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Dipartimento di Fisica - Politecnico di Milano

Coherent Raman metrology of molecular hydrogen (CHROME)

The challenge

The CH2ROME laboratory is devoted to the experimental study of the physics of the simplest and at same time the most abundant molecule in the universe, namely molecular hydrogen. H2 is a unique benchmark for molecular quantum physics 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. The experimental state of the art does not go beyond 2 10-4 cm-1: this implies a gap of almost two orders of magnitude to be filled before setting up the most stringent QED tests ever performed on a molecular system. A major obstacle is the inherent weakness of H2 absorption transitions, which are mediated by weak quadrupole moments due to the H2 symmetry.


The methodology
In the CH2ROME laboratory we address this challenge with a paradigmatic shift at a methodological level, with those transitions no longer linearly observed in an absorption regime, yet in a nonlinear way through a Coherent Raman Spectroscopy approach, for the first time in combination with Optical Frequency Combs to achieve absolute calibration of the frequency axis. The experimental setup developed for such a challenge is unique in the world: it is composed by two cw lasers providing the pump and Stokes fields needed for the Stimulated Raman Scattering process, an Er:fiber optical frequency comb for the absolute measurement of the frequency detuning between pump and Stokes, a dedicated FPGA detection chain at 100 MS/s for the real-time tracking of the beat note between the cw-lasers and the comb, a temperature-stabilized multipass cell operating in a large pressure range to host the gas and allow for an enhanced interaction length. an electro-optic modulator driven by a 10 MHz radio-frequency generator (RFG) for synchronous detection of the SRS signal, acousto-optic- modulators for the intensity stabilization of pump and Stokes beams.
Measurements are ongoing on the Q lines of the 1-0 band of H2, but other measurements are planned for purely rotational S-type transitions. Experimental spectra are fitted with sophisticated line shape models that take into account the complex collisional physics of H2.