Precision optical spectroscopy of hydrogen-like molecules
The physics of the simplest and at same time the most abundant molecule in the universe, namely molecular hydrogen, is attracting increasing attention as its simple structure allows for a full ab-initio computation of its rovibrational transition energies, including relativistic corrections and quantum-electro-dynamics contributions. Those calculations are predicted to infringe soon the 10-10 level of accuracy, which is equivalent to 10-6 cm-1 on the Q fundamental branch of H2 at around 4155 cm-1.
A major obstacle to experimentally reach similar accuracies and thus to challenge theory with experiments, is the inherent weakness of H2 absorption transitions, which are mediated by weak quadrupole moments due to the H2 symmetry. This has pushed the scientific community to primarily focus so far on transitions of overtone bands, lying in the near-infrared, where the high-quality of mirrors, lasers, modulators and detectors allows high-sensitivity absorption measurements in optical cavities. At IFN-Lecco we are pursuing the goal to address fundamental rovibrational and also rotational transitions by adoption of two different approaches.
Stimulated-Raman-Scattering metrology. This approach involves observing the target transitions in a nonlinear fashion, by adoption of pump and Stokes laser fields whose frequency detuning matches the vibrational resonance. For the first time this approach is applied in combination with an optical frequency comb to achieve absolute calibration of the frequency axis. The experimental setup measures the Stimulated Raman Loss induced by an intense Stokes laser at 1064 nm on hydrogen molecules housed in 30-m-long multipass cell to enhance the signal.
The detection is shot-noise limited and comb-referred, while experimental spectra are fitted with sophisticated line shape models relying on ab-initio quantum-scattering calculations of collisional parameters. In a recent achievement we showed this approach to be able to reach a combined uncertainty of 1.0·10-5 cm-1 (310 kHz) on the Q(1) line of the 1-0 band of H2 at ≈ 4155 cm-1, improving by 20 times the experimental benchmark and by a factor of 2 the theoretical benchmark. Measurements are planned on other lines of the 1-0 band of H2 as well as on purely rotational S-type transitions.
Cavity-ring-down spectroscopy. Cavity-ring-down-spectroscopy is a traditional linear spectroscopy approach where the intra-cavity absorption of a gaseous species is extracted from the measurement of the cavity photon lifetime. After a first demonstration, a few years ago, of a near-infrared apparatus operated in a frequency-agile rapid-scanning (FARS) scheme, we are now working at its extension to the mid-infrared by exploiting a difference-frequency-generation process.
This involves the nonlinear interaction between a powerful 1064-nm pump laser and an electro-optically tunable signal laser in the telecom region. An optical frequency comb ensures frequency calibration to the pump and signal lasers and thus to the nonlinearly generated mid-infrared beam coupled into the high-finesse optical cavity. The system is planned to be applied to the investigation of several lines od the D2 and HD hydrogen isotopologues that lie in the 3-3.5 µm region.
