  We calculate the index of refraction of sodium matter waves propagating
through a gas of nitrogen molecules. We use a recent ab initio potential for
the ground state of the NaN_2 Van der Waals complex to perform quantal
close-coupling calculations and compute the index of refraction as a function
of the projectile velocity. We obtain good agreement with the available
experimental data. We show that the refractive index contains glory
oscillations, but that they are damped by the averaging over the thermal motion
of the N_2 molecules. These oscillations appear at lower temperatures and
projectile velocity. We also investigate the behavior of the refractive index
at low temperature and low projectile velocity to show its dependence on the
rotational state of N_2, and discuss the advantage of using diatomic molecules
as projectiles.
