Diffusion-limited coalescence and annihilation in random media

We study the kinetics of diffusion-limited coalescence, A+A --> A, and annihilation, A+A --> 0, in random media consisting of disconnected domains of reaction. Examples include exciton fusion and annihilation in porous matrices and along polymer chains. We begin with an exact analysis of A+A --> A in a finite segment. This result is applied to coalescence in a random distribution of segment lengths, and the implications for coalescence and annihilation in percolation clusters and other confined geometries are then derived by means of scaling techniques. (C) 2000 American Institute of Physics. [S0021-9606(00)50712-0].