Measurement and calculation of CO (7-0) overtone line intensities

Intensities of 14 lines in the sixth overtone (7-0) band of carbon monoxide (12C16O) are measured in the visible range between 14 300 and 14 500 cm-1 using a frequency-stabilized cavity ring-down spectrometer. This is the first observation of such a high and weak overtone spectrum of the CO molecule. A theoretical model is constructed and tested based on the use of a high accuracy ab initio dipole moment curve and a semi-empirical potential energy curve. Accurate studies of high overtone transitions provide a challenge to both experiment and theory as the lines are very weak: below 2 × 10-29 cm molecule-1 at 296 K. Agreement between theory and experiment within the experimental uncertainty of a few percent is obtained. However, this agreement is only achieved after issues with the stability of the Davidson correction to the multi-reference configuration interaction calculations are addressed

Collisional effects in pure D2. Ultra accurate measurements and ab initio calculations

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CRDS measurements and ab initio calculations of collisional effects in pure D2

International audienceRecent progress in theoretical calculations of dissociation energies of H2, HD and D2 [1–2] gives predictions of the transition frequencies with uncertainty exceeding the level of 10-3 cm-1 for the first overtone band (2–0) [3]. Such predictions open a way for testing relativistic and quantum electrodynamics corrections. They give also the opportunity for searching for new physics like additional long-range hadron-hadron interactions [4]. At this level of accuracy the uncertainty of the H 2 (or its isotopologues) line position determination in Doppler limit becomes considerably affected by the line-shape effects [5] including asymmetry of the line shapes. Spectral line shapes of D2 transitions are atypical and difficult to model. First strategy for overcoming this problem is measuring the spectra at low pressures, where collisional effects are negligible [3]. However, it is experimentally challenging due to exceptionally low intensities of the quadrupole lines. Another approach is recording them at higher pressures and describe the collisions in a more sophisticated way. Here as an example of the second strategy, we present our preliminary results applied for very weak S(2) transition of deuterium in the 2-0 band, using ab initio calculations. Transition has been measured with the frequency-stabilized cavity ring-down spectroscopy (FS-CRDS) assisted by an optical-frequency comb [6,7], using experimental setup described in [8]. The line positions at high pressures, up to 1000 Torr, were measured with sub-MHz accuracy. Furthermore, to validate ab initio model, we extended our experiments to a wide range of temperatures. We compare it with ab initio quantum scattering calculations, where we obtain the generalized spectroscopic cross sections. The real and imaginary parts provide the speed-dependent collisional broadening γ(ν) and shifting δ(ν). The velocity-changing collisions, in turn, are described by hard-sphere approximation of the ab initio potential which is called the speed-dependent billiard-ball profile (SDBBP) [9]

Ultra accurate measurements and ab initio calculations of collisional effects in pure D2 .

International audienceWe present our experimental spectra of the very weak S(2) transition from the 2–0 band of molecular deuterium, measured with a frequency-stabilized cavity ring-down spectroscopy (FS–CRDS) assisted by the optical frequency comb (OFC). Experimental collisional broadening and shifting are compared with results of ab initio quantum scattering calculations

COMB-REFERENCED COHERENT RAMAN SPECTROSCOPY ON PURE H2

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Stimulated Raman scattering metrology of molecular hydrogen

International audienceFrequency combs have revolutionized optical frequency metrology, allowing one to determine highly accurate transition frequencies of a wealth of molecular species. These progresses have only marginally benefited infrared-inactive transitions, due to their inherently weak cross-sections. Here we overcome this limitation by introducing stimulated-Raman-scattering metrology, where a frequency comb is exploited to calibrate the frequency detuning between the pump and Stokes excitation lasers. We apply this approach to the investigation of molecular hydrogen, which is a recognized benchmark for tests of quantum electrodynamics and of theories that describe physics beyond the standard model. Specifically, we measure the transition frequency of the Q(1) fundamental line of H-2 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. Our comb-calibrated stimulated Raman scattering spectrometer extends the toolkit of optical frequency metrology as it can be applied, with simple technical changes, to many other infrared-inactive transitions, over a 50-5000 cm(-1) range that covers also purely rotational bands. Molecular hydrogen has a simple structure that makes it a unique benchmark for molecular quantum physics. The authors determined the transition energy of its fundamental Q(1) vibrational line with an unprecedented parts-per-billion accuracy by a novel spectrometer that combines Stimulated-Raman-Scattering with comb calibration of optical frequencies

Comb-calibrated coherent Raman spectroscopy of molecular hydrogen

International audienceHighly accurate measurements of H2 transition frequencies is fundamental for testing the quantum electrodynamics and physics beyond the standard model [1-3]. However, the retrieval of the un-perturbed line positions is very challenging since it compels to work in low pressure conditions: the achievement of high signal-to-noise ratios is then hindered by the weakness of quadrupole transition moments and by the low molecular density. Alternatively, the distortion of the line profile at higher pressure could be carefully modelled in order to compensate for speed-dependent collisional effects and for the strong Dicke narrowing. High accuracy measurements of the Q(1) transition of the pure H2 1-0 band at 4155.25 cm-1 have been performed from 0.2 to 5 atmosphere using stimulated Raman spectroscopy. An Er:fiber frequency comb has been used to calibrate the frequency difference between the pump and Stokes cw lasers involved in the Raman process. The pump laser emits at 737.8 nm and is kept fixed while the Stokes laser is scanned over 3 GHz around 1064 nm. The two beams are spatially superimposed and travel through a multipass cell filled with H2. Figure 1 (a) displays the line profiles measured at seven different pressures (the measurements at the two lowest pressures are displayed in the inset). As it can be noticed from panel (b) the retrieved widths are in a good agreement with ab-initio values based on H2-H2 quantum scattering calculations. The frequency shift, plotted in panel (c), is proportional to pressure above 1 atm and the retrieved pressure coefficient agrees well with previous results [4]. The strength of the approach which provides high signal-to-noise ratio and frequency accuracy at the same time enables the use of more advanced profile models, such as the Hartmann-Tran profile, for line shape investigation

Ultrahigh finesse cavity-enhanced spectroscopy for accurate tests of quantum electrodynamics for molecules

International audienceWe report the most accurate, to the best of our knowledge, measurement of the position of the weak quadrupole S(2) 2–0 line in ${{\rm D}_2}$D2. The spectra were collected with a frequency-stabilized cavity ringdown spectrometer (FS-CRDS) with an ultrahigh finesse optical cavity (${\cal F} = 637 000$F=637000) and operating in the frequency-agile, rapid scanning spectroscopy (FARS) mode. Despite working in the Doppler-limited regime, we reached 40 kHz of statistical uncertainty and 161 kHz of absolute accuracy, achieving the highest accuracy for homonuclear isotopologues of molecular hydrogen. The accuracy of our measurement corresponds to the fifth significant digit of the leading term in quantum electrodynamics (QED) correction. We observe $2.3\sigma$2.3σ discrepancy with the recent theoretical value