Indirectly excited fluorescence spectra
Fluorescence spectrum of thallium mixed with Hg vapor.—(1) Repetition of experiments of Franck and Cario. The illuminated end of the quartz tube containing Tl metal was placed in the hottest part of the furnace to avoid condensation. In the fluorescence spectrum all Tl lines to be expected theoretically, from 2p1-2s to 2p1-4d1 and 2p1-5d2 were identified. This excitation shows that the life of the 2p1 state is for Tl atoms of the order of 10(-6) sec. (2) Enhancing effect of inert gases. Series of spectrograms with the same tube confirmed the results of Donat. As the pressure of A or N2 was increased the intensity of λλ5350,3776 and 2768 increased rapidly reaching a maximum for 5 to 20 mm of A or 5 to 10 mm of N2. A trace of O2 neutralizes this enhancing effect. (3) Experiment with Tl vapor distilling past the window. In this case a "negative effect" was observed, the addition of a neutral gas decreasing the fluorescence. This was probably due to the interference of the inert gas on the rate of distillation and hence on the amount of Tl present near the window. (4) Experiments with closed tubes at uniform temperature. The enhancing effect was also obtained with these tubes, showing that it cannot be an indirect effect of the interference of the neutral gases with the distillation of the Tl vapor in Donat's experiments, but is definitely associated with the presence of the neutral gas. (More intense fluorescence was obtained with N2 at 5 mm and Hg at 0.16 mm than with N2 at 8 mm and Hg at 0.5 mm, the Tl being at 0.45 mm.) (5) Conclusions regarding role of Hg atoms in the 2p3 state. These results confirm the assumption of Franck and Donat as to the part the Hg atoms in the metastable 2p3 state take in the energy transfer to normal Tl atoms. Comparison of spectrograms at different partial pressures indicates that the metastable Hg atom may survive many collisions with normal A or N2 molecules but easily gives up its energy when colliding with normal Tl or Hg atoms, probably more easily to Tl than to Hg.
© 1925 The American Physical Society. Received 12 June 1925. The writer wishes to express his best thanks to Professor R. A. Millikan for the opportunity of carrying out experimental research in the Norman Bridge Laboratory, and for his stimulating interest throughout this investigation. He is also indebted to Dr. I. S. Bowen for his valuable assistance in the photographic part of the work.
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