Unimolecular dissociation as diffusion with a radiation boundary condition

Unimolecular decomposition of an isolated polyatomic molecule is discussed in terms of a diffusion equation with a radiation boundary condition, instead of a master equation with an absorbing boundary. This gives a short-cut to evaluating the mean first passage time. More important, it results in simple models, such as the model of diffusion in a linear potential solved here. This model is analytically solvable for the probability distribution and the survival probability, and may be helpful in analyzing future experiments.