Hydrogen (H2, HD and D2)

In total, only 39 H₂ transitions have been reported in the literature from laboratory measurements. These transitions are of strong astrophysical interest and hydrogen and its isotopologues are test species for the most advanced ab initio calculations. Indeed, the hydrogen molecule is probably a unique case of rovibrational spectroscopy, in the sense that the quality of the ab initio calculations is so good that the choice between theoretical values and experimental determinations (if available) deserves to be examined.

A fruitful collaboration has been initiated with two Polish theorists K. Pachucki (Warsaw) and J. Komasa (Poznan) who have performed the most advanced ab initio calculations for hydrogen. Their calculations taking into account non-adiabatic effects, relativistic corrections and quantum electrodynamics, have been compared with our CW-CRDS measurements. In our accessible range with DFB diodes, we have detected H2, HD and D2 transitions which are the lowest measured to date. Our measurements confirmed the value of the calculated positions and intensities to the experimental accuracy (0.001 cm^-1 and 2% respectively).

The ab initio calculations thus validated, we published for each species (H₂, HD and D₂) the corresponding calculated line list as shown on the figure above for H₂. Such lists in the HITRAN database format were not available until then. They have been adopted in the 2012 update of the HITRAN database.

Our publication on the subject :

D. Mondelain, S. Kassi, T. Sala, D. Romanini, D. Gatti, A. Campargue, "Sub-MHz accuracy measurement of the S(2) 2–0 transition frequency of D2 by Comb-Assisted Cavity Ring Down spectroscopy", Journal of Molecular Spectroscopy, Elsevier, 2016, 326, pp.5 - 8. ⟨10.1016/j.jms.2016.02.008⟩

S. Vasilchenko, D. Mondelain, S. Kassi, P. Čermák, B. Chomet, A. Garnache, S. Denet, V. Lecocq, A. Campargue, "The HD spectrum near 2.3μm by CRDS-VECSEL: Electric quadrupole transition and collision-induced absorption", Journal of Molecular Spectroscopy, Elsevier, 2016, 326, pp.9 - 16. ⟨10.1016/j.jms.2016.04.002⟩

Piotr Wcislo, Ha Tran, Samir Kassi, Alain Campargue, Franck Thibault, Roman Ciurylo, "Velocity-changing collisions in pure H2 and H2-Ar mixture", Journal of Chemical Physics, American Institute of Physics, 2014, 141, pp.074301. ⟨10.1063/1.4892414⟩

Samir Kassi, Iouli E. Gordon, Alain Campargue, "First detection of transitions in the second quadrupole overtone band of nitrogen near 1.44 μm by CW-CRDS with 6 × 10-13 cm-1 sensitivity", Chemical Physics Letters, Elsevier, 2013, 582, pp.6-9. ⟨10.1016/J.CPLETT.2013.07.031⟩