The UV/Vis absorption spectrum of matrix-isolated dichlorine peroxide, ClOOCl

Abstract

UV/Vis absorption spectra of ClOOCl isolated in neon matrices were measured in the wavelength range 220–400 nm. The purity of the trapped samples was checked by infrared and UV/Vis matrix spectroscopy as well as low-temperature Raman spectroscopy. At wavelengths below 290 nm, the results agree with the UV spectrum recently published by Pope et al. [J. Phys. Chem. A, 2007, 111, 4322–4332]. However, the observed absorption in the long wavelength tail of the spectrum—relevant for polar stratospheric ozone loss—is substantially higher than reported by Pope et al. Our results suggest the existence of a ClOOCl electronic state manifold leading to an absorption band similar to those of the near UV spectrum of Cl₂. The differences to previous studies can be accounted for quantitatively by contributions to the reported absorption spectra caused by impurities. The observed band in the long wavelength tail is supported by several high-level ab initio calculations. However, questions arise concerning absolute values of the ClOOCl cross sections, an issue that needs to be revisited in future studies. With calculated photolysis rates based on our spectrum scaled to previous cross sections at the peak absorption, the known polar catalytic ozone-destruction cycles to a large extent account for the observed ozone depletion in the spring polar stratosphere.