Absorption of Luminous Mercury Vapors

E. R. Metcalfe and B. Venkatesachar. On the Absorption of Light by Electrically Luminescent Mercury Vapour. Proc. Roy. Soc. 100, p. 149, 1921.

The absorption of the radiation of a mercury arc in another arc of the same kind has been investigated many times, for the most part in order to clarify the applicability of Kirchhoff’s law to the processes of radiation emission in gases under the action of an electric discharge. The current densities used in the absorbing arc in these experiments were always

of the same order as in the absent one, i.e., sufficiently considerable, yet the observed absorption was small.

The authors of the article observed a quite exceptional absorption of certain lines of the mercury spectrum in weakly luminous mercury vapors excited in the following way: mercury vapors liberated from an ordinary arc discharge were passed through a current of insignificant strength (tenths of an ampere). Under these conditions a layer of weakly luminous mercury vapor 52 cm long absorbed 8% of the incident radiation of the green line 5461 Å; the same line and $\lambda 4359$ were obtained either reversed against the continuous spectrum of a carbon arc and of the sun. The former was subjected to especially detailed photometric and interferometric study. A list is appended of visible and near-ultraviolet lines that gave strong or noticeable absorption.

While giving indications of the series affiliation of these lines, the authors, however, do not compare the results obtained with the scheme of the atomic structure of mercury, from which it follows directly that, under these conditions of electrical excitation, the majority of the mercury atoms prove to have been transferred from the normal state to one of the nearest higher quantum states, characterized by the series terms $2p_1$, $2p_2$, $2p_3$, whence alone arises the possibility of absorption of quanta $\lambda 5461$ and of other bright lines of the visible and nearest ultraviolet spectrum.

There is no doubt that in this experimental arrangement the excitation of the mercury vapors occurs twice: first in the arc discharge, then by means of the weak current, the first excited state possessing exceptional duration.

A. Terenin.