Dispersion-like lineshape observed in cavity-enhanced saturation spectroscopy of HD at 1.4  µm

Abstract

Precision measurement of ro-vibrational transitions in the electronic ground state of the hydrogen molecule can be used to test quantum electrodynamics and also to determine the dimensionless proton-to-electron mass ratio. Saturation spectroscopy of the 2-0 overtone transitions of hydrogen deuterium (HD) were measured with three cavity-enhanced spectroscopy methods. With a sensitivity at the 10 − 13 c m − 1 level, we revealed a dispersion-like lineshape instead of a conventional Lamb “dip,” which explains the significant discrepancy among previous independent measurements. The spectra can be fit well by using the Fano profile. Centers of R(1) and R(3) lines were determined as 217 105 182 111 ( 19 ) s t a t ( 240 ) s y s k H z and 220 704 305 234 ( 20 ) s t a t ( 240 ) s y s k H z , respectively.

Rotational level spacings in HD from vibrational saturation spectroscopy

“…2. This phenomenon of observing unexpected atypical line shapes was discussed in previous papers on the saturation spectroscopy of HD, either using 1f -demodulation in NICE-OHMS [16,17], cavity-ringdown spectroscopy [15], or a variety of cavity-enhanced techniques [19]. Secondly, the experimental data show that the 2f -demodulation spectra exhibit a better signalto-noise ratio (SNR) than the 1f -demodulated spectra, while the 1f spectra display a much better SNR than the direct NICE-OHMS spectra.…”

“…In the Amsterdam laboratory subsequent measurements with the NICE-OHMS (Noise-Immune Cavity-Enhanced Optical-Heterodyne Molecular Spectroscopy) technique were carried out and the dispersive-like line shape was interpreted in terms of an Optical Bloch equation (OBE) model including underlying hyperfine structure and crossover resonances in the saturation spectrum [17,18]. The Hefei group extended their studies on the observation of a dispersive line shape, which was interpreted as a Fano line shape [19]. The uncertainties associated with the observed lineshape remain to be a dominating factor and hinder full exploitation of the extreme resolution of the saturation technique, and the determination of transition frequencies at the highest accuracy.…”

The (2–0) R(0) and R(1) transition frequencies of HD determined to a 10−10 relative accuracy by Doppler spectroscopy at 80 K

“…The R (1) transition, already accurately measured in ref. 15, 19, 25 and 26 was used to validate our measurement procedure. Eighty-three fourfold interlaced spectra were recorded at liquid nitrogen temperature with frequency step of 100 MHz (matching the CRDS cavity free spectral range) leading to a final 25 MHz step resolution.…”

“…This discrepancy and the unusual Lamb dip profile were examined in two subsequent studies led by the same groups. 16,26 In Hefei, an experimental study was performed with an enhanced signal-to-noise ratio and the use of different spectroscopic techniques confirming the dispersivelike shape, fitted with a Fano profile. 26 Physical mechanisms at the origin of this Fano-type profile was very recently discussed by the same group.…”

“…16,26 In Hefei, an experimental study was performed with an enhanced signal-to-noise ratio and the use of different spectroscopic techniques confirming the dispersivelike shape, fitted with a Fano profile. 26 Physical mechanisms at the origin of this Fano-type profile was very recently discussed by the same group. 27 For the Amsterdam group, the dip was fitted according to Bloch equations formalism and introducing crossovers transitions.…”