The Low Electronic States of Simple Heteropolar Diatomic Molecules. II. Alkali Metal Hydrides

Some discussion of the nature of the $V$ level in ${\mathrm{H}}_2$ and ${\mathrm{M}}_2$ is first given, then the alkali hydrides are taken up. The upper electron state involved in the one well-known band system of each of the molecules LiH, NaH, and KH is a $V$ state. The exceptional form of its potential energy curve (shown by the abnormal band spectrum constants) can be explained satisfactorily if we suppose that the observed $N$ and $V$ curves result from the strong interaction of two zero approximation curves, one of atomic (M·H), the other of ionic (${\mathrm{M}}^{+}$ ${\mathrm{H}}^{-}$) character. The situation is analyzed in detail for LiH. It is shown that the $V$ state of LiH, while of only partially ionic character near its minimum, goes over at somewhat larger $r$ values into the pure ionic (${\mathrm{Li}}^{+}$ ${\mathrm{H}}^{-}$) curve. The $N$ state of LiH is strongly polar near its minimum, but finally goes over into a pair of neutral atoms with increasing $r$. It is concluded that the "additivity" theorem for homopolar bond energies is not fulfilled in MH (nor in MX; cf. III). This conclusion is forced by the otherwise unaccountably small dissociation energy of diatomic MH. $N$ and $V$ levels of the molecules Be${\mathrm{H}}^{+}$, Zn${\mathrm{H}}^{+}$ and the like are identified. In CuH, AgH and AuH, the presence of $d$ electrons is considered responsible for a lack of close analogy to MH.