By means of a sensitive optical method described elsewhere, we determined the quantum yield of the processI2+h&ngr;→I+Iin mixtures of I2vapor with several hundreds of mms of He, A, H2and 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 &ggr;1=1.00 in the first region, the values &ggr;2=1.14 and &ggr;3=1.10 are obtained for the two other regions, showingthat all excited I2molecules dissociate by collisions with foreign molecules.The quenching of I2fluorescence by foreign gases is thus practically never due to the dissipation process (1) I2*+X→I2+Xbut 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).
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