Theoretical prediction of the rate constant for I+O-2(a(1)Delta(g)) electronic energy transfer: A surface-hopping trajectory study

The temperature dependence; of the rate constant for the electronic energy transfer process I(P-2(3/2)) + O-2(a (1)Delta (g)) --> I(P-2(1/2)) + O-2(X (3)Sigma (-)(g)) has been studied theoretically. Seven ab initio diabatic p otential energy surfaces, four for the entrance channel and three for the e xit channel, and the coupling elements between them, were adopted. Energy t ransfer dynamics was simulated with the semiclassical surface-hopping traje ctory calculation, using Tully's "fewest switches" model for electronic tra nsition. Approximately 5 X 10(5) trajectories were statistically averaged o ver a range of impact parameters and collision energies to calculate therma l rate constants for the temperature range 10-300 K. It was found that coll isions resulting in energy transfer were dominated by;single hop trajectori es. The calculated energy transfer rate constant was found to decrease smoo thly with increasing temperature over the range 100-300 K. The predicted va lue was in excellent agreement with the experimental result for 150 K, but the calculations underestimate:room temperature data by a factor of 1.6. Th e rate-constant increases with decreasing energy because (1) long-range att ractive forces draw-slow moving collision partners together and (ii) longer lifetime of slow collisions increases the probability of surface hopping. It is also found that there is a competition between rotational relaxation of O-2(a) and electronic energy transfer. (C) 2001 American Institute of Ph ysics.