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

Abstract

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]