Absorption Cross Sections of Methane and Ammonia in the Vacuum Ultraviolet

Abstract

Absorption cross sections of methane and ammonia for electromagnetic radiation in the range from 1300 A to 374 were obtained with a two‐meter radius of curvature, grazing incidence vacuum spectrograph. For methane 85 wavelengths were measured. Starting from σ = 14 Mb (1 Mb = 10^—18 cm²) at 1300 A, the absorption first increased slowly and then rose more rapidly to a maximum of σ = 56 Mb at 960±278 A corresponding to the first ionization potential of methane. The absorption then decreased gradually to 14 Mv at 374 A. The total f‐value pertaining to the transitions of 2p‐electrons was found to be 6.1. For ammonia, 100 wavelengths were measured. Band absorption appeared at longer wavelengths. The continuous absorption began near 1210 A and extended to the shorter wavelengths. The structure of the absorption contour suggested the existence of two ionization continua: a weaker one with maximum σ = 21 Mb at 1130±20 A and a larger one with its maximum of 34 Mb at 730±20 A. These two maxima corresponded to the ionization limits of the outer electrons of ammonia. The total f‐value was found to be 5.9.