Lamb-peak spectrum of the HD (2-0) P(1) line

Abstract

A saturation spectroscopy measurement of the P(1) line of the (2-0) band in HD is performed in a sensitive cavity-enhanced optical setup involving frequency comb calibration. The spectral signature is that of a Lamb peak, in agreement with a density-matrix model description involving 9 hyperfine components and 16 crossover resonances of Λ type. Comparison of the experimental spectra with the simulations yields a rovibrational transition frequency at 209 784 242 007 (20) kHz. Agreement is found with a first-principles calculation in the framework of nonadiabatic quantum electrodynamics within 2σ, where the combined uncertainty is fully determined by theory.