High-resolution electronic spectra of yttrium oxide (YO): The D²Σ⁺-X²Σ⁺ transition

The D²Σ⁺-X²Σ⁺ electronic absorption spectrum of the astrophysically relevant yttrium oxide (YO) molecule has been recorded for the first time in the 400-440 nm region using laser induced fluorescence. YO molecules are produced by corona discharge of oxygen between the tips of two yttrium needles in a supersonic jet expansion. An unambiguous spectroscopic identification of the D²Σ⁺-X²Σ⁺ transition becomes possible from a combined analysis of the moderate-resolution laser excitation spectrum and dispersed fluorescence spectrum. We have also performed multi-state complete active space second order perturbation theory calculations on the first six doublets of YO, and the results support our assignment of the D²Σ⁺ state. Accurate spectroscopic constants for D²Σ⁺ν' = 0 and 1 levels have been determined from a rotational analysis of the high resolution spectra that are recorded with a resolution of ∼0.018 cm^-1. Severe perturbations are observed in the experimental spectra and are considered to originate from interactions with at least one nearby ^2/4Π electronic state, e.g., the undetected C²Π state. We have also measured the radiative lifetimes of B²Σ⁺ν' = 0, and D²Σ⁺ν' = 0 and 1 states, based on which the B²Σ⁺-X²Σ⁺ (0, 0) and D²Σ⁺-X²Σ⁺ (0/1, 0) band oscillator strengths have been determined.