2004 年 14 巻 1 号 p. 20-29
Theories for diffusion-controlled reaction (DCR) dynamics based on the diffusion equation (DE) and the Fokker-Planck-Kramers equation (FPKE) have been reviewed, and the validity of the theories has been examined in the picosecond time region by comparing them with computer simulations in liquid Ar. At the initial a few picoseconds, the dynamics of DCR is not explained by the Smoluchowski theory based on DE. Although the Collins-Kimball treatment for DE works practically well, FPKE should be used for detailed discussion. The effect of the initial distribution of reactants is significant in this time range. At times longer than ten picoseconds, the effects of the inertia and the initial distribution are not important, and the dynamics of DCR is well explained by the Smoluchowski theory. A possibility of the application of the DE-based theories in interpreting the reaction dynamics in supercritical fluids has been discussed briefly.