Primary and Subsidiary Condon Loci of Molecular Spectra

THE Franck–Condon principle 1 predicts that the vibrational quantum numbers v ′, v ″ of the strongest bands of a molecular spectrum lie on a family of open double-limbed quasi-parabolic curves in the v ′ v ″ plane which are often roughly coaxial about v ′ = v ″. The curves are called the primary (outermost) and subsidiary (inner) Condon loci. The loci are indicated on a number of recently published Franck–Condon factor arrays 2–4 . The smaller the difference Δ r e between equilibrium internuclear separations r ′ e and r ″ e of the molecular potentials involved, the narrower are the loci and the closer is the apex of the primary locus to (0, 0). The greater Δ r e , the more open the loci, and the further the apex of the primary locus is from (0, 0) down the diagonal v ′ = v ″. The apices of the subsidiary loci are often spaced roughly two quantum numbers apart down the diagonal axis.