Cross sections for rotational relaxation in alkali dimer molecular beams

Abstract

A sudden-freeze model for the extraction of effective rotational relaxation cross sections from nozzle beam expansions is refined by introducing a temperature-dependent cross section σ rot ( T). Since (i)σ rot ∝ exp ( − Δ E j ), (ii) the rotational energy level spacing Δ E J ∝ J and (iii) J* ∝ T 1 2 it is assumed that σ rot ( T) ∝ exp [− α ( T/Θ r) 1 2 ]. Here, J* stands for the most probable value of J and Θ r= B/k, where B is the rotational constant. The value of α was determined from a semi-logarithmic plot of directly measured values of σ rot ( J) versus ( T/Θ r) 1 2 . The resulting sudden-freeze-model expression for σ rot ( T rot) was used to deduce values of σ rot for Li/Li 2 and Na/Na 2 from measurements in nozzle-beam expansions. A combined plot of the directly measured values of σ rot ( J) and the deduced values of σ rot ( T rot) exhibits a common semi-logarithmic dependence on J*. Rotational relaxation cross sections deduced from quasi-classical trajectory calculations for Na/Li 2 ( J = 5, J= 20) collisions at E coll = 100 meV agree quantitatively with the predicted values.