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.