The continuous absorption spectrum of hydrogen iodide

For a molecule that exhibits a band spectrum it is possible to calculate the course of the potential energy internuclear distance curve for the upper state, from an analysis of the fine structure of the spectrum. In the present investigation, the upper potential energy curve has been calculated for a molecule which shows only continuous absorption, from the extinction coefficients of the absorption. The method employed is that used by Goodeve and Taylor in the interpretation of the continuous absorption spectrum of hydrogen bromide. From the convergence limit of a band system an exact correspondence is obtained with the heat of dissociation of the molecule into either excited or normal atoms. With molecules that absorb continuously, the “threshold of absorption” has sometimes been taken to give a value for this heat. This simple conclusion has, however, been found to be unsound, as the position of the “threshold” depends on the smallest amount of absorption that can be measured.