ROTATIONAL ENERGY-TRANSFER IN VIBRATIONALLY EXCITED ACETYLENE(X)OVER-TILDE (1)SIGMA(G)(V-2''=1,J'') - DELTA-J PROPENSITIES

A complete set of state-to-state rotational energy transfer rate constants has been measured for acetylene-acetylene collisions at room temperature under single collision conditions. Initial rotational states (J(i)=5,7,...,25) were prepared and final states (J(f)=1,...,25) interrogated. The measurements were carried out in a typical gas phase pump and probe arrangement. The initial vibrationally excited state was prepared by stimulated Raman pumping using strong e-branch transitions. State preparation via this branch produces an isotropic spatial distribution of the excited state which is important for data analysis. Narrow bandwidth lasers ensure single rotational state selectivity. The rotational distribution after collisions is monitored by time-delayed laser-induced fluorescence via the ($) over tilde A (1)A(u) (v(3)'= 1) <-- ($) over tilde X (1) Sigma(g) (v(2)'' = 1) transition. In general, the rate constants decrease exponentially with the transferred rotational energy. The complete rate constant matrix can consistently be described by a simple parameter set within the dynamical infinite order sudden power approximation. Tn addition to this general behavior a significant Delta J propensity of the rate constants is observed. Using the energy corrected sudden approximation with a power law basis an excellent match, reproducing the Delta J propensities, to the rate constant matrix is obtained, again with a single set of parameters.