Herzberg continuum cross section of oxygen in the wavelength region 193.5–204.0 nm and band oscillator strengths of the (0, 0) and (1, 0) Schumann–Runge bands

Abstract

Cross sections of oxygen at 300 K have been obtained from photoabsorption measurements at various pressures, from 50 to 760 Torr (1 Torr = 133.3 Pa), at wavelength intervals of 0.0002 nm throughout the region 193.5–204.0 nm with a 6.65-m photoelectric scanning spectrometer equipped with a 2400 line/mm grating and having an instrumental full width at half maximum of 0.0013 nm. The spectral features contributing to absorption in this region are the discrete lines of some Schumann–Runge bands and underlying dissociation continua consisting of the weak Herzberg continuum of O₂ and a pressure dependent continuum involving two molecules of O₂. Analysis of the pressure dependence of the total continuum cross section in window regions between the discrete lines yields the Herzberg continuum cross section, which decreases monotonically from 1.4 × 10⁻²¹ cm² at 193.58 nm to 5.1 × 10⁻²⁴ cm² 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 the values used in many photochemical stratospheric models, and consistent with recent in situ stratospheric cross-section measurements. The acceptance of reduced Herzberg continuum cross sections significantly affects photochemical modelling predictions. Band oscillator strengths f(0, 0) = 2.95 × 10⁻¹⁰ and f(1, 0) = 3.13 × 10⁻⁹ of the (0, 0) and (1, 0) Schumann–Runge bands have been determined by direct numerical integration of the measured discrete cross section remaining after subtraction of the continua from the total cross section. Improved wavenumber measurements of the (0, 0) and (2, 1) Schumann–Runge bands are given.