Cluster-enhanced X-O2 photochemistry (X=CH3I, C3H6, C6H12, and Xe)

The effect of a local environment on the photodissociation of molecular oxygen is investigated in the van der Waals complex X-O-2 (X=CH3I, C3H6, C6H12, and Xe). A single laser operating at wavelengths around 226 nm is used for both photodissociation of the van der Waals complex and simultaneous detection of the O(P-3(J),J=2,1,0) atom photoproduct via (2+1) resonance enhanced multiphoton ionization. The kinetic energy distribution (KED) and angular anisotropy of the product O atom recoil in this dissociation are measured using the velocity map imaging technique configured for either full (crush) or partial (slice) detection of the three-dimensional O(P-3(J)) atom product Newton sphere. The measured KED and angular anisotropy reveal a distinct difference in the mechanism of O atom generation from an X-O-2 complex compared to a free O-2 molecule. The authors identify two one-photon excitation pathways, the relative importance of which depends on IPx, the ionization potential of the X partner. One pathway, observed for all complexes independent of IPx, involves a direct transition to the perturbed covalent state X-O-2(A('3)Delta(u)) with excitation localized on the O-2 subunit. The predominantly perpendicular character of this channel relative to the laser polarization detection, together with data on the structure of the complex, allows us to confirm that X partner induced admixing of an X+-O-2(-) charge transfer (CT) state is the perturbing factor resulting in the well-known enhancement of photoabsorption within the Herzberg continuum of molecular oxygen. The second excitation pathway, observed for X-O-2 complexes with X=CH3I and C3H6, involves direct excitation into the (3)(X+-O-2(-)) CT state of the complex. The subsequent photodissociation of this CT state by the same laser pulse gives rise to the superoxide anion O-2(-), which then photodissociates, providing fast (0.69 eV) O atoms with a parallel image pattern. Products from the photodissociation of singlet oxygen O-2(b (1)Sigma(+)(g)) are also observed when the CH3I-O-2 complex was irradiated. Potential energy surfaces (PES) for the ground and relevant excited states of the X-O-2 complex have been constructed for CH3I-O-2 using the results of CASSCF calculations for the ground and CT states of the complex as well as literature data on PES of the subunits. These model potential energy surfaces allowed us to interpret all of the observed O(P-3(J)) atom production channels. (c) 2007 American Institute of Physics.