Photodissociation and Photoionization of Oxygen (O_2) as Inferred from Measured Absorption Coefficients

Absorption coefficients, k, of O2 between 1320 A and 200 A were measured using a 2-meter grazing incidence vacuum spectrograph. Three weak dissociation continua above 1040 A have been investigated. The “first continuum” between 1320 A and 1270 A had a maximum k-value of 13 cm−1 at 1295 A; another one between 1220 A and 1096 A was found to have a k=0.23 cm−1, and the third one between 1096 A and 1040 A had a value of about 3 cm−1. The absorption near Lyman α and in various other “atmospheric windows” in adjacent regions is given. Between 1040 A and 740 A strong absorption was observed due to diffuse bands, which were superimposed over the first ionization continuum with a maximum of 100 cm−1 at 920 A. The main ionization continuum had a long wavelength limit at 683 A and a flat maximum of 590 cm−1 at 510 A. (In addition low absorption coefficients of less than 1 cm−1 have been found in N2 between 1040 A and 910 A.) As an application of these data, a survey calculation on ionospheric absorption was made which showed that the formation of the E layer at 100 km may be due to the absorption of solar radiation by the first ionization continuum of O2. It is also shown that solar radiation between 1220 A and 1100 A can reach the height of the D layer by passing through the indicated atmospheric windows.