The band spectra of the alkaline earth halides. I.—CaF, SrF

Abstract

Band spectra of the alkaline earth halides were familiar to the earliest spectroscopic workers. Most of these spectra can be produced in both the flame and arc. The fluorides of calcium, strontium and barium all give interesting groups of bands which present obvious regularities, and these have been measured under high dispersion by several observers. The chlorides, bromides, and iodides have not been measured with such precision, although from time to time low dispersion measurements seem to have been made. In 1921 Datta made a systematic study of the fluorides of calcium, strontium and barium, and, added to these, data of the bands of magnesium fluoride. He gives an excellent plate of photographs of all these spectra and it has, therefore, not been considered necessary to reproduce any spectrograms in the present paper. Constant reference will, however, be made to this plate. Datta’s work was principally directed to the discovery of numerical relationships between the molecular weights and corresponding band spectra of these similar compounds. With this in view he labelled the band groups in each spectrum A 1 , A 2 , A 3 , ..., B 1 B 2 , B 3 , ..., C 1 , C 2 , C 3 , etc., so that analogous groups in the spectra of the various molecules should have the same designation. His selection of analogous spectra was largely based on their general appearance, and Datta gives several numerical relationships of an empirical character on the basis of this assignment. This work was completed before the applica­tion of the quantum theory to band spectra became prevalent, and it, therefore, needs revision in the light of our present knowledge. For convenience, how­ ever, Datta’s designation of the spectra by these letters will be used in referring to them. Mecke has recently made a quantum analysis of many of these bands and recognised distinct band systems in the fluorides of all the alkaline earths. He has expressed these systems by formulæ of the usual type, and discussed the band structure, the doublet separation, and the vibration frequency shown by this series of similar molecules. The present paper is a continuation and extension of Mecke’s work on the analysis of these spectra. The whole of the known band spectra of the fluorides of Mg, Ca, Sr, and Ba have been completely analysed and ordered in regard to their gross structure.