Rotational energy levels in the ground vibrational state of methane with kHz-level accuracy from comb-referenced double-resonance and Lamb-dip spectroscopies

TL;DR

This paper achieves kHz-level precision in measuring methane's ground state rotational energy levels using advanced comb-referenced spectroscopic techniques, overcoming previous limitations due to selection rules.

Contribution

It introduces two frequency-comb-referenced sub-Doppler methods to accurately determine methane's ground state energies, including forbidden transitions.

Findings

Ground state rotational energy levels determined with kHz accuracy

Hamiltonian fit yields term values up to J=12

Both allowed and forbidden transitions measured

Abstract

Methane is a key spherical-top molecule, yet restrictive selection rules for one-photon transitions have prevented determination of its ground state (GS) energies with state-of-the-art kHz-level accuracy. We report the GS rotational energy level differences with kHz-level accuracy from two frequency-comb-referenced sub-Doppler methods: optical-optical double-resonance spectroscopy in the ${\Lambda}$-type configuration, and Lamb-dip spectroscopy of allowed and forbidden transitions. A Hamiltonian fit to the data yields GS term values with rotational numbers up to $\it{J}$ = 12 with kHz level accuracy.