On the True Absorption of Light
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On the True Absorption of Light
Günter Cario. Über Entstehung wahrer Lichtabsorption und scheinbare Koppelung von Quantensprüngen. Zs. f. Ph., 10, 3, p. 185, 1922.
In accordance with the terminology proposed by Klein and Rosseland, the author calls collisions of the first kind those collisions (with fast electrons and atoms) as a result of which the outer electron of an atom is removed to one of the higher quantum orbits, i.e., the atom passes into an “excited” state. Collisions of the second kind are those in which the excited atom again passes into the normal state.
state, but the excess energy contained in it is not radiated, and is instead distributed among the other atoms participating in the collision.
By such collisions of the second kind, Cario attempts to explain, first, the weakening of the resonance radiation of mercury vapor as a result of adding neutral gases (observed for the first time by Wood), and, second, the phenomenon he discovered of the sensitization of metal vapors.
First part. The explanation of the indicated phenomenon is based on the assumption that every collision of an excited atom with an unexcited one is a collision of the second kind, i.e., that in each such collision one of the atoms capable of becoming a center of radiation loses this possibility. Calculating what diameter the excited atom must have in order that the corresponding number of its collisions (according to the kinetic theory of gases) could, under the assumption made, explain the observed weakening of the resonance radiation upon addition of gases, the author finds for the diameter of the excited mercury atom a value from 3 to 5.5 times greater than the diameter calculated from experiments on internal friction. The nonconstancy of the value of the diameter (the apparent increase in the case of admixture of light gases, \(He\)) proves that the assumption made is fulfilled only approximately (with light admixtures, evidently, not all collisions take away the energy of the excited atom); nevertheless, the mean value obtained for the diameter of the excited mercury atom, in agreement with the foundations of Bohr’s model and confirmed by the measurements of Füchtbauer and Joos (Ph. Zs, 23, p. 73, 1922) on line broadenings upon addition of neutral gases, compels one to consider that, in general, collisions of excited molecules with unexcited ones are almost 100% collisions of the second kind.
In Wood’s experiments (Ph. Zs., 13, p. 353, 1912) air served as the admixture. Cario, in his experiments, added noble gases (\(Ar\), \(Ne\)—\(He\)) in order to exclude the possibility of chemical interactions between mercury and the admixture. In addition, in his measurements the excitation of mercury vapor was produced not by means of a second mercury lamp (as in Wood’s), but by means of a stream of electrons of suitable velocity (Franck’s method). By this device it was possible greatly to increase the intensity of the glow. The first portions of the gas being mixed in even somewhat intensified the glow, evidently by creating more favorable conditions for collisions of the electrons with mercury atoms (elastic reflection of electrons from atoms of \(Ar\) and \(He\) in all directions).
The change in the intensity of the glow was determined by means of successive photographs of the spectrum; photometric measurement of the plates was carried out with Hartmann’s microphotometer.
Second part. As an admixture to mercury vapor there is chosen a substance satisfying the following requirements: its excitation potential is less than the excitation potential of mercury (corresponding to the line \(2536.7 A^\circ\)), but close to it; its ionization potential is greater than the excitation potential of mercury; the line \(2536.7 A^\circ\) lies outside the region of absorption of the admixture. In such a case one may hope that the excitation energy of the mercury atom, passing in collisions to the atoms of the added substance, will be capable of exciting (but not ionizing) them and will cause the glow of the admixture. A suitable substance is thallium vapor (\(Tl\)). Indeed, heating in one end of a sealed quartz vessel \(Hg\) (to \(100^\circ\)), and in the other \(Tl\) (to \(800^\circ\)), Cario succeeded in obtaining a mixture of \(Hg\) and \(Tl\) vapors which revealed the expected phenomenon: illuminating the mixture with the line \(2536.7 A^\circ\) and photographing the resonance radiation, he, with an exposure of one hour, obtained simultaneously with the mercury lines the following thallium lines:
| \(\lambda\) | Series scheme | Transition |
|---|---|---|
| \(5351\ A^\circ\) | corresponding to the series scheme | \(1.5s — 2p_1\) |
| \(3776\) | corresponding to the series scheme | \(1.5s — 2p_2\) |
| \(3530\) | corresponding to the series scheme | \(3d_2 — 2p_1\) |
| \(3519\) | corresponding to the series scheme | \(3d_1 — 2p_1\) |
| \(3230\) | corresponding to the series scheme | \(2.5s — 2p_1\) |
| \(2918\) | corresponding to the series scheme | \(4d_1 — 2p_1\) |
Control experiments: elimination or weakening of the line 2536.7 correspondingly eliminates or weakens the indicated lines; pure vapor of Tl, despite intense illumination, gives no traces of the mentioned lines.
The lines that appear are in good agreement with the serial scheme and the selection principle (azimuthal and inner quantum numbers). The appearance of the line \(4d_1—2p_1\), when a direct transition from the fundamental orbit \(2p_2\) to the orbit \(4d_1\) is impossible (for this would require more energy than is present in the excited mercury atom), compels one to suppose that, upon collision, the electron is thrown from the orbit \(2p_2\) to \(2p_1\), where it remains, since the reverse transition to \(2p_2\) is impossible by the selection principle. The next collision throws it onto the orbit \(4d_1\), and thus the indicated line appears. Therefore the orbit \(2p_1\) must be regarded as a metastable fundamental orbit, the existence of which is confirmed, among other things, by the absorption line \(5351A^0\) \((2p_1—1,5s)\) observed at a temperature of \(500^\circ\).
Experiments with Ag serve as a supplement to the experiments with Tl. In the mixture Hg, Tl, Ag the lines of Ag appear.
\[ \lambda = 3281A^0 \;\; \cdot\cdot\cdot\cdot\cdot\cdot\cdot\cdot\cdot\cdot\cdot\cdot\cdot\cdot\cdot\cdot\cdot\cdot\cdot\cdot \;\; 2p_1 — 1,5s \]
\[ 3383A^0 \;\; \cdot\cdot\cdot\cdot\cdot\cdot\cdot\cdot\cdot\cdot\cdot\cdot\cdot\cdot\cdot\cdot\cdot\cdot\cdot\cdot \;\; 2p_2 — 1,5s \]
In the absence of Tl the lines of Ag are greatly weakened. In exactly the same way, the relative intensity of the Tl lines changes considerably in the presence of Ag. This phenomenon is explained by the assumption that the most favorable conditions for the transfer of energy occur when the energy of the exciting atom does not differ greatly from the energy of the atom being excited. (Hence, for Tl, jumps to \(2,5s\), \(3d_1\), \(3d_2\) are more probable than to \(1,5s\).) The excitation of Ag takes place better not directly from Hg, but through the intermediary of Tl. Conversely, some Tl lines (the \(3776A^0\) line of the series \(1,5s — 2p_2\)) are more intense in the presence of Ag, since the energy of excited Ag is closer to the energy required for its excitation than is the energy of Hg.
The work reported on, which served as the dissertation of Cario, carried out under Franck in Göttingen, is of very great interest, since from the experimental side it approaches the question of the possibility of distributing energy, absorbed in the amount of a quantum of a definite frequency, among different atoms by means of collision. The absence of indisputable facts of this kind has greatly hindered, among other things, the rational explanation of deviations from Einstein’s photochemical law of equivalence.
Gr. Landsberg.