Note on the series spectra of the aluminium sub-group

Abstract

The arc spectra of the elements of the aluminium sub-group (gallium, indium, and thallium) have been arranged in series consisting of widely spaced doublets, but, in contrast with the series of the alkali metals, the convergence frequency of the principal series is less than that of the sub-ordinate ones. The relations between the ionisation and resonance potentials and the limits and lines of series have been fully established in the group of alkali, and alkaline earth, elements, but a difficulty has been pointed out by Messrs. Foote and Mohler in connection with thallium, which is the only element of this group for which critical potentials have been obtained. Foote and Mohler found for thallium the values 1·07 and 7·3 volts for the resonance and ionisation potentials, respectively corresponding to a line of wave-number 8669 and a series limit of 59145. The series data, however, show no terms higher than the common limits of the sharp and diffuse series, namely, 41471·5 and 49264·2, which correspond to critical potentials of 5·12 and 6·08 volts respectively. Foote and Mohler, therefore, suggested that the supposed principal series (denoted by 1σ - mπ 1, 2 in Fowler's report) was really a derived series, 2σ - mπ π 1,2 , and that a series, 1σ - m 1, 2 , remained to be discovered. In such a series most of the lines would lie in the far ultra-violet, and their positions may be approximately calculated by re-numbering the terms in the known sharp series. This calculation leads to the value 92707 for the supposed 1 σ term, and the least refrangible line of the series would have a wave-number about 51200 ( λ 1952 A.), corresponding to ionisation and resonance potentials of 11·4 and 6·3 volts respectively.