A weak absorption in iodine vapour was reported by earlier workers to not obey Beer's Law, and was attributed to the dimer I₄. The ultraviolet spectrum of iodine vapour was reinvestigated in this study on a more quantitative basis. The extinction coefficients have been found to be independent of concentration of iodine (10 ̄⁵ to 10 ̄² M), pressure of an inert gas (up to 1 atm), and temperature (25°C to 220°C, except for the usual temperature broadening). The absorption continuum (maximum 2694±3

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... A; oscillator strength 4.98 ±. 05 x 10 ̄⁴) must be due to a transition in the free molecule to a repulsive state correlating with either ²P3/2 + ²P3/2 or ²P3/2 + ²P1/2 atoms. The identification of the upper state involved and of the mechanism allowing the transition to occur were not possible with the available evidence. The previous identification of I₄ in solution and of Br₄ in the vapour phase, determined by similar ultraviolet continua, are discussed in relation to the lack of evidence for I₄ in the vapour phase found in this study.