Line Intensities in the Hydrogen Chloride Fundamental Band

Abstract

Absorption intensities in the lines of the HCl fundamental band for tube-lengths from 0.0998 to 2.97 cm.—The transmission of columns of HCl 0.0998, 0.169, 0.248, 0.54, 0.996, and 2.97 cm long in the region of its fundamental vibration-rotation band at 3.5μ was measured with a bismuth-silver vacuum thermopile. The light was resolved by a quartz spectrometer. The curves give indirect evidence of the isotopic doubling and yield fairly accurate absolute and relative values of the intensities of the absorption lines. The values given are checked by several independent methods of calculation. The results confirm the predicted asymmetry in the intensities of corresponding lines in the $P$ and $R$ branches as suggested by Kemble's theory, the summation rule, and the new formulation of the quantum theory, and, further, verify the quantitative predictions of these theories when the statistical weights are suitably chosen. The particular choice favored by the data is the selection $p_M=1, 3, 5, \cdots$, if the lines are assumed to be singlets (and $p_M=2, 4, 6, \cdots$, if the lines are assumed to be spectroscopic doublets on the presumption that the summation rule is valid for this case). The lines are apparently narrower and deeper than heretofore supposed, and it is demonstrated that the exponential law of transmission is not applicable to the transmission curves even after these have been corrected by the usual slit-width correction formulas.