The Spectrum of Mercury below the Ionization Potential

Abstract

Spectrum of mercury excited by electrons of energy 7 to 10.4 volts.—When mercury vapor is bombarded by electrons of energy below the ionizing potential 10.4 volts, previous spectroscopic investigation has detected the emission of only the lines $1S-2p_2$ and $1S-2P$, whereas according to the Bohr theory there are many other lines which correspond to changes of orbit involving less energy change than 10.4 volts, which should be emitted line by line as the impacting energy is increased so as to be sufficient to take the electrons to the proper orbits. The failure to observe these lines is now shown to be due to the effect of space charge in lowering the energy of the impacting electrons except at the immediate surface of the anode, and by focussing light from the surface of the anode onto the slit of a quartz spectrograph, spectrograms have been secured showing the development of the spectrum of mercury in the stages predicted by the theory and at approximately the theoretical voltages. At 7 volts only $1S-2p_2$ appears, at 8.4 volts 4 new lines due to electrons returning from the $2S$ or $2s$ levels, at 8.9 volts 8 new lines due to electrons returning from the $3d_1$, $3d_2$, $3d_3$, $3D$ levels; at 9.9 volts 16 more lines; etc., in full agreement with the theory and with the photo-electric experiments of Franck and Einsporn.