On Collisions of the “Second Kind”

J. Franck. Einige aus Theorie von Klein und Rosseland zu ziehende Folgerungen über Fluoreszenz, photochemische Prozesse und die Electronenemission glühender Körper. Zsch. f. Ph., IX, p. 259, 1922.

In a paper published a year ago, Klein and Rosseland showed that, in the collision of electrons with atoms, alongside collisions in which the kinetic energy of the electron is converted into the quantum energy of an excited atom (collisions of the first kind), there must also exist collisions in which the reverse transition occurs: the energy of the excited molecule is not radiated, but passes into the kinetic energy of those electrons with which it collides. Franck extends these ideas also to the case of collisions of atoms with one another, for in temperature radiation one can see direct proof of the existence of collisions of the first kind between atoms, and those thermodynamic considerations which, according to Klein and Rosseland, lead to the necessity of the existence of collisions of the second kind if collisions of the first kind take place, retain their force also for the case of collisions of atoms with atoms. (Direct experimental proof of such collisions of the second kind is found, among other places, in the dissertation of Cario, carried out under Franck’s supervision; see below.)

Applying the indicated ideas to various physical phenomena leads the author to a number of conclusions, although it must be noted that he confines himself only to qualitative representations, without developing a quantitative theory. Thus he satisfactorily explains the effect of impurities on the character of the fluorescence of vapors; under monochromatic excitation of pure iodine, for example, (at low pressure) only resonance lines are emitted; the addition of gaseous impurities entails collisions of the second kind of excited iodine atoms with impurity atoms, as a result of which part of the energy is expended and, consequently, the part capable of being radiated \((h\nu)\) becomes smaller (transition to a lower “quantum” state), so that lines of lower frequency must appear, which is indeed confirmed by experiment. The same considerations make it possible to avoid the difficulties arising in explaining the features of the resonance radiation of mercury, which consist in the fact that among the presumed electron transitions there is a transition from the orbit \(2P\) to \(2p_2\), which, according to the selection principle, cannot occur in radiation. In the new theory this transition appears as the result of collisions of the second kind, with the excess energy not being radiated but passing into the kinetic energy of the colliding atoms. As applied to photochemical phenomena, Franck’s ideas explain the deviations from Einstein’s law of equivalence observed by Warburg in those cases when \(h\nu < Q/N\),

where \(Q\) is the heat effect per gram-molecule, and \(N\) is the number of atoms in a mole, and also provides experimental grounds for the views expressed earlier by Stark concerning the nature of the action of sensitizers in photochemical reactions.

Finally, in the phenomenon of the emission of electrons by incandescent bodies, collisions of the second kind may also play a role; in this case the observed Maxwellian distribution of velocities of the emitted thermions is explained by the fact that the liberation of electrons from the atoms of the substance occurs as a result of thermal dissociation, and does not presuppose free electrons, whose existence, especially in metal oxides, is, as is known, very doubtful.

Tr. Landsberg.