Thirty-three band heads of ICl were measured in absorption and found to consist of two series, one of them converging at $\nu =17410\mathrm{}\pm 30$ ${\mathrm{cm}}^{-1\mathrm{}}$ ($\lambda =5744\mathrm{}\mathrm{A}$). Two heads of a third series were faintly in evidence just beyond the convergence, fading out in the region of continuous absorption. The maximum of continuous absorption is about 4800A. Intensity variations with temperature indicate that all three series come from excited states of normal ICl, the observed convergence coming from the first vibrational level. The vibration frequency of normal ICl is 382 ${\mathrm{cm}}^{-1\mathrm{}}$. Interpreted according to the theory of Franck our results indicate that ICl dissociates in light (1) into unexcited atoms, (2) into an unexcited iodine atom and a chlorine atom in the $2{}_{}{}^2{}_{}{}^{}P_1^{}{}_{}{}^{}$ state. The convergence corresponding to dissociation into chlorine and excited iodine was not found. Combined with the optical data of Kuhn on ${\mathrm{Cl}}_2$ and ${\mathrm{I}}_2$ our results require an evolution of 3.8 Cal. for the reaction ICl=½${\mathrm{I}}_2$+½${\mathrm{Cl}}_2$, assuming the observed convergence to correspond to process (1). Thermodynamic data give 3.6 Cal. for the same reaction. The two faint heads of the third series probably correspond to (2). The thermodynamic measurements are susceptible of determination with accuracy sufficient to exclude definitely other possible interpretations.

• Received 16 July 1927

DOI:https://doi.org/10.1103/PhysRev.30.598