Rydberg states with a 2Π core: The b3Π i and C1Π states of DCl

Abstract

The rotational structure in the lowest Rydberg complex of hydrogen chloride, [ X 2Π]4 sσ, was reinvestigated. The study is limited to the spectrum of D 35Cl, the HCl bands being too diffuse for a detailed analysis of second-order effects. The Λ-type doubling in both component states, b 3Π i and C 1Π, is small since it arises from the uncoupling of the core rather than Rydberg orbital angular momentum. It can be interpreted in terms of pure precession relations that are known to exist between the ground and first excited states of DCl +. By contrast, the spin-orbit interactions, also originating in the core, are strong. In addition to distorting the triplet splitting in b 3Π, they lead to an avoided crossing between the nearly coinciding levels b 3Π 0 ( v = 1) and C 1Π 1 ( v = 0). They are also responsible for anomalies in the b 3Π 0 ← X 1Σ + R-branch intensities of DCl as well as of HBr, DBr, and HI. From the J values at the observed R-branch minima we have estimated the ratio μ ∥ μ ⊥ of the transition moments associated with the excitation of a 3 pσ or 3 pπ core electron to the 4 sσ Rydberg orbital of DCl and, correspondingly, of the other hydrogen halides.