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Dipartimento di Fisica - Politecnico di Milano

Coherent Raman metrology of molecular hydrogen (CHROME)

The challenge

The main focus of the CH2ROME laboratory is 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. The state-of-art accuracy of these calculations is of few parts per billion, corresponding to the sub-MHz level for rovibrational lines that belong to the fundamental 1-0 band. It was only recently that experiments have been able to challenge this theoretical benchmark (https://arxiv.org/abs/2207.03998): this happened in our CH2ROME laboratory, where the transition frequency of the Q(1) H2 line at ≈ 4155 cm-1 was determined with a combined uncertainty of 1.0·10-5 cm-1 (310 kHz), improving by 20 times the previous experimental benchmark and providing one of the most stringent QED tests ever performed on a molecule.

 

The methodology

We address the above challenge by replacing absorption spectroscopy with a nonlinear approach based on Coherent Raman Spectroscopy (CRS). This allows to overcome the intrinsic weakness of quadrupole rovibrational transitions, the only ones allowed in homonuclear species such as H2.  For the first time CRS measurements are performed in combination with Optical Frequency Combs to achieve absolute calibration of the frequency axis and a final frequency accuracy of 50 kHz (sub-part-per-billion level). Over the vertical axis the spectrometer benefits from shot-noise limited detection, signal enhancement via multipass cell, active flattening of the spectral baseline and measurement times of few seconds over spectral spans larger than 10 GHz. Under these conditions an efficient averaging of Raman spectra is possible over long measurement times with minimal distortion of spectral lineshapes. 

 

Current results and perspectives

We recently measured the transition frequency of the Q(1) fundamental line of H2 around 4155 cm-1 with few parts-per-billion uncertainty, which is comparable to the theoretical benchmark of ab initio calculations and more than a decade better than the experimental state of the art. Measurements are now ongoing on other Q and S lines of the 1-0 band of H2, also at different temperatures to reinforce the robustness of the global fitting procedure of the measured spectra used to extrapolate transition center frequencies at zero pressure. As an outlook, we are going to i) equip the spectrometer with different pump lasers to be able to address a two decades-spanning frequency range, from 50 to 5000 cm-1, that covers all fundamental rovibrational bands as well as purely rotational bands, ii) perform metrology of many other Infrared-inactive rotational and rovibrational lines, also of other molecular species such CO2, N2, C6H6, iii) reduce the uncertainty budget through nonlinear signal enhancement, up to a factor of 100, by replacement of the multi-pass cell with hollow-core photonic crystal fibers that offer much narrower laser beam cross-sections.