  In this paper we consider the process of diffraction dissociation in deep
inelastic scattering producing a small mass. This process is analyzed by the
calculation of $\bar q\, q$ and $\bar q\, q\, G$ production. We show that the
small distance contributions ($r_{\perp}\,\propto\,1/Q$) to the longitudinal
polarised virtual photon dominate the diffractive channels. Formulae for the
cross section using the gluon density are written within the framework of
perturbative QCD. We show that the production of small masses by a transverse
polarised photon is concentrated at moderate values of $r_{\perp} \,\sim\,1
GeV^{-1}$, where the pQCD approach is valid. It is shown that only $\bar q\, q$
pair production contributes to diffraction dissociation with $\b\,>\,$ 0.4 and
the possibility to extract the values of the gluon density from the
measurements in this kinematic region is discussed. Shadowing corrections are
assessed for both longitudinal and transverse polarised photons and estimates
of the different damping factors are given. The relation between diffractive
production and the corrections to $F_2$ is alluded to. The evolution of the
diffractive structure function is studied and a solution to the difractive
evolution equations is proposed.
