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 ¹Π and ³II, T ³Σ⁺, V ¹Σ⁺, or their Ω-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 ³II₁, ³II₀+, and ¹II, but with Ω-s coupling. It is further concluded (in contrast to what is found in the halogen spectra) that the observed absorption goes mainly to the ³II₁ and ¹II levels rather than the ³II₀+. 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 ¹Σ⁺ curve, derived from a d⁹s² excited silver atom and an unexcited X atom, with curves of the types V ¹Σ⁺ and especially Q ³Π_0⁺; but that near its minimum state B is essentially Ag (d⁹s²) ·{} 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 d⁹s²,²D).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 X^- of M⁺{X}^{{-}}$ 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 MX⁺) 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 ³Π₁, Q ¹Π, and T(³Σ⁺)₁, and two 0⁺ levels each of which is an approximately 50, 50 mixture of Q ³Π_0⁺ with V ¹Σ⁺.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 X₂ 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.