Ionization and Excitation Potential of Nitrogen.

E. Brandt, Über die Ionisierungs- und Anregungs-Spannung des Stickstoffs /Zsch. f. Ph. 39, VIII. 32, 1922.

Brandt’s work was carried out by the usual method. The accelerating potentials were brought up to 100 V. Corrections for contact potential difference and the like were not calculated, but were introduced, as is usually done, by means of measurements on a standard gas, which

served as He. In the region between 7.5 and 8.2 V, a large number of bends of the current curve were measured (with an accuracy of up to 0.01 V), corresponding to the lines of the ultraviolet radiation of \(N_2\). These lines constitute precisely the region of the band spectrum of nitrogen. Thus, the study of the structure of bands in the far ultraviolet becomes possible with the aid of the method of ionization points. To be sure, the results obtained so far are not yet accurate enough to give a clear quantitative picture of the structure of the band.

The values obtained by the author, \(17.75 \pm 0.1\ \mathrm{V}\), \(25.41 \pm 0.1\ \mathrm{V}\), \(30.72 \pm 0.2\ \mathrm{V}\), correspond, according to his indications, to the ionization potential and to two higher stages of ionization. These figures must be reduced by 0.7 V according to the correction for He given by Franck. Unfortunately, the data cited by Brandt are insufficient to say with certainty to what processes the quantities observed by him corresponded. Assuming that 17 V corresponds to the splitting of the molecule and the ionization of one of the atoms \((N_2 \to N + N + \Theta)\), and 30 V to the ionization of both atoms \((N_2 \to N^+ + N^+ + 2\Theta)\), I estimate the heat of dissociation of nitrogen at 4 V, i.e. only slightly greater than the heat of dissociation of \(H_2\). That this quantity must be greater than for hydrogen is consistent with Langmuir’s observations on the difficulty of thermal dissociation of nitrogen. However, this quantity—the heat of molecular dissociation, so important for photochemical calculations—would be desirable to know with greater certainty.

Gr. Landsberg.