Resonance-enhanced multiphoton ionization and time-of-flight product imaging have been used to study the O 3 (X 1 A 1)ϩh→O(1 D 2)ϩO 2 (1 ⌬ g) channel of the UV photodissociation of ozone at 235, 245, 255, 265, 275, 285, 298, and 305 nm. At all wavelengths, the vibrational populations, the spatial anisotropy parameter ␤, and the O(1 D 2)͉m j ͉ populations were determined. The corresponding vibrational populations of O 2 (1 ⌬ g) were peaked at vϭ0. The spatial anisotropy parameter was determined for each vibrational level and changed monotonically from about 1.2 at 235 nm to 1.7 at 298 nm. At all wavelengths, ͉m j ͉ populations were peaked at ͉m j ͉ϭ0. A full density matrix method was used to determine the a q (2) (p) parameters at 255 and 298 nm, where most of the signal was found to be from parallel, incoherent excitation. The data support a dissociation mechanism in which excitation occurs to a state of AЈ symmetry and there is substantial bending of the ozone before dissociation.

Photolysis wavelength dependence of the translational anisotropy and the angular momentum polarization of O 2 ( a Δ g 1 ) formed from the UV photodissociation of O 3The energy distribution, angular distribution, and alignment of the O ( 1 D 2 ) fragment from the photodissociation of ozone between 235 and 305 nm Resonance-enhanced multiphoton ionization coupled with time-of-flight product imaging has been used to study the O 3 (X 1 A 1 )ϩh→O(2p 3 P J )ϩO 2 (X 3 ⌺ g Ϫ ) product channel in the UV ͑ultraviolet͒ photodissociation of ozone at photolysis wavelengths of 226, 230, 233, 234, 240, and 266 nm. These imaging experiments, together with a measurement of the branching ratio into the different spin orbit states of the O atom, allowed the determination of the yields of the O 2 product in vibrational states greater than or equal to 26 as a function of wavelength. It was found that at 226, 230, 233, 234, and 240 nm, the yield was 11.8Ϯ1.9%, 11.5Ϯ1.2%, 8.2Ϯ2.0%, 4.7Ϯ1.8%, and 0.6Ϯ0.1%, respectively.

Resonance-enhanced multiphoton ionization with time-of-flight product imaging has been used to study the SO 2 + hν f SO( 3 Σ -) + O( 3 P 2 ) channel in the ultraviolet photodissociation of sulfur dioxide at photolysis wavelengths between 202 and 207 nm. These imaging experiments allowed the determination of the vibrational populations of the SO( 3 Σ -) fragment at several wavelengths. A change in the vibrational populations occurs from 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 distributed for longer wavelength dissociation. The changes in the internal energy distribution 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.

Two different techniques, vacuum ultraviolet laser-induced fluorescence ͑VUV-LIF͒ spectroscopy and two-dimensional ͑2D͒ ion counting product imaging, have been used to determine the bond energy for the dissociation of jet-cooled O 3 into O(1 D)ϩO 2 (a 1 ⌬ g). The photofragment excitation ͑PHOFEX͒ spectrum for O(1 D) products is recorded by detecting the VUV-LIF signal associated with the 3s 1 D 0-2p 1 D transition at 115.22 nm while scanning the photolysis laser wavelength between 305 and 313 nm. A clear cutoff corresponding to the appearance threshold into O(1 D) ϩO 2 (a 1 ⌬ g) is observed in this PHOFEX spectrum. The 2D image of the O(1 D) products from the O 3 photolysis near 305 nm is measured using an ion-counting method, with the detection of O(1 D) atoms by ͓2ϩ1͔ resonance enhanced multiphoton ionization ͑REMPI͒ at 205.47 nm. The kinetic-energy distribution obtained from the 2D image shows rotational structure due to the O 2 (a 1 ⌬ g ,vЉϭ0) fragment. The bond energy into O(1 D)ϩO 2 (a 1 ⌬ g) has been obtained from the rotational assignments in the kinetic-energy distribution. The two different experimental approaches give consistent results and an accurate value of the bond dissociation energy into O(1 D) ϩO 2 (a 1 ⌬ g) is found to be 386.59Ϯ0.04 kJ/mol. The standard heat of formation of O 3 , ⌬ f H 0 (O 3)ϭϪ144.31Ϯ0.14 kJ/mol, has also been calculated from the bond energy obtained, in conjunction with thermochemical data for O 2 molecule and O atom. The uncertainty for the ⌬ f H 0 (O 3) value obtained in the present study is smaller than the previous value which has been used widely.

Gas-phase photodissociations of cyclopropyl iodide were conducted at 266 and 279.7 nm, and the radical products were probed by multiphoton ionization, with imaging of the resulting ions and their corresponding electrons. Solution-phase photodissociations of cyclopropyl iodide were also conducted with TEMPO-trapping of the radical dissociation products. In both gas and solution phases, allyl radical was found to be a direct product of the cyclopropyl iodide photodissociation. CASSCF calculations indicate that the allyl radical could be formed directly from photoexcited cyclopropyl iodide by way of two surface crossings between open- and closed-shell potential energy surfaces. Each surface crossing represents a point of potential bifurcation in the reaction dynamics. Thus, cyclopropyl iodide that is excited to a 1(n,σ*) state can remain on an open-shell surface and generate the cyclopropyl radical and an iodine atom or can cross to a closed-shell (ion-pair) surface. The cyclopropyl cation that results from the surface crossing can undergo barrierless ring opening to the allyl cation before crossing back to an open-shell surface to generate allyl radical and an iodine atom. In this manner, both cyclopropyl radical and allyl radical can be formed as direct products of cyclopropyl iodide photodissociation.