  Using Hollowood's conjecture for the S-matrix for elementary solitons in
complex $a_n^{(1)}$ affine Toda field theories we examine the interactions of
bound states of solitons in $a_2^{(1)}$ theory. The elementary solitons can
form two different kinds of bound states: scalar bound states (the so-called
breathers), and excited solitons, which are bound states with non-zero
topological charge. We give explicit expressions of all S-matrix elements
involving the scattering of breathers and excited solitons and examine their
pole structure in detail. It is shown how the poles can be explained in terms
of on-shell diagrams, several of which involve a generalized Coleman-Thun
mechanism.
