The photoelectric effect of molten tin and two of its allotropic modifications
The photoelectric effect of molten tin and two of its allotropic modifications (β and γ) is investigated. An apparatus is described which permits the measurement of the photoelectric emission at the different changes of state. Because of the small photoelectric sensibility of tin, a Hoffmann electrometer had to be used with a sensitivity of ca 10-18 coul./sec. The range of temperature was 50°-600°C. The range of wave-lengths from 2300Å to 2900Å. Taking all precaution to obtain monochromatic light and also taking into account the changes in the absorption of the metal, it was found that the photoelectric emission is independent of temperature as long as the phase of the cathode does not change. On the other hand, a change of state changes very definitely both its intensity and threshold, so that the denser the phase, the larger the work function. The threshold for liquid tin was found at hν0=2925±10Å; for γ-tin (hexagonal) 2820±10Å; for β-tin (tetragonal) 2740±10Å. The photoelectric curve of crystallization shows that the hexagonal modification starts to be unstable below 200°±2°C; and, also, that the speed of transformation is very slow. On account of the fact that the transformation can be followed photoelectrically, it is possible to have a criterion for the cleanliness of the surface. It seems probable that the quantum-equivalent ηe changes also with the phase, so that the denser phase has the larger ηe.
©1929 The American Physical Society. Received December 1928. In conclusion, the author desires to express his appreciation of the kind interest which Dr. Millikan took in this work and his thanks to Mr. E. McMillan for the valuable help which he gave to the work for one year. A more detailed description of this subject will be published in the Zeitschrift für Physik. [Initially] [r]eported at the meeting of the American Physical Society at Pasadena, Calif. December 8, 1928.