THE HYDROGEN–CHLORINE SYSTEM IN THE MM PRESSURE RANGE II. THE VIBRATIONAL GROUND STATE STUDIED BY SELF-ABSORPTION

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Abstract
Studies of energy distribution among reaction products, through the agency of infrared chemiluminescence, have previously only yielded information concerning the distribution among vibrationally excited states. In the work described here it was shown that self-absorption of the infrared emission could be used as a measure both of the relative and the absolute amount of product present in the vibrational ground state, ν = 0. Two independent methods were used to measure the extent of self-absorption. The first method relied on measurements of an apparent deviation from Boltzmann-type rotational intensity distribution within the ν(1–0) and ν(2–1) bands. The second method depended on measurements of an apparent deviation of the relative intensity of corresponding H35Cl and H37Cl isotopic lines from the natural abundance ratio. (It was shown, at the same time, that the isotopic reactions H + 35Cl2 → H35Cl + 35Cl and H + 37Cl2 → H37Cl + 37Cl have the same rate constant, within ±10%). A third method of measuring the extent of self-absorption, which depends on the detection of an anomaly in the relative intensity of P- and R-branch emission lines is discussed.The self-absorption method was applied to the study of the hydrogen–chlorine system in the 1–2 mm Hg pressure range (see also Part I). Mean partial pressures of HClν=0 ~ 10−2 mm Hg were measured in individual rotational states to an accuracy of ca. ±10%, using an optical path length of 20 cm. The rotational distribution in ν = 0 corresponded to a temperature of 1150 ± 150 °K (the uncertainty in this figure encompasses two independent methods of estimating the self-absorption), as compared with 1300 ± 100 °K for all vibrationally excited states (Part I).