Lamb-dip spectroscopy of buffer-gas-cooled molecules

Abstract

Nowadays, buffer-gas cooling represents an invaluable option to produce cold stable molecules, both in view of secondary cooling/trapping strategies towards the achievement of quantum degeneracy and for fundamental studies of complex molecules. From this follows a demand to establish a pool of specialized, increasingly precise spectroscopic interrogation techniques. Here, we demonstrate a general approach to Lamb-dip ro-vibrational spectroscopy of buffer-gas-cooled molecules. The saturation intensity of the selected molecular transition is achieved by coupling the probe laser to a high-finesse optical cavity surrounding the cold sample. A cavity ring-down technique is then implemented to perform saturation sub-Doppler measurements as the buffer (He) and molecular gas flux are varied. As an example, the (ν1+ν3) R(1) ro-vibrational line in a 20 Kelvin acetylene sample is addressed. By referencing the probe laser to a Rb/GPS clock, the corresponding line-center frequency as well as the self and foreign (i.e., due to the buffer gas) collisional broadening coefficients are absolutely determined. Our approach represents an important step towards the development of a novel method to perform ultra-precise ro-vibrational spectroscopy on an extremely wide range of cold molecules. In this respect, we finally discuss a number of relevant upgrades underway in the experimental setup to considerably improve the ultimate spectroscopic performance.