Dissociation of Excited Iodine Molecules

By means of a sensitive optical method described elsewhere, we determined the quantum yield of the process $${\mathrm{I}}_2\mathrm{+h\nu \to }\mathrm{I}+\mathrm{I}$$ in mixtures of I₂ vapor with several hundreds of mms of He, A, H₂ and N₂, in three spectral regions: (1) In the continuum, i.e., on the short wave‐length side of the convergency limit at 4990A; (2) in the band region immediately behind this limit, on its long wave‐length side; and (3) in the band region further away from the limit. If the quantum yield is assumed to be γ₁=1.00 in the first region, the values γ₂=1.14 and γ₃=1.10 are obtained for the two other regions, showing that all excited I₂ molecules dissociate by collisions with foreign molecules. The quenching of I₂ fluorescence by foreign gases is thus practically never due to the dissipation process (1) I₂^*+X→I₂+X but entirely to the dissociation process (2) I₂^*+X→I+I+X. The process (2) apparently occurs in most cases at the very first kinetic collision of excited iodine with a foreign molecule; only in the case of helium ten or more collisions are necessary to produce dissociation according to (2).