Precision spectroscopy with cold stable molecules
Thanks to the advent of optical frequency comb (OFC) synthesizers based on femto-second mode-locked lasers, the field of precision spectroscopy is experiencing an extraordinary growth. This has allowed to devise more and more challenging experiments aiming at testing fundamental physics laws with unprecedented sensitivity. Since the ultimate resolution attainable in any spectroscopic measurement is limited by the interaction time between the particle under investigation with the radiation field, such experiments would draw enormous benefit from the ability of interrogating extremely slow molecules, as produced by the emerging cooling/trapping techniques. For this purpose, based on a two-stage pulse tube cryo-cooler, we have realized a buffer-gas-cooling machine where the molecular species of interest is brought to translational and rotational temperatures near 1 K via collisions with a helium buffer gas in a cryogenic cell. Then, by expansion in a high vacuum, a decelerated molecular beam is formed, subsequently further collimated by means of an electrostatic hexapole lens. In parallel, a second, chip-based source of Stark-decelerated molecular beams is under construction. For the spectroscopic interrogation of such samples, excitation of two-photon Ramsey fringes will be used, where an ultra-narrow-linewidth mid-infrared probe laser is phase-locked to a specially-developed OFC that is ultimately referenced to the Cs primary standard via the National Optical Fiber Link. In this frame, we envisage a new generation of low-energy tests of the Standard Model (time variation of fundamental constants and axion dark matter detection) as well as of high-precision studies of astrophysical phenomena on a laboratory scale.
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 buffergas-
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.