[en] An extension of the diffusion model is proposed for describing the intermediate processes and the compound nucleus formation in heavy ion collisions. The model describes the intermediate processes and fusion in terms of the formation and the evolution of long-lived dinuclear molecular complex (DMC) and its subsequent decay by fragmentation. The colliding ions can be trapped into the pocket of the entrance channel nucleus-nucleus potential and a DMC is formed. This DMC acts as a doorway state towards formation of a completedly equilibrated compound nucleus. It evolves through the exchange of nucleons to different dinuclear configurations. At each stage of its evolution, there is a finite probability to emit fragments into outgoing channels by thermal penetration over the barrier. The doorway states that do not fragment relax into a compound nucleus configuration and are identified as the fusion yield. The model is applied to the analysis of Si + C collisions for which the fusion cross section and the orbiting data are available for a wide range of bombarding energies. 7 refs., 2 figs