Herzberg continuum cross section of oxygen in the wavelength region 193.5‐204.0 nm: New laboratory measurements and stratospheric implications

The absorption cross section of oxygen at 300 K and at pressures in the range 50‐760 torr has been measured throughout the wavelength region 193.5‐204.0 nm with a 6.65 m scanning spectrometer having an instrumental full width at half maximum of 0.0013 nm. The absorption features in this region are the discrete lines of some Schumann‐Runge bands and two underlying dissociation continua, viz., the weak Herzberg continuum of oxygen and a pressure dependent continuum involving two molecules of oxygen. The instrumental width is small enough to permit measurement of the total continuum cross section, as a function of pressure, at a number of wavelengths at which interference from the discrete lines is negligible. Analysis of the pressure dependence of the total continuum cross section yields the Herzberg continuum cross section, which decreases monotonically from 14 barn at 193.58 nm to 5.5 barn at 204.06 nm. At all wavelengths in this region, our values of the Herzberg continuum cross section are smaller than previous laboratory values, much smaller than values used in many photochemical models, and consistent with recent in situ stratospheric values. The acceptance of reduced Herzberg continuum cross sections implies significant changes in photochemical modelling predictions of the stratospheric concentrations and altitude profiles of ozone, chlorofluorocarbons and nitrous oxide.