Absorption cross section measurements of oxygen in the wavelength region 195–241 nm of the Herzberg continuum

Abstract

The continuum cross section of oxygen at 296–300 K has been measured with a resolution of 0.13 nm throughout the wavelength region 205–241 nm with oxygen pressures from 5 to 760 torr and optical path lengths from 13.3 and 133m. The three processes contributing to the observed cross section are absorption into two kinds of continua, viz. the Herzberg continua of O 2 and a pressure-dependent continuum involving two molecules of O 2, and Rayleigh scattering. Extrapolation of the observed cross section to zero pressure yields the continuum cross section of O 2, from which the calculated Rayleigh scattering is subtracted to give the Herzberg continuum absorption cross section of O 2. Our previous continuum cross sections [Cheung et al. (1984) Can. J. Phys. 62, 1752], obtained from studies at high resolution (0.0013 nm) between the Schumann-Runge absorption lines in the region 194–204 nm, are here adjusted for Schumann-Runge line-wing contributions. These adjusted cross sections are compared with those calculated from computed Franck-Condon densities and a transition moment extrapolated from that calculated from our more accurate cross section measurements at longer wavelengths. Our calculated cross section in the region 195–237 nm is similar in shape to that calculated by Saxon and Slanger [ J. geophys. Res., 91, 9877] from the transition moments computed ab initio by Klotz and Peyerimhoff [ Molec. Phys., 57, 573]. Our values of the Herzberg continuum cross section of oxygen, tabulated at 1 nm intervals in the region 195–241 nm, increase from 6.3 × 10 −24 cm 2 at 195 nm to a maximum of 6.6 × 10 −24 cm 2 at 201 nm and then decrease to 0.85 × 10 24 cm 2 at 241 nm. Our results agree with those in the region 205–225 nm covered by the most recent previous laboratory study [Johnston et al. (1984) J. geophys. Res. 89, 11661] and are consistent with values in the region 200–220 nm spanned collectively by three in situ stratospheric studies [Frederick and Mentall (1982) Geophys. Res. Lett. 9, 461; Herman and Mentall (1982) J. geophys. Res. 87, 8967; Anderson and Hall (1983) J. geophys. Res. 88, 6801]. The larger Herzberg continuum cross sections found in another in situ stratospheric study [Pirre et al. (1984) Geophys. Res. Lett. 11, 1199] are in definite disagreement with our laboratory values, certainly in the region 205–214 nm. Our Herzberg continuum cross sections in the region 195–241 nm are significantly lower than those previously used in many photochemical stratospheric modelling calculations. Acceptance of our cross sections in such models will affect markedly the calculated altitude profiles of ozone, nitrous oxide, chlorofluorocarbons, and other trace stratospheric species.