On the Ionization Potential of He.

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

As was indicated in the preceding abstract, when slowly moving electrons collide with an atom, an elastic impact first occurs, in which the electron does not give up its energy. If the velocity of the electrons is gradually increased, then finally, at a certain electron energy, the electron, on colliding with the atom, 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 a series of lines belonging to the absorption series, one can observe the shortest wavelength, determined by the relation that the electron has the energy \(h\nu_\infty\) (where \(\nu_\infty\) is the greatest frequency in the series), corresponding to the ionization energy. Since determinations of \(h\nu_r\) and \(h\nu_\infty\) lead to a test of Bohr’s theory, the authors have carried out 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.

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

N. Lazarev.