CROSS-SECTIONS FOR ROTATIONAL RELAXATION IN ALKALI DIMER MOLECULAR-BEAMS

A sudden-freeze model for the extraction of effective rotational relax ation cross sections from nozzle beam expansions is refined by introdu cing a temperature-dependent cross section sigma(rot)(T). Since (i) si gma(rot) is-proportional-to exp (-DELTAE(J)), (ii) the rotational ener gy level spacing DELTAE(J) is-proportional-to J and (iii) J is-propor tional-to T1/2 it is assumed that sigma(rot)(T) is-proportional-to exp [-alpha(T/THETA(r))1/2]. Here, J stands for the most probable value o f J and THETA(r) = B/k, where B is the rotational constant. The value of alpha was determined from a semi-logarithmic plot of directly measu red values of sigma(rot)(J) versus (T/THETA(r))1/2. The resulting sudd en-freeze-model expression for sigma(rot)(T(rot)) was used to deduce v alues of sigma(rot) for Li/Li2 and Na/Na2 from measurements in nozzle- beam expansions. A combined plot of the directly measured values of si gma(rot)(J) and the deduced values of sigma(rot)(T(rot)) exhibits a co mmon semi-logarithmic dependence on J. Rotational relaxation cross se ctions deduced from quasi-classical trajectory calculations for Na/Li2 (J = 5, J = 20) collisions at E(coll) = 100 meV agree quantitatively w ith the predicted values.