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

Abstract

The effect of a local environment on the photodissociation of molecular oxygen is investigated in the van der Waals complex X -O 2 ͑X=CH 3 I, C 3 H 6 , C 6 H 12 , and Xe͒. 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 O͑ 3 P J , J =2,1,0͒ atom photoproduct via ͑2+1͒ resonance enhanced multiphoton ionization. The kinetic energy distribution ͑KED͒ 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 O͑ 3 P 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 ͑AЈ 3 ⌬ 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 ͑CT͒ 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 =CH 3 I and C 3 H 6 , 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 eV͒ O atoms with a parallel image pattern. Products from the photodissociation of singlet oxygen O 2 ͑b 1 ⌺ g + ͒ are also observed when the CH 3 I-O 2 complex was irradiated. Potential energy surfaces ͑PES͒ for the ground and relevant excited states of the X -O 2 complex have been constructed for CH 3 I-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 O͑ 3 P J ͒ atom production channels.

Singlet oxygen photogeneration from X–O2 van der Waals complexes: double spin-flip vs. charge-transfer mechanism

“…In that work singlet oxygen O 2 ( 1 D g ) was generated in a pulsed discharge. The images of the photofragment O atoms were found to be similar to those observed earlier 5,6 in photodissociation of complexes X-O 2 . Revealing process (1) indicates the formation of singlet oxygen O 2 ( 1 D g ) after UV-excitation of complexes X-O 2 .…”

“…A similar image observed earlier for complexes X-O 2 with X = C 3 H 6 , CH 3 I in ref. 5 was also reinterpreted to be due to photodissociation of singlet oxygen via process (1). 6 This assignment is confirmed by the results recently published by Farooq et al 8 who studied photodissociation of singlet oxygen with velocity map imaging.…”

“…This radiation at 226.223 nm (hereafter l vac ) has been used simultaneously for excitation of the complex and for REMPI (2 + 1) of appearing O( 3 P 0 ) atoms. This state was used for detection because the results obtained previously 5,6 definitely indicate a higher yield of it as compared with 3 P 1 and 3 P 2 states of O atoms arising in photodissociation of singlet oxygen. The second setup used in Novosibirsk is described elsewhere.…”

“…Among the revealed channels differing by kinetic energy and angular anisotropy of O atom recoil directions there was a channel with kinetic energy of O atoms equal to about 0.7 eV observed for complexes with partner molecules X = C 3 H 6 , CH 3 I and numbered as C5. 5 Formerly this channel was attributed to the photodissociation of a superoxide anion O 2 À + hn -O( 3 P j ) + O À , which was supposed to appear via excitation of a complex into a charge-transfer (CT) state X + -O 2 À with further photodecomposition of CT by the same pulse X + -O 2 À + hn -X + + O 2 À . 5 Oxygen atoms with a similar image have been later observed to appear after photodissociation of the complex of isoprene with oxygen C 5 H 8 -O 2 .…”

“…5 Formerly this channel was attributed to the photodissociation of a superoxide anion O 2 À + hn -O( 3 P j ) + O À , which was supposed to appear via excitation of a complex into a charge-transfer (CT) state X + -O 2 À with further photodecomposition of CT by the same pulse X + -O 2 À + hn -X + + O 2 À . 5 Oxygen atoms with a similar image have been later observed to appear after photodissociation of the complex of isoprene with oxygen C 5 H 8 -O 2 . 6 Two-laser arrangement of experiment in this case allowed us to testify the earlier assumption about the formation of ionic intermediates of O atoms in this C5 channel.…”