Li I spectra in the 4.65–8.33 micron range: high-Lstates and oscillator strengths

Abstract

Context. Infrared (IR) astronomy capacities have rapidly developed in recent years thanks to several ground- and space-based facilities. To take advantage of these capabilities efficiently, a large amount of atomic data (such as line wavenumber, excited-level energy values, and oscillator strengths) are needed. These data are incomplete, in particular, for lithium whose abundances are important for several astrophysical problems. Aims. No laboratory-measured spectra of Li I have been reported for wavelengths longward of 6.6 microns. We aim to find new Li I lines in the 4.65–8.33 microns range due to transitions between states with high orbital momentum (l ≥ 4) and to determine the excitation energies of these states. Methods. The Li I lines were studied using the time-resolved Fourier transform infrared spectroscopy of a plasma created by the laser ablation of a LiF target in a vacuum. The classification of the lines was performed by accounting for oscillator strengths (f -values) calculated using quantum defect theory (QDT). The adequacy of QDT for these calculations was checked by comparison with the available experimental and theoretical results. Results. We report four new Li I lines in the 900–2200 cm –1 range that allow us to extract the excitation energies of the 6g, 6h, and 7h states of Li I, which have not been measured before. We also provide a large list of QDT-calculated f -values for Li I in the range of 1–20 microns.