The band spectra of helium and hydrogen at low temperatures

Abstract

The theoretical effect on a band spectrum of a change in the temperature of the emitting gas or vapour is already well known. In a normal band the intensities of the component lines of a series at first increase with increasing values of the rotational quantum number (m), so that for some definite value ofm(not necessarily integral) there is a maximum of intensity, after which the intensity gradually decreases to zero. A curve between the intensity and m should have the same general form as the curve of the Maxwellian distribution of the angular velocity, since the most important factor in the relative intensities of the components is the number of molecules in the different possible states of rotation. At low temperatures the number of molecules with large values of the rotational energy is reduced, with the result that band components with large values of m are reduced in intensity or disappear altogether. At the same timethe point of maximum intensity should be shifted to a lower value ofm.Helium. In the present investigation the bands described by Curtis (1) in the spectrum of helium have been studied at the three temperatures given by surrounding the discharge tube with water, liquid air, and liquid hydrogen respectively. For certain series the change in the quantum number corresponding to the maximum intensity is very clearly shown, while for other series the first line is the most intense at all temperatures, and the lowering of the temperature has only the effect of reducing the relative intensity of the higher members of the series.