The Limiting Frequency in the Spectrum of Helium, Hydrogen, and Mercury Vapor in the Extreme Ultraviolet Region.

(Richardson and C. Bazzoni. The limiting Frequency in the spectra of Helium, Hydrogen and Mercury in the Extreme Ultra-Violet. Phil. Mag. 34. p. 285 1917).

As is known, the use of quartz lamps and prisms makes it possible to obtain light waves down to 1850 Å on ordinary photographic plates. The use of plates without gelatin, which absorbs short waves, enabled Schumann to extend this region to 1230 Å; Lyman, in 1914, by replacing the prism with a concave grating, brought it down to 900 Å, and in 1916, by filling the chamber with helium instead of hydrogen, he succeeded in establishing waves of length 600 Å. On the other hand, the greatest length of soft X-rays is about 1 Å. Consequently, between light rays and X-rays there remained a gap of 600 Å. The investigations of Richardson and Bazzoni have considerably reduced this gap.—It is clear that when we pass to very short waves, we must take into account the circumstance that they are absorbed not only by solid bodies but also by the gas in which they propagate. Therefore one must strive for the shortest possible path of the ray in the gas (in Lyman’s experiments the ray traveled a path of about 2 m), and it is necessary that the gas be as pure as possible, while the apparatus must be constructed so that it contaminates the gas as little as possible. Of course, under such conditions, for detecting short waves one has to abandon the photographic method.

Richardson and Bazzoni used the photoelectric effect for this purpose. Namely, radiation excited by thermions in helium was directed onto a copper plate; the electrons emitted under the influence of this radiation, with the aid of a magnetic field, described a path of known radius. From the relations existing here one can determine the velocity of the emitted electrons, and, using Einstein’s well-known equation

\[ \frac{1}{2}mv^2 = V.e = h(\nu-\nu_0) \]

(where \(m\) is the mass of the electron, \(v\) its velocity, \(e\) its charge, \(V\) the potential, \(\nu\) the frequency of the exciting radiation, \(\nu_0\) the frequency of oscillations of the electron in the plate), one can calculate \(\nu\)—the frequency of the oscillations of the radiation under whose influence the electron was emitted with velocity \(v\).

Under these conditions the apparatus consisted of a quartz tube with appropriate electrodes. They succeeded in detecting waves of 420 Å.

By means of this method they attempted to determine the limiting frequencies in the spectra of helium, hydrogen, and mercury vapor. For helium they obtained limiting wavelengths from 470 to 420 (most probably 420), for hydrogen from 830 to 950 (most probably 900), and for mercury vapor from 1000 to 1200 Å. Moreover, these wavelengths do not depend on the exciting potential. Substituting in Einstein’s equation \(V.e=h\nu\) the values for \(V\) (the ionizing potential) taken from Bohr’s theory (for helium 29.3,

for hydrogen 13.6 and for mercury vapor 10.5 volts) we can obtain for the limiting wavelengths the values: 422, 909, and 1180 Å, which differ extremely little from those observed by Richardson and Bazzoni.

T. Molodii.