Measurement of neutral gadolinium oscillator strengths using dual-comb absorption spectroscopy in laser-produced plasmas

Abstract

The oscillator strengths of 43 transitions of neutral gadolinium from 530 to 535 nm were determined based on high-resolution absorption spectra measured by probing a laser-produced plasma using time-resolved dual-comb spectroscopy. Absorption spectra were measured at various time delays, ranging from 33 to 252 μs after the onset of the plasma formation, over which the temperature and number density varied as the ablation plume evolved. A Boltzmann analysis was used to determine the excitation temperature and column density of Gd in the ablation plume, based on measured absorption from 20 Gd I spectral lines with known oscillator strengths. Oscillator strengths were then determined for additional dipole-allowed Gd I transitions identified in the absorption spectra, 19 of which were previously unreported. Oscillator strengths of all measured transitions were compared with literature values when available and show good agreement in most cases. Time-resolved measurement of atomic absorption in the cooling plasma provides access to a range of excitation temperatures and optical densities for the atoms in the ablation plume, allowing repeated measurements under different conditions, thereby improving confidence in the results. Our results also highlight that the high spectral bandwidth and resolution capabilities of dual-comb spectroscopy make it well-suited to measuring the dense optical spectrum of Gd, and the technique is applicable to other elements, especially lanthanides and actinides.