INTRODUCTION
Our proposal is to measure the frequency of a suitable molecular ro-vibrational transition relative to the clock hyperfine transition in the Cs electronic ground state. Then, the following relationship between the time evolution of our observable and that of β is derived
where S denotes the molecular species, μB the Bohr magneton and μCs the magnetic dipole of the Cs nucleus. In combination with the current constraints on the temporal variance of α and μCs/μB, as inferred from precise atomic clock frequency comparisons, the above equation can be used to set an upper limit to the temporal variation of β.
CAVITY RING-DOWN SPECTROSCOPY OF A BUFFER-GAS-COOLED ACETYLENE BEAM
We demonstrate continuous-wave cavity ring-down spectroscopy of a partially hydrodynamic molecular beam emerging from a buffer-gas-cooling source. Specifically, the (v1+v3) vibrational overtone band of acetylene (C2H2) around 1.5 micron is accessed using a narrow-linewidth diode laser stabilized against a GPS-disciplined rubidium clock via an optical frequency comb synthesizer. As an example, the absolute frequency of the R(1) component is measured with a fractional accuracy of 1x10e-9. Our approach represents the first step towards the extension of more sophisticated cavity-enhanced interrogation schemes, including saturated absorption cavity ring-down or two-photon excitation, to buffer-gas-cooled molecular beams.
In the near future, further developments can come from the implementation of other cavity-enhanced advanced interrogation schemes, like direct frequency comb spectroscopy. As for metrology applications, a state-of-the-art optical frequency standard, as delivered by an actively stabilized fiber link, is being finalized; its combination with the present BGC setup, together with an ultra-high-resolution spectroscopic interrogation technique, may produce new sets of ultra-precise frequency measurements on cold molecules at the electron volt energy scale.
DOPPLER-FREE SATURATED ABSORPTION SPECTROSCOPY OF FLUOROFORM
We report on absolute measurements of saturated absorption line-center frequencies of room-temperature trifluoromethane using a quantum cascade laser at 8.6 μm and the frequency modulation spectroscopy method. Absolute calibration of the laser frequency is obtained by direct comparison with a mid-infrared optical frequency comb synthesizer referenced to a radio-frequency Rb standard. Several sub-Doppler transitions falling in the v5 vibrational band are investigated at around 1158.9 cm−1 with a fractional frequency precision of 8.6x10e−12 at 1-s integration time, limited by the Rb-clock stability. The demonstrated frequency uncertainty of 6.6x10e−11 is mainly limited by the reproducibility of the frequency measurements.
Further improvements in the frequency measurements can be obtained using a 77-K hermetic gas cell operating at a pressure of 1 Pa or even lower. The method above proposed can be immediately extended to realize a compact and fully transportable
molecular-gas-cell optical frequency standard in the mid-infrared with a potential accuracy at the 10e−12 level.
REMPI-ASSISTED ABSORPTION SPECTROSCOPY OF METASTABLE CARBON MONOXIDE
High-resolution spectroscopy in the 1–10 μm region has never been fully tackled for the lack of widely tunable and practical light sources. Indeed, all solutions proposed thus far suffer from at least one of three issues: they are feasible only in a narrow spectral range; the power available for spectroscopy is limited; the frequency accuracy is poor. Here, we present a setup for high-resolution spectroscopy, whose approach can be applied in the whole 1–10 μm range. It combines the power of quantum cascade lasers (QCLs) and the accuracy achievable by difference frequency generation using an orientation patterned GaP crystal. The frequency is measured against a primary frequency standard using the Italian metrological fibre link network. We demonstrate the performance of the setup by measuring a vibrational transition in a highly-excited metastable state of CO around 6 μm with 11 digits of precision.
It has been suggested that the simultaneous monitoring of atomic and molecular transitions in various laboratories around the world can shed light on topological defect dark matter. The fibre link network that is being developed on European scale, together with methods like the one presented here can contribute to such investigations. Moreover, the fibre network together with state-of-the-art mid IR photonics and innovative techniques of molecular beam manipulation brings atomic-level accuracies within reach. In particular, the 10e−16 accuracy allowed by the fibre link can become the standard accuracy for measurements in the mid IR (10e13 Hz), when molecular cooling techniques will allow for 1-second interaction times.