On the parafermionic scattering of an integrable Heisenberg model with impurity

We have studied the massive parafermionic sector of an integrable spin-$s$ chain with an impurity of spin-$s^{'}$ in presence of a magnetic field. The effect of the impurity is encoded in a computable parafermionic impurity scattering amplitude and its contribution to the finite size properties of the free energy is discussed. As an example we compute the non-integer degeneracy of the ground state. For the simplest model of $s=1$ and $s^{'}=1/2$, its interpretation in terms of conformally invariant boundary conditions is presented.