissociation. The changes in the internal energy di stribution are attributed to participation of two different predissociation mechanisms. Our data suggest that the predissociation mechanism below 203. 0 nm involves an avoided crossing with the repulsive singlet state (1)A(1). The O-3(X (1)A(1)) + hv --> O(2p, P-3(J)) + O-2(X (3)Sigma (-)(g)) product channel in the UV photodissociation of ozone has been investigated at photo lysis wavelengths of 226, 230, 233, 234, 240, and 266 nm. At 226, 230, 233, 234, and 240 nm, the yield of the O-2 product in vibrational states greate r than or equal to 26 was 11.8 +/- 1.9%, 11.5 +/- 1.2%, 8.2 +/- 2.0, 4.7 +/ - 1.8, and 0.6 +/- 0.1%, respectively. Two-dimensional ion counting product imaging has also been used to determin e the bond energy for the dissociation of jet-cooled O-3 into O(D-1) + O-2( (1)Delta). The bond dissociation energy into O(D-1) + O-2((1)Delta) was fou nd to be 386.59 +/- 0.04 kJ/mol. The standard heat of formation of O-3 is c alculated to be -144.31 +/- 0.14 kJ/mol. State-selective differential cross sections for rotationally inelastic scat tering of NO (J(i) = 0.5, 1.5, F --> J(f) = 2.5-12.5, F-1 and J(f) = 1.5-9. 5, F-2) from He and D-2 measured by crossed molecular beam ion imaging are reported. The images typically exhibit a single broad rotational rainbow ma ximum that shifts from the forward to the backward scattering direction wit h increasing DeltaJ. The angle of the rainbow maximum was lower at a given DeltaJ for D-2 than for He as a collision partner. At a collision energy of similar to 500 cm(-1), primarily the repulsive part of the potential surfa ce is probed, which can be modeled with a 2D hard ellipse potential. This m odel for rotationally inelastic scattering is shown to qualitatively match the experimental differential cross sections. A more advanced CEPA PES for NO + He does not give substantially better agreement with the experiment.