Rotational level spacings in HD from vibrational saturation spectroscopy

TL;DR

This study measures high-precision rotational and vibrational transition frequencies in HD using saturation spectroscopy, revealing that experimental spacings are slightly larger than theoretical predictions, thus providing valuable data for molecular physics.

Contribution

First high-precision saturation spectroscopy measurements of specific HD ro-vibrational transitions, including the previously unobserved P(3) line, with implications for testing molecular theory.

Findings

Measured rotational level spacings with high accuracy

Observed systematic deviations from theoretical calculations

Provided new frequency data for HD vibrational transitions

Abstract

The R(1), R(3) and P(3) ro-vibrational transitions in the (2-0) overtone band of the HD molecule are measured in Doppler-free saturation using the technique of NICE-OHMS spectroscopy. For the P(3) line, hitherto not observed in saturation, we report a frequency of $203\,821\,936\,805\,(60)$ kHz. The dispersive line shapes observed in the three spectra show strong correlations, allowing for extraction of accurate information on rotational level spacings. This leads to level spacings of $\Delta_{(J=3)-(J=1)}= 13\,283\,245\,098\,(30)$ kHz in the $v=0$ ground state, and $\Delta_{(J=4)-(J=2)}= 16\,882\,368\,179\,(20)$ kHz in the $v=2$ excited vibration in HD. These results show that experimental values for the rotational spacings are consistently larger than those obtained with advanced ab initio theoretical calculations at 1.5$\sigma$, where the uncertainty is determined by theory. The same holds for the vibrational transitions where systematic deviations of 1.7-1.9$\sigma$ are consistently found for the five lines accurately measured in the (2-0) band.