Rotationally Resolved (1 + 1′) Resonance-Enhanced Multiphoton Ionization (REMPI) of CaR (R = H, D) in Supersonic Beams: CaR X ²∑⁺ (v″ = 0) → CaR* B ²∑⁺ (v′ = 0,1) → CaR⁺ X ¹∑⁺

We report on a laser-spectroscopic study of the CaH and CaD radicals in a supersonic beam, generated in a reaction of laser-sputtered Ca atoms and a pulsed beam of H₂ or D₂, seeded in helium. The rotational levels of the electronic ground-state X ²∑⁺ (v″ = 0, N″ ≤ 10) were probed by two-color resonance ionization mass spectrometry-(1 + 1′) resonance-enhanced multiphonon ionization (REMPI)-via the intermediate B ²∑⁺ (v′ = 0, 1) rovibronic levels. Rotational constants for the B-state were derived, including the spin splitting parameters γ_v′, which have been determined in the case of CaD for the first time: γ₀ = -0.409(7) cm^-1 and γ₁ = -0.398(8) cm^-1 for CaD (B ²∑⁺), and γ₀ = -0.792(15) cm^-1 and γ₁ = -0.765(16) cm^-1 for CaH (B ²∑⁺). The comparison between the rotational level energies of the F₁ and F₂ spin substates of the two isotopomers revealed the onset of perturbation in CaH B ²∑ ⁺(v′ = 0), at N′ ≃ 0, by the presence of the vibronic state A ²∏ (v′ = 1). Although this perturbation is documented for v′ ≥ 1, it had not been previously recognized for v′ = 0. The rotational populations observed in X ²∑ ⁺(v″ = 0) for the two isotopomers could be associated with different average rotational temperature values for CaD and CaH: T _rot = 52 (± 7) and T_rot = 85 (± 11) K, respectively.