Emission and absorption in the infra-red spectra of mercury, zinc, and cadmium

In a previous paper the writer pointed out that well-marked absorption bands exist in the infra-red region of the spectrum caused by passing white light through non-luminous mercury vapour. These bands occur at λ 1·014, λ 1·129. and λ 1·205, the first and third of these being especially strong. This investigation has since been extended further into the infra-red with mercury vapour, and it has also been repeated with the vapours of zinc and cadmium in place of that of mercury. The results serve to establish at least a partial parallelism in the behaviour of the three metals, the resemblance being most marked in the region of the first line of the series represented by v = (2·5, S)—( m , P), at which wave-length absorption takes place in the case of all three metals with extremely small vapour-pressure. On account of the ease with which the wave-length corresponding to v = (2·5, S)—( m , P) was absorbed, it was suspected that it should be emitted under electronic bombardment. At the suggestion of Prof. McLennan an investigation was undertaken with mercury vapour, in order to determine the speed which electrons must be given in order to stimulate emission of the wave-length λ 1·014. If we apply the quantum relation ve = hv to the frequency of this wave-length, we get V = 1·26 volts. In the experiments which will be described herein, the wave-length λ 1·014 was actually emitted with a voltage as low as 5 volts. There were strong indications that even a lower voltage would suffice to stimulate emission of the wave-length, but under the conditions of the experiment the radiations of this wave-length when emitted were reabsorbed by the intervening layers of mercury vapour.