Na I spectra in the 1.4–14 micron range: transitions and oscillator strengths involving f-, g-, and h-states

Abstract

Context. Compared with the visible and ultraviolet ranges, fewer atomic and ionic lines are available in the infrared spectral region. Atlases of stellar spectra often provide only a short list of identified lines, and modern laboratory-based spectral features for wavelengths longer than 1 micron are not available for most elements. For the efficient use of the growing capabilities of infrared (IR) astronomy, detailed spectroscopical information on atomic line features in the IR region is needed. Aims. Parts of the infrared stellar (e.g., solar) spectra in the 1200–1800 cm-1 (5.6–8 μ m) range have never been observed from the ground because of heavy contamination of the spectrum by telluric absorption lines. Such an infrared spectrum represents a great challenge for laboratory observations of new, unknown infrared atomic transitions involving the atomic levels with high orbital momentum and their comparison with the available spectra. Methods. The vapors of excited Na i atoms are produced during the ablation of the salt (sodium iodide, Na i) targets by a high-repetition rate (1.0 kHz) pulsed nanosecond ArF laser ExciStar S-Industrial V2.0 1000, pulse length 12 ns, λ = 193 nm, output energy of 15 mJ, fluence about 2–20 J/cm2 inside a vacuum chamber (average pressure 10-2 Torr). The time-resolved emission spectrum of the neutral atomic potassium (Na i) was recorded in the 700–7000 cm-1 region using the Fourier transform infrared spectroscopy technique with a resolution of 0.02 cm-1 . The f -values calculated in the quantum-defect theory approximation are presented for the transitions involving the reported Na i levels. Results. This study reports precision laboratory measurements for 26 Na i lines in the range of 700–7000 cm-1 (14–1.4 μ m), including 20 lines not measured previously in the laboratory. This results in newly observed 7h, 6h, and 6g levels, and improved energy determination for ten previously known levels. The doublet structure of the 4f level has been observed for the first time. For transitions between the observed levels, we report calculated f -values that agree reasonably well with experiment.Conclusions. The recorded Na i line features agree with the data from the available solar spectrum atlases. The energy values of Na i 4s, 4p, 5p, 6p, 4f, 5f, and 5g levels extracted from our spectra have lower uncertainties as compared to the values reported several decades ago, but the latter values slightly differ from ours.