The 5d states of barium hydride; BaH and BaD

Abstract

The A′²Δ, A ²Π and B ²Σ⁺ states of BaH and BaD which correlate with Ba …6s5d ³D have been observed in infra-red laser-induced fluorescence from E ²Π and in thermal emission to the ground state, X ²Σ⁺. The spectra were recorded interferometrically using a high resolution Fourier transform spectrometer. Observations include v = 0 and 1 of the complex, both for BaH and BaD. A global analysis of these levels is given in terms of an effective hamiltonian for a d-complex. The observed interactions require a 15 × 15 representation of the complex of BaH, with the use of experimental data for v = 2 taken from the work of other authors. However an independent 5 × 5 representation of each vibrational level is shown to describe the BaD spectrum with very good accuracy, in particular the complicated local A′²Δ ∼ A ²Π perturbations. The spectra of both molecules are reproduced to within the experimental precision. Deperturbed energies and rotational constants, and values of the interaction constants are given. A consistent set of molecular constants is also reported for the X ²Σ⁺ and E ²Π states of BaH and BaD, and for the perturbing level v = 5 in B ²Σ⁺ of BaD. A simple analysis of the interaction constants indicates that A′²Δ is essentially a covalent state which correlates with Ba (6sσ5dδ), and that A ²Π is largely, ∼ 80 per cent, covalent (6sσ5dπ). The results do not allow an estimate of the configuration of B ²Σ⁺. Some rotational structure of the 0-0 bands of the forbidden systems, A′²Δ-X ²Σ⁺, of BaH and BaD are included in the overall fit: it is unusual in that the bands seem to be limited to only three branches, ^Q R ₁₂, ^P Q ₁₂ and ^R Q ₂₁.