Ultraviolet Absorption of Iodine Vapor

Abstract

The regularity of development of the ultraviolet absorption spectrum of iodine between $\lambda 1900$ and $\lambda 3413$ from shorter to longer wave-lengths with increasing temperature and pressure indicates that the bands all belong to one system. The large range of conditions under which bands studied by Pringsheim and Rosen and those found by Kimura and Miyanishi are both found leads to the same conclusion. The "long thin bridge" joining these two regions is explained as the result of the coincidence of $\Delta G^{\prime }=70\mathrm{}$ ${\mathrm{cm}}^{-1\mathrm{}}$ and $\Delta G^{\prime \prime }=210\mathrm{}$ ${\mathrm{cm}}^{-1\mathrm{}}$, causing successive $v^{\prime }$ progressions to coincide. This is supported by the manner in which successive $v^{\prime }$ progressions join on to the single series. A similar explanation probably holds for the single series of spacing 90 ${\mathrm{cm}}^{-1\mathrm{}}$ at the red end of the Pringsheim and Rosen bands, which breaks up near $\lambda 2850$ into the bands found associated with the $\lambda 3413$ "continuum" in fluorescence, emission and absorption. This continuum forms the absorption maximum at the long wave-length limit of the spectrum, the red end. It is therefore neither an electron affinity nor ionic recombination nor any other atomic spectrum. These results agree well with Mulliken's theory of this system.