Intensity measurements in the band spectrum of lithium

Abstract

The band spectrum of lithium in the red and blue-green regions has been analysed in absorption by Wurm. The blue-green bands have also been investigated by Harvey and Jenkins. These bands have an intensity alternation, and, as the latter authors show, 1·50, 1·67 and 2·00 are the only values among those predicted by the theory as possible for the ratio of the strong to the weak lines, which need to be considered in connection with their experi­mental results. Reasons were given by them for accepting 1·67 as the true value of the intensity ratio, which would signify that the angular momentum of the Li 7 -nucleus is 3/2. As this result was not in agreement with unpublished data of Wurm and as the interpretation of the hyperfine structure in the green lithium spark line with the assumption of a nuclear spin of 3/2 was not entirely satisfactory, new measurements appeared to be desirable. The apparatus used in this research was in principle the same as that of the former investigators. The absorption vessel was a steel tube about 80 cm. long and 2 cm. in diameter, bearing glass windows at its ends. The lithium vapour was produced by heating electrically about 60 cm. of the length of the tube, keeping its ends water-cooled. The light of a tungsten spiral lamp was sent through the tube and brought to a focus on the slit of the 6 m. grating of the institute. With this arrangement photographs were taken in the first order of the grating, whereas for second order plates the tungsten lamp was replaced by an electric arc to reduce the time of exposure, which varied from one-half to one hour.