Extended line list for the B2Σ+–X2Σ+ band system of CaF for use in laser-induced plasma studies

Abstract

Calcium monofluoride (CaF) is an important species for the sensitive detection of fluorine with laser-induced breakdown spectroscopy (LIBS). A line list for the B2Σ+–X2Σ+ transition system of CaF was calculated and extended from v ≤ 20 to v = 35 using empirical potential functions obtained through a direct-potential fit approach with observed transition frequencies. The line strengths were predicted using the scaled transition dipole moments calculated ab initio. The simulated intensities and transition positions with the present line list agree well with the spectra of high-resolution molecular spectroscopy from the literature and of LIBS from this work. Based on the simulated spectra of the CaF B–X system, the derived temperatures with the LIBS spectra vary across different wavelength regions. For example, with the same spectrum of delay time t = 13 μs, the obtained temperatures with λ ≈ 514–529 nm and λ ≈ 514–570 nm are 4000 and 5500 K, respectively. However, this discrepancy decreases with increasing delay time. For t = 23 μs, the obtained temperatures (3700 ± 200–4000 ± 100 K) are already in agreement within the estimated uncertainties. Self-absorption in the spectrum also influences the simulated temperature. Excluding the influences from self-absorption, the derived temperature (6000 ± 200 K) with the CaF molecular spectrum of λ ≈ 529–542 nm of t = 13 μs is close to the ionization temperature (6600 K) obtained with the Saha–Boltzmann fit from the observed atomic/ionic lines of Ca in the plasma. The present line list can be adopted in plasma diagnostics and in sensitive detections of fluorine with the CaF spectrum in hot environments.