  We study the spectral and scattering theory of three body dispersive systems,
which include a massless particle and a two body non-relativistic pair, along
with two body short interactions among the three particles. We prove local
decay estimates and propagation estimates, which are then used to prove
asymptotic completeness for all non-threshold negative energies of the system.
By adding (trace class) non-particle-conserving interactions, this models
ionization processes as well. In particular, if the massless particle is
interacting only through the non-particle-number conserving interactions, no
ionization is possible if the total energy of the system is negative, as
predicted by the formal analysis of the photo-electric effect.
