Pre-equilibrium Components of the Fluctuation Cross Section

It is shown that the fluctuation cross section for the generalized-exciton or nested-doorway model can be obtained explicitly and exactly in the limit that doorways of successive classes have very different widths, ${\Gamma }_n\gg {\Gamma }_{n+1\mathrm{}}$, and that the doorways of a given class are overlapping, ${\Gamma }_n>D_n$. The result is given in terms of experimentally observable quantities, and explicitly separates the compound and precompound contributions. It contains the results of previous, more specialized, models as limiting cases.