Absorption Spectrum of Iodine Bromide

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It is shown by means of a vibrational analysis that the absorption spectrum of iodine bromide is analogous to that of iodine chloride and that an interpretation similar to that recently proposed by the writer and Gibson for ICl is applicable. The infrared bands of IBr discovered by Badger and Yost are classified as a $^{3}\ensuremath{\Pi}_{1}\ensuremath{\leftarrow}^{\ensuremath{'}}\ensuremath{\Sigma}$ transition. The $^{3}\ensuremath{\Pi}_{{0}^{+}}\ensuremath{\leftarrow}^{\ensuremath{'}}\ensuremath{\Sigma}$ system is observed as a faint set of bands in the red, not hitherto reported, which exhibit the same type of predissociation as the corresponding bands of ICl. Transitions to a state which originates at the maximum of the $^{3}\ensuremath{\Pi}_{{0}^{+}}$ state and which dissociates to yield normal iodine and excited bromine atoms are observed as a strong system of partially diffuse bands. Cordes' assignment of these bands to two systems is not confirmed. In the present work the vibrational quantum numbering for each system is deduced from measurements of the isotope effect due to bromine. The heat of dissociation of IBr is 1.808\ifmmode\pm\else\textpm\fi{}0.001 volts. For the four molecules ${\mathrm{I}}_{2}$, IBr, ICl, and ${\mathrm{Br}}_{2}$ it is shown that the separation of the $^{3}\ensuremath{\Pi}_{{0}^{+}}$ and $^{3}\ensuremath{\Pi}_{1}$ states is roughly equal to two-thirds of the mean $^{2}P$ multiplet widths of the constituent atoms. The total $^{3}\ensuremath{\Pi}$ widths are then probably greater and the $^{3}\ensuremath{\Pi}_{2}$ state somewhat lower than has been supposed.