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The electronic levels of the nitrogen molecule fall into two groups, the triplet and singlet systems, the latter including the ground state, the two being unconnected by any known optical transition. The most direct method of determining the relative position of the two groups is to measure the excitation potential of one of the Positive (triplet) systems of bands, usually, for convenience, the Second Positive system. The standard determination is that of Sponer whose results have been generally accepted as giving the most reliable value. Two earlier experimenters give only rough values, and, in addition, apply no correction for the contact potential of the electrodes. This Sponer and subsequent workers have done by determining also the excitation potential of some other atom, the value of which was calculable from spectroscopic data, deducing the additive correction to be applied in this case, and assuming that it remained the same when the nature of the gas in the apparatus was altered. The method used by Sponer was to take spectrograms of the Second Positive bands excited by electrons of known energies at intervals of 0⋅2 volt, and compare the photographs visually to detect the first appearance of the bands. The calibration for contact potentials, however, was done visually with a pocket spectroscope, so that Sponer’s estimate of her accuracy may be a little optimistic. The value obtained was 13⋅0 ± 0⋅3 volts. An obvious improvement was effected by Ornstein and Langstroth who used the well-known Utrecht technique to compare intensities at different voltages. The observations in the neighbourhood of the excitation potential are, however, rather sparse, and the resulting extrapolation to zero intensity too lengthy to be very accurate. Their limits of error are not stated, but some idea of the accuracy can be obtained from the fact that the excitation potentials of the (0, 2), (1, 3) and (2, 4) bands are given as being the same (13⋅4 volts) while spectroscopic data indicate that they should extend over a range of 0⋅48 volt. A further source of error which becomes appreciable when this more refined method of measuring the intensities is used, is the possibility of a change in the spatial distribution of the glow discharge with the voltage, which would produce spurious variations in the intensity voltage curve. The method of using a defined pencil of electrons as used by Hanle and his co-workers, takes cognizance of this, but although Thieme has determined the excitation function for the Second Positive system over the range 16-30 volts, he quotes no value for the excitation potential. An estimate from the published curves would be 13⋅6 ± 0⋅5 volts (this for the (1, 4) band), but the result is not very reliable since the extrapolation is again very lengthy.

The excitation potential of the nitrogen second positive bands