  In this paper we describe a formalism for generating exclusive final states
in diffractive excitation, based on the optical analogy where diffraction is
fully determined by the absorption into inelastic channels. The formalism is
based on the Good--Walker formalism for diffractive excitation, and it is
assumed that the virtual parton cascades represent the diffractive eigenstates
defined by a definite absorption amplitude. We emphasize that, although
diffractive excitation is basically a quantum-mechanical phenomenon with strong
interference effects, it is possible to calculate the different interfering
components to the amplitude in an event generator, add them and thus calculate
the reaction cross section for exclusive diffractive final states. The
formalism is implemented in the DIPSY event generator, introducing no tunable
parameters beyond what has been determined previously in studies of
non-diffractive events. Some early results from DIS and proton-proton
collisions are presented, and compared to experimental data.
