The absorption of light in caesium vapour in the presence of foreign gases

Abstract
The absorption of caesium vapour on the short wave-length side of the series-limit, alone, and in the presence of helium, has been described in earlier papers. It was shown that helium, at a pressure of a few centi metres of mercury, greatly reduced the absorption at all wave-lengths and also produced significant changes in the shape of the absorption curve. No theoretical explanation of these results has been given. At this stage of the investigation it appeared more desirable to obtain measurements of moderate accuracy on a number of gases, rather than to attempt very detailed and accurate measurements on one other gas. The present paper contains results on the absorption of caesium in the presence of the following gases:—neon, argon, krypton, xenon, nitrogen, hydrogen, deuterium, and benzene. All these gases give effects qualitatively similar to those produced by helium. It is still not possible to give a detailed theory of the results, but some empirical generalizations emerge from the extensive data now available. Experimental The methods used for the control of temperature were similar to those previously described. The photometric technique was the same except for one small alteration. In the experiments on the absorption of caesium in the presence of helium, the absorption was measured by comparison with certain grids. The reduction of light produced by these grids had previously been measured and they were used in the experiments on the absorption of caesium in vacuum. Some of the present experiments were carried out in this way, but in the later experiments the absorption of caesium in the presence of the foreign gas was compared more directly with the absorption of caesium in vacuum, by recording both kinds of absorption spectra on the same plate. Suitable calibration spectra were also included. This direct comparison eliminated the possibility of systematic errors due to imperfect temperature control, etc. It also reduced the effect of random errors.

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