  The contribution to diffraction dissociation of virtual photons due to
quasi-elastic scattering of the $q$-$\bar q$ component is calculated in the
framework of the QCD dipole picture. Both longitudinal and transverse
components of the cross-section are given. It is shown that, at fixed mass of
the diffractively produced system, quantum mechanical interference plays an
important r\^ ole. Phenomenological consequences are discussed.
