Absorption Coefficients of Nitrogen in the 1000–580 Å Wavelength Region

Abstract

Absorption coefficients of molecular nitrogen have been measured with a photoelectric scanning technique using an intense, stabilized Hopfield helium‐continuum light source as background. These measurements differ from previous investigations where line sources were used as background. The measurements have been done with an instrumental bandwidth of 0.5 Å over a pressure range of 0.040 to 0.17 mm Hg using an absorption cell placed behind the exit slit of a vacuum monochromator. Corrections were made for the pressure gradient in the flow‐absorption cell, for the scattered light, and for fluorescence. From 1000 to 796 Å, the first ionization threshold (N₂⁺, X ²Σ_g⁺), a number of sharp, intense bands are present with no evidence of a dissociation continuum. Below 796 Å, the ionization continuum appears to rise slowly to 400 cm^—1 at 742 Å (N₂⁺, A ²II_u), but this region is also crowded with many intense bands. Most of these appear to be strongly preionized. Between 742 Å and the threshold of the B ²Σ_u⁺ state at 661 Å, the ionization continuum increases from roughly 400 to 780 cm^—1, and the pre‐ionized Hopfield Rydberg series is the main feature. At 661 Å, the absorption coefficient rises abruptly to 980 cm^—1 and continues at approximately this constant value to 580 Å. The present results are compared whenever possible with previous measurements and discussed in relation to photo‐ionization measurements. Application of this data to calculation of absorption processes in the upper atmosphere is discussed.