Comb-referenced Doppler-free spectrometry of the $^{200}$Hg and $^{202}$Hg intercombination line at 254 nm

TL;DR

This paper demonstrates high-precision Doppler-free spectroscopy of mercury isotopes at 254 nm using a frequency comb, achieving unprecedented accuracy in line center frequencies and isotope shift measurements.

Contribution

It introduces a novel wavelength-modulated, saturated absorption technique for sub-Doppler spectroscopy in the deep UV with improved accuracy and reduced uncertainties.

Findings

Measured absolute line center frequencies with 8 and 15 kHz uncertainty.

Achieved the most accurate isotope shift measurement of 5,295,570 ± 15 kHz.

Reduced statistical and systematic uncertainties compared to previous studies.

Abstract

We report on precision spectroscopy of the 6s$^2$ $^1$S$_0\to$6s6p $^3$P$_1$ intercombination line of mercury in the deep ultraviolet, by means of a frequency-comb referenced, wavelength-modulated, saturated absorption technique. This method allowed us to perform sub-Doppler investigations with an absolute frequency axis at 254 nm, while ensuring a relatively high signal-to-noise ratio. The absolute line center frequencies of the $^{200}$Hg and $^{202}$Hg bosonic isotopes were measured with a global uncertainty of 8 and 15 kHz (namely, 6.8$\times$10$^{-12}$ and 1.3$\times$10$^{-11}$, in relative terms), respectively, the statistical and systematic components being significantly reduced as compared to past determinations. This remarkable result was achieved also thanks to an in-depth study of the AC stark effect. Furthermore, we found the most accurate $^{200}$Hg-$^{202}$Hg isotope shift ever obtained before, namely, $5295570\pm15_{stat}\pm8_{syst}$ kHz.