IN a series of experiments on the excitation of nitrogen bands by a controlled electronic bombardment, I have observed, as Dr. Kaplan did1, the great modifications of the spectrum which accompany any change in the experimental conditions. As the pressure decreases, the intensity of the negative system of bands (emitted by N2+ ions) increases in comparison with that of the second positive system. On account of the rarity of collisions, the average energy of the exciting particles (ions or electrons) rises. progressively and, at the same time, the probability of excitation of the negative bands becomes higher2.
NATURE, 139, 1112 (June 26, 1937).
C.R. Acad. Sci., 204, 489 (1937).
C.R. Acad. Sci., 204, 993 (1937).