On the Ionization Potential of He

(J. Franck and P. Knipping, Die Jonisierungsspannungen des Heliums. Phys. ZS. 20 p. 481. 1919).

As was indicated in the preceding abstract, when slowly moving electrons collide with an atom, at first an elastic impact is obtained, in which the electron does not give up its energy. If the velocity of the electrons is gradually increased, then finally, at a definite electron energy, upon collision with the atom the electron loses all its energy, and it is emitted in the form of rays determined by the relation \(h\nu_r\) (\(h\)—Planck’s constant, \(\nu_r\)—the frequency of the longest waves of the absorption series). This radiation often coincides with resonance radiation, and therefore the potential corresponding to \(h\nu_r\) is called by the authors the resonance potential. Further, from the series of lines belonging to the absorption series, one can observe the shortest wavelength, determined by the relation that the electron has energy \(h\nu_\infty\) (where \(\nu_\infty\) is the greatest frequency in the series), corresponding to the ionization energy. Since the determinations of \(h\nu_r\) and \(h\nu_\infty\) lead to a test of Bohr’s theory, the authors undertook an extensive and precise experimental investigation of this question. For the resonance potential the authors found 20.5 volts; for the ionization potential, 25.4 volts was found.

As a general result, the conclusion is obtained that the helium atom is in reality considerably less stable than Bohr’s theory requires.

N. Lazarev.