The CaOD, d¹,³Δ-a³Π System: Sub-Doppler Spectrum, Rotational Analysis, and Deperturbation

The CaO "Orange Arc Bands" are shown to contain transitions from both D 1 Δ and d 3 Δ i upper states into both a 3 Π i and A ' 1 Π lower states, as well as other not yet identified electronic band systems. More than 2000 lines in the 605-613 nm region are resolved by sub-Doppler Intermodulation Spectroscopy. Of these lines, more than 900 are definitively assigned (vibration, rotation, Ω, absolute parity) to the 0-0 and 1-1 bands of the D, d 1,3 Δ- a 3 Π system. This is the first full rotational analysis of an alkaline earth monoxide triplet-triplet system. Assignments are based on combination differences from the sub-Doppler excitation spectra, supplemented by information from resolved laser induced fluorescence spectra. The analysis is guided by model Hamiltonians for the strong d 3 Δ ∼ D 1 Δ perturbation and for Λ-doubling in a 3 Π. The D 1 Δ state lies below d 3 Δ; the first known reversal of the usual ordering of 1,3 Δ states derived from a π 3 π' configuration. Spin-orbit interaction between 3 Δ 2 and 1 Δ causes the ordering of the d 3 Δ i spin components to be irregular, with Ω = 2 highest and Ω = 3 lowest. The two 1,3 Δ Ω = 2 components are nearly 50-50 mixtures of singlet and triplet character; this mixing may represent an important pathway for spin-interconversion in CaO.