Fourier transform emission spectroscopy and ab initio calculations on the visible spectrum of AlD+

The spectrum of the AlD+ isotopologue has been investigated at high resolution in the 27, 000 − 29, 000 cm−1 region using a Fourier transform emission spectroscopy technique. The AlD+ molecules were produced within a water−cooled aluminum hollow−cathode lamp in the presence of 1.5 Torr of Ne and 0.8 Torr of ND3. The (0, 0) and (1, 1) bands belonging to the A 2Π − X 2Σ +system were recorded with an instrumental resolution of 0.03 cm−1 . In total, almost 500 rotational frequencies were measured with an absolute accuracy of about 0.005 cm−1 . It improved the experimental accuracy of the determined frequencies by the factor 10 compared to the previous work [J Phys B: Mol Phys 1984;17:L861-L866]. The rotational analysis has shown irregularities in the Λ− doubling splitting of the A2Π v = 0, 1. Consequently, the A2Π state has been represented by the rotational term values, while the regular X2Σ + state by the molecular constants. The causes of the irregularities were identified in the interaction between the A2Π and the B2Σ + states, which lies about 3720 cm−1 above. Supporting ab initio quantum chemical calculations, including spin−orbit effects, reproduce the observed spectroscopic constants including the small energy splittings due to spin−rotation interactions (for 2Σ + states) and Λ−doubling.