The Photo-Ionization of the Vapors of Caesium and Rubidium

A modification of Foote and Mohler's space charge method for the measurement of photo-ionization was devised which compensated out spurious effects due to fluctuations of temperature and increased the useful sensitivity of the space charge to positive ions about one hundred fold. By measuring simultaneously photo-ionization and light intensities the following results have been obtained. In caesium, the ionization per unit light intensity $B_{\nu }$ varies with frequency $\nu$ (over the range 2200A to 3130A) according to the equation $B_{\nu }=\frac{\mathrm{const}}{{\nu }^3(\nu -{\nu }_0)}$ This relation also represents fairly well the experimental observations of $B_{\nu }$ in rubidium. Thus, the continuous absorption resulting in ionization decreases with wave-length much more rapidly than the analagous absorption beyond the Lyman and Balmer series limits as calculated on the wave mechanics by Sugiura, Oppenheimer, and Reiche. The results, moreover, are not even qualitatively similar to Hargreaves' calculation of the variation with frequency of $B_{\nu }$ for lithium. Assuming the principle of detailed balance it follows from these experiments that the effective collision capture cross-sections of caesium and rubidium ions for electrons vary inversely as the square of the energy of the electrons relative to the ions. This is the Thomson recombination law and has been derived on the wave mechanics by Oppenheimer for the recombination of protons and electrons. The existence of photo-ionization by absorption of principal series lines in caesium, so clearly exhibited by Mohler, Foote and Chenault, has been confirmed and also has been observed in rubidium.