Test of Theoretical Absorption Band Model Approximations

Abstract

Three approximations to theoretical absorption band models have recently been proposed by Plass. Experimental studies of total band absorptances for N2O, CO2, CH4, and CO show that only the “weak line approximation” is actually completely fulfilled; the “strong line approximation” is realized only for large values of total absorptance corresponding to near saturation. The validity of the “nonoverlapping approximation” could not be determined easily from the data except under conditions for which the strong line approximation would also be expected to apply. Considerable deviation from this approximation was found and has been attributed to the presence of lines that remain weak even for large absorber concentrations. Quantitative tests of this hypothesis were made on the 2224 cm−1 N2O band and the CO fundamental at 2143 cm−1. Absorptance near the R branch maximum of the CO fundamental represented the only close approach to complete fulfillment of the strong line approximation. Since the R branch of CO is nearly free of secondary lines, it is concluded that the strong line approximation can be realized experimentally only for “clean” spectral regions.