Excitation of Mercury Vapor by Electron Impact.

J. Franck and E. Einsporn. Über die Anregungspotentiale des Quecksilberdampfes.
Zeitschr. f. Phys. 2, p. 18, 1920.

Numerous investigations of the ionization and excitation of radiation in mercury vapor by the method of subjecting it to a stream of slow electrons have shown the existence, in the mercury atom, of a characteristic absorption of the kinetic energy of the incident electron when it reaches three definite values, corresponding to the electron’s passage through potential differences of 4.7; 6.7 and 10.4 volts. The first two values, called resonance potentials, are accompanied by the emission, by mercury vapor, of ultraviolet $\lambda 2537$ and $\lambda 1\,850$; the last value is the ionization potential of mercury. These results are in complete agreement with the scheme of the mercury atom derived, on the basis of Bohr’s theory, from the serial structure of the mercury spectrum. Namely, on absorbing an energy of 4.9 volts, the atom passes from the normal state, characterized by the serial term $1S$, into the state $2P_2$; the return to the initial state is accompanied by the emission of a quantum of frequency $1S—2P_2$, i.e. of the line 2537. An analogous process occurs at $6.7\ V$, where the serial symbol of the transition is $1S—2P$. At $10.4\ V$ there occurs the complete detachment of one of the electrons of the atom, i.e. ionization.

The question naturally arises why, of all the complex set of possible states of the mercury atom that manifest themselves in the optical spectrum, under electron impact only two occur, namely: \(1S—2P_2\) and \(1S—2P\). The work under review has answered this question by discovering, in addition to the two indicated ones, another 15 resonance potentials in the interval from \(4.6\,V\) (1st resonance potential) to \(10.38\,V\) (ionization potential). The method employed contains nothing new and is based on measuring the photoelectric effect on an iron electrode under the influence of the radiation of mercury vapors subjected to the action of an electron stream that has passed through a certain variable potential difference; only the secondary influences on the accuracy of the readings have been eliminated. Of the spectral lines calculated from these 18 values of the potentials, 13 find their place in the series, and consequently also in the atomic scheme. All transitions originate from the state \(1S\), i.e. the normal state. Noteworthy is the appearance of the transitions \(1S—2P_1\) and \(1S—2P_3\), which are completely unknown optically.

Despite the weak expression of some of the resonance potentials presented, the work undoubtedly represents a step forward, giving grounds for Franck to see in this method a necessary supplement to spectral methods in the region of ultraviolet radiation.

A. Terenin.