Absorption and Resonance Raman Study of the 2 B 1 (X)- 2 A 2 (A) Transition of Chlorine Dioxide in the Gas Phase Anthony P. Esposito, Todd Stedl, Hannes Jo ´ nsson, and Philip J. Reid* Department of Chemistry, UniVersity of Washington, P.O. Box 351700, Seattle, Washington 98195 Kirk A. Peterson Department of Chemistry, Washington State UniVersity, 2710 UniVersity DriVe, Pullman, Washington 99164-4630 ReceiVed: NoVember 9, 1998; In Final Form: January 27, 1999 The photochemical reaction dynamics of chlorine dioxide (OClO) are investigated using absorption and resonance Raman spectroscopy. The first Raman spectra of gaseous OClO obtained directly on resonance with the 2 B 1 - 2 A 2 electronic transition are reported. Significant scattering intensity is observed for all vibrational degrees of freedom (the symmetric stretch, bend, and asymmetric stretch), demonstrating that structural evolution occurs along all three normal coordinates following photoexcitation. The experimentally measured absorption and resonance Raman intensities are compared to the intensities predicted using both empirical and ab initio models for the optically active 2 A 2 surface. Comparison of the experimental and theoretical absorption spectra demonstrates that the frequencies and intensities of transitions involving the asymmetric stretch are well reproduced by the empirical model characterized by a double-minimum along the asymmetric stretch. However, the ab initio model is also found to reproduce a subset of the experimental intensities. In addition, the extremely large resonance Raman intensity of the asymmetric stretch overtone transition is predicted by both models. The results presented here taken in combination with the model for the 2 A 2 surface in condensed environments suggest that the phase-dependent photochemical reactivity of OClO is due to environment-dependent excited-state structural evolution along the asymmetric stretch coordinate. Introduction The photochemistry of chlorine dioxide (OClO) is of current interest in atmospheric chemistry because of the participation of this compound in the reactive chlorine reservoir as well as its potential role in stratospheric ozone layer depletion. 1-5 Photoexcitation of OClO results in either the formation of ClO ( 2 Π) and O ( 3 P g ), or Cl ( 2 P u ) and O 2 ( 1 Δ g , 3 Σ g ). 1,6-58 In addition, the Cl and O 2 products may be preceded by the formation of the peroxy isomer, ClOO, produced by the photoisomerization of OClO. 1,15,16,18,20,25,33,55 The intriguing aspect of this photo- chemistry is that the partitioning between product pathways is phase-dependent. For example, the quantum yield for Cl formation (Φ Cl ) is ∼0.04 in the gas phase, 10-14,37 but increases to near unity in low-temperature matrices and on sur- faces. 33,40,41,49-54,59 Solutions represent an intermediate case with Φ Cl ≈ 0.1 in water. 16,18,20-23,27,29,30 Elucidating the origin of this phase dependence is currently the central problem in OClO photochemistry. Recent work regarding OClO photochemistry has demon- strated that the structural evolution on the optically prepared 2 A 2 surface is intimately coupled to photoproduct forma- tion. 21,28,32,56-58 In particular, evolution along the asymmetric stretch coordinate is believed to be critical in defining the photoproduct quantum yields. Two models for the 2 A 2 potential energy surface along the asymmetric stretch coordinate in the gas phase have been proposed. Through analysis of the rotationally resolved electronic absorption spectrum of gaseous OClO, Richard and Vaida demonstrated that the unusually large intensity observed for transitions involving the asymmetric stretch can be reproduced by including an energy barrier along this coordinate (i.e., a double-minimum surface as depicted in Figure 1A). 7 In contrast, ab initio studies performed by Peterson and Werner suggested that the potential energy surface along the asymmetric stretch coordinate is roughly harmonic with appreciable anharmonic coupling to the symmetric stretch (Figure 1B). 56-58 We recently completed the resonance Raman intensity analysis of OClO in water and cyclohexane where the curvature of the 2 A 2 surface in these solvents was deter- mined. 28,31,32 We found that the 2 A 2 surface along the asym- metric stretch is significantly different in these solvents relative to either model of the gas-phase potential energy surface (Figure 1C). Specifically, the limited resonance Raman intensity of the asymmetric stretch overtone transition was found to be consis- tent with a harmonic potential along this coordinate with ω e g 750 cm -1 . In solution, limited structural evolution occurs along the asymmetric stretch; however, significant structural evolution along this coordinate is predicted by both gas-phase models. Given this difference in excited-state dynamics, we suggested that the increase in quantum yield for Cl formation in condensed environments originates from the preservation of C 2V symmetry on the optically prepared excited state. 28,32,57 The photochemical model outlined above is consistent with all of the experimental and theoretical results presented to date; however, it relies on comparison of 2 A 2 surface descriptions derived from various techniques (i.e., absorption, resonance * To whom correspondence should be addressed. E-mail: preid@ chem.washington.edu. Phone: (206) 528-1127. 1748 J. Phys. Chem. A 1999, 103, 1748-1757 10.1021/jp984368i CCC: $18.00 © 1999 American Chemical Society Published on Web 03/09/1999