A study of an unusual group of bands lying just beyond the convergence of the ordinary visible absorption bands of iodine chloride has revealed an interesting case of predissociation, and has made possible a new interpretation of the electronic states in accordance with Mulliken's theory of the halogens.

Electronic states and dissociation products. Spectroscopic evidence provided by the new band systems, together with thermodynamic data recently obtained by Yost and McMorris, definitely prove that the dissociation products of the ordinary absorption system (system I) are normal atoms. The upper level of this system is now interpreted as a ${}_{}{}^3{}_{}{}^{}\Pi _1^{}{}_{}{}^{}$ state, an interpretation which is supported by the recent analysis of these bands by Curtis. The 3, 1, 2, 0, and 3, 0 band of a new system (system II) have been analyzed. These bands show $P$ and $R$ branches only, and furnish rotational constants for the normal state which are in good agreement with those obtained by Curtis from bands of system I. The upper electronic state lies 3600 ${\mathrm{cm}}^{-1\mathrm{}}$ above the ${}_{}{}^3{}_{}{}^{}\Pi _1^{}{}_{}{}^{}$ state, appears to be derived from normal iodine and excited chlorine (${}_{}{}^2{}_{}{}^{}P_{\frac{1}{2}}^{}{}_{}{}^{}$) atoms, and is classified as the ${\mathrm{O}}^{+}$ member of the ${}_{}{}^3{}_{}{}^{}\Pi$ multiplet. The existence of this state is in satisfactory confirmation of Mulliken's theory of the halogens. The 4,1 and 4,0 bands of system II appear as bands of continuous absorption, except for certain lines, and at this point a new series of closely spaced vibrational levels begins, giving rise to faint and diffuse band heads (system III) which converge to give normal iodine and excited chlorine (${}_{}{}^2{}_{}{}^{}P_{\frac{1}{2}}^{}{}_{}{}^{}$) atoms as dissociation products. The rotational structure of this system, as observed in absorption, is incomplete, consisting of $P$ and $R$ branches extending over narrow ranges of $J$ values, and all lines are diffuse except the central lines in these short series. The sharp lines in these series can also be interpreted as belonging to system II.

Predissociation. These phenomena can be explained by assuming that the predissociation, which sets in sharply above the $v^{\prime }=3\mathrm{}$ level of System II (${}_{}{}^3{}_{}{}^{}\Pi _{0^{+}}^{}{}_{}{}^{}\leftarrow {}_{}{}^1{}_{}{}^{}\Sigma$), is due to interaction with a repulsive ${\mathrm{O}}^{+}$ state derived from two normal atoms, and that, at the intersection of the potential energy curves, a new set of levels (system III) originates. In absorption transitions to this new ${\mathrm{O}}^{+}$ state result in sharp lines only where the rotational levels coincide, at the same rotational quantum number, with predicted levels of the original ${}_{}{}^3{}_{}{}^{}\Pi _{0^{+}}^{}{}_{}{}^{}$ state. This is interpreted as due to momentary existence of the molecule in the ${}_{}{}^3{}_{}{}^{}\Pi _{0^{+}}^{}{}_{}{}^{}$ state as an intermediate step in the absorption process, a step which is followed either by predissociation giving normal atoms, or by a radiationless transition into the new ${\mathrm{O}}^{+}$ state, the latter being highly probable only when rotational levels with the same $J$ value are coincident or very close.

• Received 2 April 1932

DOI:https://doi.org/10.1103/PhysRev.40.529