The continuous absorption spectrum of hydrogen bromide

Abstract

Some molecules absorb light continuously at the high-frequency (high energy) end of a band system, while others show a similar continuous absorption without any related band system. Molecules of the halogens belong to the first type: their upper electronic levels are stable and show a series of vibrational sub-levels. From an analysis of the fine structure of the band spectrum, it is possible to calculate the course of the potential energy-internuclear distance curve for the upper state. Furthermore, the convergence limit of the band system corresponds exactly to the heat of dissociation energy of the products thus leads to the normal heat of dissociation of the molecule. It has sometimes been concluded from the absence of band systems with molecules of the second type, such as the hydrogen halides, that no minima exist in the potential energy curves of the upper state in these cases. Furthermore, the coincidence between the position of the convergence limit of a band system and the "threshold" of continuous absorption had led to the conclusion has not been substantiated by recent theoretical developments; but these, on the other hand, give us a method of calculating the course of the upper potential energy curve from the extinction coefficient for hydrogen bromide have been measured accuratley over a wide range of conditions, and the approximate upper potential energy curve has been calculated.