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.

Critical review of ultraviolet photoabsorption cross sections for molecules of astrophysical and aeronomic interest

“…The absorption cross section of methane between 1000 A and 1600 A has been measured by Wilkinson and Johnston [1950], Ditchburn [1955], Sun and Weissler [1955], Laufer and McNesby [1965], and Watanabe et al [1953c]. The latter results are contained in a technical report from the Air Force Cam- Watanabe et al is extremely good in the region of overlap of these two measurements, agreeing within ±3%.…”

“…and 1000 A; solid line, Metzger and Cook [1964a]; dashed line, Watanabe and Sood [1965]; +, Sun and Weissler [1955] ; X, Walker and Weissler [1955a], results were obtained with the helium continuum light source at instrumental bandwidths of 0.5 and 0.3 A, respectively. The results shown in Figure 46 are most disturbing, as the groups differ both in magnitude and shape as to the resultant absorption cross section curve.…”

“…In the interval between 600 A and 1000 A, photoabsorption cross sections have been measured by Metzger and Cook [1964a[ ], Rustgi [1964, Schoen [1962], and Wainjan et al. [1955], in addition to the measurements already discussed by Ditchburn [1955] and Sun and Weissler [1955]. All of these additional measurements used photoelectric techniques.…”

“…Since the absorption cross section would appear to be continuous in this spectral region, the bandwidths that were used should not be important, and this should be one region where the cross sections can be compared most favorably. Metzger and Cook [1964a], Rustgi [1964], and Sun and Weissler [1955] do not give an estimate of the accuracy of their photoabsorption cross sections. Wainjan et al [1955] give an error for each emission line that they used; the average error would appear to be of the order of ±10%.…”

Photoionization of atoms and molecules.

“…Sun and Welssler [68] and DItchburn [69] have measured the continuous absorption to 400A and 300A respectively. Wainfan,Walker and Welssler [38] have measured the efficiency of photoionization and the photoionlzatlon cross section to 5OOA.…”