Iodine Revisited

(1) Previous estimates of the vertical energies of the valence‐shell states of the iodine molecule are revised in the light of newer information. Correspondingly, a complete set of estimated potential curves is drawn for those valence‐shell states which dissociate into ground‐configuration atoms. In the light of this set of curves, conclusions in several recent papers are reviewed and in some cases revised. LeRoy's $0_g^{+}$ excited state is reconsidered. Predissociations of the 2431, ${}^3{\Pi }_{0^{+}u}\mathrm{(B)}$ state are discussed. It is definitely concluded that the state mainly responsible for magnetic predissociation is the corresponding ${}^3{\Pi }_{0^{-}u}$ state and not a ${}^3{\Sigma }_u^{+}{\text{(0}}^{-})$ state. It is, however, possible that a ${}^3{\Sigma }_u^{+}{\text{(0}}^{-})$ state, probably that of the 1441 configuration, makes some contribution to the magnetic predissociation near $\text{υ\hspace{0.17em}=\hspace{0.17em}10}$ . Spontaneous predissociation and collision‐induced predissociation are also discussed. (2) Previous calculations for the estimation of the wavefunctions, relative energies, and transition probabilities from the normal state 2440, ${}^1{\Sigma }_g^{+}\mathrm{(X)}$ , of the several states of the 2431 configuration, are revised in the light of newer evidence. It is concluded that of the fairly strong visible absorption, perhaps 20% is due to the transition ${}^1{\Pi }_u\mathrm{\hspace{0.17em}\leftarrow \hspace{0.17em}X}$ , the rest being ${}^3{\Pi }_{0^{+}u}\mathrm{\hspace{0.17em}\leftarrow \hspace{0.17em}X}$ and a little ${}^3{\Pi }_{\text{1u}}\mathrm{\hspace{0.17em}\leftarrow \hspace{0.17em}X}$ . It is concluded that the ${}^3{\Pi }_{0^{+}u}\mathrm{\hspace{0.17em}\leftarrow \hspace{0.17em}X}$ transition moment results largely from case c mixing of 2431, ${}^3{\Pi }_{0^{+}g}$ into $X$ and relatively little directly from the mixing of 1441, ${}^1{\Sigma }_u^{+}$ into ${}^3{\Pi }_{0^{+}u}$ , but that a cross term arising from the joint effect of these two admixtures is also important. (3) Contrary to some writers, the conclusion that the intense uv room temperature absorption bands peaking at about 1825 Å (part of the Cordes bands) belong to the transition 1441, ${}^1{\Sigma }_u^{+}\mathrm{\hspace{0.17em}\leftarrow \hspace{0.17em}X(D\hspace{0.17em}\leftarrow \hspace{0.17em}X)}$ of the VN type, is strongly reaffirmed. The corresponding 1441, ${}^3{\Sigma }_u^{+}{\text{(1}}_u)$ transition is identified with the weak I₂ continuous absorption peaking near 2700 Å. Possible minor overlapping contributions from other weakly allowed transitions are discussed. The extension of the D ← X bands to longer wavelengths in high‐temperature absorption is reviewed. An equation given by Wieland, aside from some uncertainty in the vibrational numbering, accurately describes the vibrational pattern of the $D$ levels. The high‐temperature absorption bands with low‐frequency edges at $\text{λ 3427}$ and $\text{λ 3263}$ (Skorko bands) are, respectively, attributed to the transitions 1432, ${}^3{\Pi }_{\text{2g}}\text{\hspace{0.17em}←\hspace{0.17em}2431}$ ,