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 $2p_1-2s$ to $2p_1-4d_1$ and $2p_1-5d_2$ were identified. This excitation shows that the life of the $2p_1$ state is for Tl atoms of the order of ${10}^{-6\mathrm{}}$ 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 ${\mathrm{N}}_2$ was increased the intensity of $\lambda \lambda 5350,3776 \mathrm{and} 2768$ increased rapidly reaching a maximum for 5 to 20 mm of A or 5 to 10 mm of ${\mathrm{N}}_2$. A trace of ${\mathrm{O}}_2$ 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 ${\mathrm{N}}_2$ at 5 mm and Hg at 0.16 mm than with ${\mathrm{N}}_2$ 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 $2p_3$ state. These results confirm the assumption of Franck and Donat as to the part the Hg atoms in the metastable $2p_3$ 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 ${\mathrm{N}}_2$ molecules but easily gives up its energy when colliding with normal Tl or Hg atoms, probably more easily to Tl than to Hg.