Measurement of Weak Absorption by Molecules*

Abstract

To measure small concentrations of molecules, particularly in a metastable state, we have developed a form of double-beam absorption spectrometer. This instrument allows one to use as an absorber a tube in which a dc discharge creates unstable species. The design provides that light emitted from this discharge shall not interfere with the absorption measurement. We use as a background source a second discharge tube emitting the band system whose absorption we wish to observe. We measure the absorption of a single vibrational band of known probability of transition. A detailed analysis of the relative intensities of all the lines constituting the band is then necessary in order to extract the population of the metastable level.