ELECTRONIC ENERGY TRANSFERS BETWEEN IODINE AND OTHER MOLECULES' JOHN R. BATES2
Department o j Chemistry, University of Michigan, Ann Arbor, Michigan Receiced October 15, 19.96
Resonance fluorescence of atoms and molecules is quenched in the presence of foreign gases. The case of mercury has long been studied because of the simplicity of the emission and absorption process which gives rise to the fluorescence. These results have been a considerable aid in the understanding of photochemical processes. Among molecular fluorescences the simplest and most examined is that of iodine vapor (2). The quenching of iodine fluorescence excited by white light has been measured for various gases. It is most interesting to observe that argon, hydrogen, nitrogen, oxygen, chlorine, and iodine are rather efficient in deactivation of iodine molecules, the order of efficiency being that given. At first sight complete deactivation would seem to be a rather improbable act. It is known that a shift from electronic to translational energy of collision is unlikely except in small amounts. This is well illustrated by the work of Franck and Wood (l),where iodine fluorescence excited by the 5462 mercury line was practically undiminished in total intensity, although considerably altered in wave-length distribution in the presence of 10 mm. of helium. Helium being able to take only small amounts of energy in the form of translation, there is little tendency to total deactivation. In considering the removal of the electronic-vibrational energy of iodine by vibrational excitation of the foreign gas we run into the same difficulty. Unless there is a fortuitous correspondence of yibrational levels between the colliding molecules, there is still a considerable amount of energy which must be transformed to kinetic form. However, there are two facts which point the way toward the answer to the problem. Turner showed that the iodine vapor illuminated with light of wave length greater than the convergence limit showed the presence of 1 Presented a t the Symposium on Alolecular Structure, held a t Princeton University, Princeton, New Jersey, December 31, 1936 to January 2, 1937, under the auspices of the Division of Physical and Inorganic Chemistry of the American Chemical Society. Present address: Catalytic Development Co., 1608 Walnut St., Philadelphia, Pa. 57
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iodine atoms. Furthermore the kinetics of reactions involving the halogens have the same form above and below the convergence limit. Thus the deactivation process must result in atom formation. The situation is represented graphically in figure 1, where are drawn the potential energy curves for the normal and excited states of iodine. Between the two there is an energy gap where only totally repulsive states,
D = 12,400 c m
FIG.1. Potential energy curveslfor the normal and excited states of iodine
with a continuous series of energy states, are possible. Such a state is shown by the dotted line. This explains the quenching results, since we are a t once freed from any necessity of correspondence in energy levels, with one exception. There must exist levels in the quenching molecule which will lower the energy of the iodine molecule to the energy range of the continuous states. I n other words 15,600 c n r l > Eel - Evib > 12,400 cm.-l
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vhere Eel is the energy of the excited iodine molecule and E,ib a vibrational level of the quenching molecule. A simple calculation shows that hydrogen will hare either zero or one such level for the different ranges of white light excitation, nitrogen one or two, oxygen two or three, chlorine and iodine a much larger number. This is thus the order of increasing efficiency: the greater the number of levels, the higher the quenching efficiency. This would seem to be a rather better explanation than the generally accepted one of deactivation to the normal state, with greater efficiency the more electronegative the gas. A more exact treatment, involving the breadth of levels and collision frequency, is defeated in effect by a lack of knowledge of collision cross sections. It is also interesting to observe that in a recent paper Rabinowitch and Wood (3) showed that the recombination of iodine atoms was affected by hydrogen, nitrogen, and oxygen with increasing efficiency. It is obvious that a similar argument might apply to this case. SUMMARY
The quenching efficiencies of iodine fluorescence can be accounted for only by the assumption that the deactivation results in dissociation, which is also in agreement with other results. REFERESCES AND WOOD:Phil. Mag. 21, 265 (1911). (1) FRANCK (2) See, for example, GRIFFITHAND MCKEOWN: Photo Processes in Gaseous and Liquid Systems, Chapter V. Longmans, Green and Co., New York (1929). (3) RABIKOWITCH AXD WOOD:J. Chem. Physics 4, 502 (1936). (4) WOODAND FRANCK: Phil. Mag. 21, 309, 314 (1911).
