CLASSICAL TRAJECTORY SIMULATION OF THE CLUSTER ATOM ASSOCIATION REACTION I-ARN-]I2+NAR .2. DIFFUSION OF CAPTURED IODINE AND EVAPORATIVE COOLING OF I2(I)

This is Part II of a series of papers in which we address the role of microscopic solvation in the association reaction between a free iodin e atom and an iodine doped van der Waals cluster: I + I (Ar)n --> I2 nAr. The influence of microscopic solvation on the I + I to I2 reacti vity, reaction mechanism, energetics, and product energy partitioning is the major focus of our study. The overall reaction for I + I (Ar)12 --> I2 + 12Ar can be characterized by three fundamental processes: ( 1 ) capture of the incident iodine atom by the I(Ar)12 cluster; (2) di ffusive migration of the captured I atom on the surface or in the inte rior of the cluster, leading ultimately to an encounter with the other I atom to form a highly excited I2 molecule; (3) vibrational relaxat ion of the nascent I2 product, leading to evaporative cooling and dec omposition of the cluster. Part I [J. Chem. Phys. 98, 8551 (1993)] dea lt with the capture process. This article focuses on the chemical dyna mics of the subsequent processes of diffusion, vibrational energy tran sfer, and evaporative cooling. The stabilization of the chemically act ivated I2 molecule through evaporative cooling eliminate the need of a third body collision as required in isolation gas phase recombinatio n. The overall distribution of final energies is nonstatistical for th e chemically activated I2Arn. The final vibrational energy of I2 exhi bits a nonthermal structure even after all the argon atoms are evapora ted. In addition to monoatomic sequential evaporation, a ''fissioning' ' mechanism, leading to the formation of at least one multiatom fragme nt, is observed. The relationship between structure and dynamics is ex plored. The dynamics of vibrational relaxation, diffusion of the captu red iodine, evaporation, and fragmentation pattern, final I2 energy pa rtitioning are found to be strongly dependent upon structure and tempe rature of the doped cluster. A spectroscopic experimental verification of the above observations is also proposed.