Photoionization and Absorption Cross Sections of O2 and N2 in the 600- to 1000-Å Region

Abstract

Absolute photoionization and absorption cross sections of O2 and N2 have been measured in the 600- to 1000-Å region using a 1-m Seya—Namioka scanning vacuum ultraviolet monochromator with a resolution of 0.5 Å in the first order and the Hopfield continuum background light source. A path length of 40.6 cm was used with simultaneous detection of ion current and photon flux. The absolute photon flux was determined by calibrating the ion chamber with the rare gases Ar, Kr, and Xe. In O2, the spectrum at longer wavelengths consisted of band absorption superimposed on a continuum, and, in the short wavelength region, by the continuum alone. Many of the O2 bands were broad, and absorption coefficients were independent of pressure. In N2, the spectrum showed many closely spaced narrow absorption bands down to about 750 Å, broad pre-ionized bands of the Hopfield Rydberg series to about 670 Å, and a smooth ionization continuum at shorter wavelengths. Pressure dependence of the coefficients in the region of the narrow bands was found to be a minimum if the measurements were made at absorption minima. Strong absorption continua were found underlying the band structure below 820 Å, and several weak continua were observed in the longer wavelength region. For each molecule, the photoionization cross section was similar to the absorption cross section but of different magnitudes, indicating an ionization efficiency of less than 100%. Strong gas fluorescence was observed in N2 when the wavelength of the incident radiation was decreased to 661 Å. The fluorescent radiation in O2 was weaker than in N2 and showed a gradual rise in intensity below 736.5 Å.