Abstract: We consider abstract massless Pauli-Fierz models describing a small quantum system linearly coupled with a field of scalar massless bosons. The typical example is the confined Nelson model.

We prove a Mourre estimate outside from the eigenvalues of the Hamiltonian. Using an extension of the Mourre method, we deduce from the Mourre estimate a limiting absorption principle. We also prove that the expectation value of the number operator is finite for all eigenvectors. In contrast with previous works our results are valid for arbitrary coupling constant and for all energies. This is joint work with Vladimir Georgescu and Jacob Schach-Moller.