The third positive carbon and associated bands

Abstract

The band spectra associated with carbon are numerous and many of then were known to the earliest workers in spectroscopy. They divided the bands into two main divisions, the positive and the negative bands. Thus they recognised the first positive carbon bands which are now called the Swan bands, the second positive bands which we now call the Ångström bands, the third positive and the fourth positive carbon bands, Among the negative bands known to them were the first negative or Deslandres' bands which are now known to be due to the ionised CO (CO + ) molecule. With the exception of the first positive bands or Swan bands, all the positive bands are now attributed to the neutral CO molecule. In recent years several new band systems have been added to the positive list, and two, the comet-tail bands or low pressure carbon bands, and the Baldet-Johnson combination bands, to the negative band systems. The history of the correlation of all these band systems is an interesting one and is adequate proof of the extraordinary usefulness of the quantum theory in the interpretation of molecular spectra. In this paper we are mainly concerned with the third positive carbon bands and such other bands as are usually associated with them. These bands were originally measured by Deslandres. Wolter could not obtain some of them which therefore were considered as spurious. Johnson and Birge gave the quantum interpretation of these bands, which fell into three n " progressions, viz., n' = 0, 1 and 4, the last two being spurious according to Wolter. Duffendack and Fox proved that the bands forming the n' = 4 progression belong to a new system of bands which they called 3A, with an initial electronic level higher than that of the third positive bands. All these bands have been obtained on all the plates taken in the first order of a 21-foot Rowland grating, by the present writer. The superficial structure of all the bands forming the same n" progression is the same but differs from the structure of the rest of the bands. It is therefore suggested that the bands forming the n" = 1 progression also arise from an initial electronic level different from that of the third positive bands. These may be called the 5 B bands. It is scarcely necessary to say that all these band systems have the same final electronic level.