Line shape parameters for the first rotational lines of HD in helium

Publication in "molecular astrophysics" by Franck Thibault, Raúl Z Martínez, Dionisio Bermejo and Piotr Wcislo.

The deuterium hydride molecule is of great importance for astrophysics and planetology.
The measurement of the D/H ratio is fundamental for understanding the evolution of the Universe
from the standard Big Bang nucleosynthesis as well as for understanding the delivery
of water to Earth’s oceans. Since molecular hydrogen is the major constituent
element of the giant planets the relative HD abundance is well suited to determine
this ratio. Compared to H2 which has no electric dipole moment in its ground electronic state,
HD molecule is the simplest heteronuclear system that possesses a (weak) dipole moment and
is thus optically active in the far IR. The helium atom is the most abundant one is
such environments and the HD-He interacting pair the simplest one after H2-He allowing
theoretical calculations including relativistic and quantum electrodynamics effects.

In the present work, we report theoretical and experimental line-shape parameters for
He-perturbed pure rotational HD lines. Besides the usual pressure broadening and shift
parameters leading to a Lorentzian profile, we also report their speed dependencies
and Dicke parameters. The theoretical values, obtained from close-coupling quantum
dynamical calculations, are for the R(j=0-3) lines and S(j=0-2) and temperatures from
10 to 500 K. The measurements, performed using stimulated Raman spectroscopy, were done for
the S(j=0-2) rotational Raman lines at 77, 195 and 298 K. The comparison of our calculations
with pressure broadening and line shift coefficients available in the literature for the
studied R lines is good if one considers the various contributions leading to the linewidth.