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

Some discussion of the nature of the V level in H₂ and M₂ 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 (M⁺{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 (${Li}⁺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 BeH⁺, ZnH⁺ 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.