Doppler-free ablation fluorescence spectroscopy of Ca for high-resolution remote isotopic analysis

Abstract

To develop remote isotopic analysis for the nuclides with small isotope shifts, Doppler-free fluorescence spectroscopy of Ca was performed using laser ablation plumes. Counter-propagating laser beams from two external cavity diode lasers were used to irradiate the plume in order to excite the ground-state Ca atoms to the ¹D₂ state through a double resonance scheme of ¹S₀ → ¹P₁ → ¹D₂. Subsequently, we measured fluorescence spectra associated with the relaxation from the ¹D₂ to ¹P₁ states. The linewidth measured at 1 ms delay after ablation under helium gas pressure of 70 Pa was found to be less than 70 MHz, which was about 1/30 of the linewidth of the Doppler-limited fluorescence spectrum. A broad Gaussian pedestal was observed at less than 600 μs delay in the temporal variation in fluorescence spectra, and it was most likely due to the velocity-changing collision. Additionally, the pressure broadening rate coefficient for the second-step ¹P₁ → ¹D₂ transition was determined to be 46.0 MHz per torr from the spectra measured under various gas pressures. We evaluated analytical performances such as linearity of the calibration curve, limit of detection, and measurement accuracy using fluorescence signals of three naturally occurring Ca isotopes (i.e., ⁴⁰Ca, ⁴²Ca, and ⁴⁴Ca). The limit of detection of isotopic abundance was estimated to be 0.09% from the 3σ criteria of the background. These results suggest that this spectroscopic technique is promising for remote isotopic analysis of nuclides with small isotope shifts.