Low Electronic States of Simple Heteropolar Diatomic Molecules: III. Hydrogen and Univalent Metal Halides

Electronic structures for the electronic states involved in the near ultraviolet spectra of AX are discussed (X=halogen, A=hydrogen or univalent metal). The upper electron levels involved are mostly among a set of levels $Q ^{1}\ensuremath{\Pi}$ and $^{3}\mathrm{II}$, $T ^{3}\ensuremath{\Sigma}^{+}$, $V ^{1}\ensuremath{\Sigma}^{+}$, or their $\ensuremath{\Omega}\ensuremath{-}s$ or case $c$ equivalents. Tentative potential energy curves for states $Q$, $T$ and $V$ of HI, AgI, and NaI are shown in Figs. 1-3. It is concluded that the observed HX absorption (continuous) involves the three $Q$ levels corresponding to $^{3}\mathrm{II}_{1}$, $^{3}\mathrm{II}_{0}$+, and $^{1}\mathrm{II}$, but with $\ensuremath{\Omega}\ensuremath{-}s$ coupling. It is further concluded (in contrast to what is found in the halogen spectra) that the observed absorption goes mainly to the $^{3}\mathrm{II}_{1}$ and $^{1}\mathrm{II}$ levels rather than the $^{3}\mathrm{II}_{0}$+. The $N$ and $V$ potential energy curves of HX obtained here are compared (see text) with those given by Pauling; for HI there is good agreement, but for HBr, HCl, HF the $V$ curves go increasingly higher than those of Pauling.For AgX, it is concluded that the upper potential energy curve of the observed bands (curve $B$) contains a shallow minimum followed, as $r$ is increased, by a maximum, beyond which it sinks to an asymptote about 1 ev lower. It is concluded that curve $B$ is probably a resultant of an interaction of a stable $^{1}\ensuremath{\Sigma}^{+}$ curve, derived from a ${d}^{9}{s}^{2}$ excited silver atom and an unexcited X atom, with curves of the types $V ^{1}\ensuremath{\Sigma}^{+}$ and especially $Q ^{3}\ensuremath{\Pi}_{{0}^{+}}$; but that near its minimum state $B$ is essentially Ag (${d}^{9}{s}^{2}$) \ifmmode\cdot\else\textperiodcentered\fi{} X in character, corresponding to a stable union between an Ag atom with a $d$ valence electron and an X atom. It is concluded that transitions to the three $Q$ levels in AgX probably involve continuous absorption, as in HX and MX. The various band systems of CuX and AuX at long wave-lengths are ascribed to electron levels derived mainly from metal atoms in which a $d$ electron has been excited (state ${d}^{9}{s}^{2},^{2}D$).For MX (M=alkali metal), it is concluded, in agreement with previous workers, that the process of light-absorption in the near ultraviolet has approximately the effect of transferring an electron from ${\mathrm{X}}^{\ensuremath{-}}$ of ${\mathrm{M}}^{+}$${\mathrm{X}}^{\ensuremath{-}}$ to the M atom, leaving two neutral atoms M and X which tend to fall apart (case $c$ separate-atom coupling). The thus indicated lack of valence attraction between neutral M and X probably involves a thoroughgoing failure in MX of the additivity rule for homopolar bond energies. Possible reasons for this probable failure (mainly, highly unequal size and energy of valence orbitals of M and X) are discussed. The electronic structures of normal and excited MX (also M${\mathrm{X}}^{+}$) are discussed theoretically both from the molecular orbital viewpoint and from that of atomic orbitals, and the occurrence of case $c$ coupling is shown to be theoretically reasonable from both standpoints. The upper levels involved in the absorption are identified as three 1 levels each of which is probably a case $c$ mixture of $Q ^{3}\ensuremath{\Pi}_{1}$, $Q ^{1}\ensuremath{\Pi}$, and $T{(^{3}\ensuremath{\Sigma}^{+})}_{1}$, and two ${0}^{+}$ levels each of which is an approximately 50, 50 mixture of $Q ^{3}\ensuremath{\Pi}_{{0}^{+}}$ with $V ^{1}\ensuremath{\Sigma}^{+}$.Franck's empirical spectroscopic criterion for distinguishing between atom-molecules and ion-molecules is discussed. From the present analysis of the electronic states involved in the spectra of HX, AgX, and MX, and from previous work on ${\mathrm{X}}_{2}$ and XY, it is concluded that, for molecules containing halogen atoms, the criterion does not have a systematic theoretical basis but, insofar as it is valid, depends on the occurrence of certain rather special and fortuitous circumstances.