The effect of collisions on the rotational angular momentum of diatomic molecules studied using polarized light

We report results of an experimental study of the changes in the alignment of the rotational angular momentum of diatomic molecules during elastic collisions.  The experiment involved collisions of diatomic lithium molecules in the A¹Σ_u⁺ excited electronic state with noble gas atoms (helium and argon) in a thermal gas phase sample. Polarized light for excitation was combined with detection of polarization-specific fluorescence in order to achieve magnetitic sublevel state selectivity.  Our experimental results show that elastic M changing collisions are allowed, and we measure the collisional rate for such process in J = 1 rotational level. Furthermore, we show that the elastic M-changing collision rate is more than a factor of four times smaller than the inelastic, ΔJ = +2, J-changing collision rate for collisions of Li₂ A¹Σ_u⁺(v = 5, J = 1) molecules with either argon or helium atoms.  In other words, it is significantly more difficult for a collision with a noble gas atom to change the orientation of the molecular rotation vector than to change the magnitude of the rotation vector.