TIME-DEPENDENT OPTICAL POTENTIALS - NON-MARKOVIAN AND DOORWAY APPROACHES

We propose new approximations to the time-dependent optical potential coupling that part of the Hilbert space in which standard truncated cl ose coupling equations are solved (P space) and the complementary Q sp ace. We define a dynamical basis in P space allowing for a transparent separation of P-Q and Q-Q coupling from P-P coupling, thus ensuring t hat we account for the latter exactly within any approximate scheme. S tudying the time structure of the evolution operator in Q space, we ar e led to propose a non-Markovian scheme going beyond the instantaneous approximation. Alternatively, a topological study of the coupling bet ween P and Q space leads to a doorway space approximation consisting i n a dynamical extension of the P space by that pan of the Q space to w hich it is directly coupled.