Unmasking blended optogalvanic lines: composite spectrum center-of-gravity metrology with a Monte–Carlo uncertainty chain

Optogalvanic (OG) spectra of heavy atoms frequently manifest as blended clusters of isotopic and hyperfine components. We present an analysis chain that (i) fits a physics-aware composite Voigt model, (ii) defines the reported line center as the cluster’s center-of-gravity (COG), and (iii) propagates calibration and dynamic effects together with fit statistics by a hierarchical Monte–Carlo (MC). An effective two-pole RC model quantifies bias from the combination of laser scan and lock-in detection. When applied to the $$^{9}\textrm{D}_{4}\rightarrow {}^{11}\textrm{F}_{3}$$ spin-forbidden transition of neutral Gd near 726 nm, the method yields a corrected COG with combined standard uncertainty at the tens-of-megahertz level despite sub-200 kHz statistical fit noise. The workflow is compact, reproducible, and portable to other OG/hollow-cathode spectra.