Lamb dip saturated cavity ring-down spectroscopy of gadolinium isotopes in a DC glow discharge

We demonstrate a cavity ring-down spectroscopy (CRDS) system using a direct-current (DC) hollow-cathode glow discharge plasma as an atomic source for isotope-resolved spectroscopy of gadolinium (Gd). This DC glow discharge CRDS provides a compact and simple alternative to conventional inductively coupled plasma (ICP)-based CRDS, while maintaining sufficient stability for high-sensitivity measurements. Saturation effects, more pronounced in atomic than molecular CRDS due to large absorption cross sections, were captured using a saturation model applied to ring-down transients. Saturated CRDS enabled clear observation of Doppler-free lamb dips in Gd spectra, highlighting its capability to resolve narrow spectral features. Combined analysis of Doppler-broadened absorption and lamb dip components allowed estimation of isotopic ratios for the even Gd isotopes, consistent with natural abundances, except for ¹⁵⁵Gd and ¹⁵⁷Gd. These results demonstrate the feasibility of applying saturated CRDS to precise isotope ratio measurements of rare-earth elements, and suggest the potential applicability of the technique to other metal atoms, including uranium, plutonium, and other transuranic elements.