The band spectrum of water vapour.—III

Abstract

In previous papers, details have been given of bands in the spectrum of water vapour, including the arrangement of the bands 2608 and 3428 into P, Q, and R branches. Measurements have also been given of a series of singlet lines lying close to the head of the band 2811 and similar in structure to the singlet series observed by Watson near the head of the band 3064. The advances which have recently been made in the theory of band spectra throw a considerable amount of light on their structure, and in the present communication it is proposed to apply the new methods to the analysis of the water-vapour bands. The conception which has been emphasised by several writers in the least few years is the close analogy which exists between molecular and atomic spectra. The electronic state of a molecule is characterised by the same features as are found in atoms, and in particular is associated with an electronic quantum number corresponding to Sommerfeld's inner quantum number for an atom. The outer electrons in a molecule are responsible for energy changes which are comparable in their nature and amount with those associated with the outer electrons of "corresponding" atoms. Mulliken, in a series of papers§ in the 'Physical Review,' has discussed at length this analogy between atomic and molecular spectra, and in the sixth paper of the series has proposed a suitable notation which will be employed here.