Absorption and Resonance Raman Study of the ²B₁(X)−²A₂(A) Transition of Chlorine Dioxide in the Gas Phase

Abstract

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 2B1−2A2 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 2A2 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 2A2 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.

A 2 A 2 ←X 2 B 1 absorption and Raman spectra of the OClO molecule: A three-dimensional time-dependent wave packet study

“…In the following calculations the wavelengths 368.9 and 360.3 nm are used. 6 ϭ I Ϫ S ϩE 0 and S is the frequency of the scattered light. The parameter ⌫ is introduced to damp the wave function to avoid too long time propagation and is set to 15 cm Ϫ1 .…”

“…Fragmentation into ClO (X 2 ⌸)ϩO( 3 P) and Cl( 2 P)ϩO 2 ( 3 ⌺ g , 1 ⌬ g , 1 ⌺ g ), as well as the wavelength dependence of the branching ratio of the dissociation, have been studied with the molecular beam technique. [2][3][4][5][6] The allowed parallel transition from the ground X 2 B 1 state to the A 2 A 2 state gives rise to a strong absorption band in the visible with a maximum at ϳ350 nm. As expected from the elongated bond lengths and a smaller bond angle of the excited A 2 A 2 state, symmetric stretch and bend excitation on the upper surface were observed in the experimental absorption spectrum.…”

“…II the numerical method is described. The calculated absorption and Raman spectra for excitation wavelengths of 368.9 and 360.3 nm are presented and compared with experimental spectra 6,7 in Sec. III.…”

Understanding the Phase-Dependent Reactivity of Chlorine Dioxide Using Resonance Raman Spectroscopy

“…Figure 2 presents the resonance Raman spectrum of gaseous OClO obtained with 368.9-nm excitation. 27 The spectrum is dominated by transitions involving the symmetric stretch (ν 1 ) and bend (ν 2 ). Most interesting is the substantial intensity at 2190 cm -1 corresponding to the asymmetric-stretch overtone transition.…”

“…Through analysis of the gas-and solution-phase absorption and resonance Raman data, a detailed description of excited-state structural evolution that occurs following OClO photoexcitation has been developed. 27,[29][30][31][32] A schematic depiction of this evolution is presented in Figure 3. In both the gas phase and solution, significant excitedstate structural evolution occurs along the symmetricstretch and bend coordinates.…”