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Molecular hydrogen has evolved into a benchmark quantum test system for fundamental physics. Recent independent sub-Doppler determinations [1,2] of the weak dipole R(1) transition in the (2,0) overtone band of HD at λ∼1.38 μm yield a discrepancy of 900 kHz or 9σ in combined uncertainty. We present measurements using Noise-Immune Cavity-Enhanced Optical Heterodyne Molecular Spectroscopy (NICE-OHMS), where the laser is locked to a Cs absolute frequency standard via an optical frequency comb [1]. The obtained saturation spectrum is found to exhibit a composite and pressure-dependent line shape, involving a Lamb-dip and a Lamb-peak. We propose an explanation of this behavior based on the effects of crossover saturation resonances in the hyperfine structure, which is made quantitative with a density matrix calculation. We expect to resolve the outstanding discrepancy between [1] and [2], which will enable the comparison of the measured R(1) transition frequency with the latest theoretical calculations [3].
[1] F. M. J. Cozijn, P. Dupré, E. J. Salumbides, K. S. E. Eikema, and W. Ubachs, Phys. Rev. Lett. 120, 153002 (2018).
[2] L.-G. Tao, A.-W. Liu, K. Pachucki, J. Komasa, Y. R. Sun, J. Wang, and S.-M. Hu, Phys. Rev. Lett. 120, 153001 (2018).
[3] P. Czachorowski, M. Puchalski, J. Komasa, and K. Pachucki, Phys. Rev. A 98, 052506 (2018).
