Theoretical prediction of the rate constant for I+O2(a1Δg) 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 potential energy surfaces, four for the entrance channel and three for the exit channel, and the coupling elements between them, were adopted. Energy transfer dynamics was simulated with the semiclassical surface-hopping trajectory calculation, using Tully's "fewest switches" model for electronic transition. Approximately 5 X 10(5) trajectories were statistically averaged over a range of impact parameters and collision energies to calculate thermal rate constants for the temperature range 10-300 K. It was found that collisions resulting in energy transfer were dominated by;single hop trajectories. The calculated energy transfer rate constant was found to decrease smoothly with increasing temperature over the range 100-300 K. The predicted value was in excellent agreement with the experimental result for 150 K, but the calculations underestimate:room temperature data by a factor of 1.6. The rate-constant increases with decreasing energy because (1) long-range attractive 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 Physics.