NONLINEARITY AND TRAPPING IN EXCITATION TRANSFER - DIMERS AND TRIMERS

We study the interplay between nonlinearity in exciton transport and t rapping due to a sink site for the dimer and a chain trimer by a numer ical integration of the discrete nonlinear Schrodinger equation. Our r esults for the dimer show, that the formation of a self-trapped state due to the nonlinear coupling increases the lifetime of the exciton su bstantially. Self-trapping can be enhanced by the sink for short times , but for long times it disappears due to the presence of the sink. In the trimer consisting of a sub-dimer extended by a sink site there ex ists a transition between states localized on the two sites of the sub -dimer before for larger nonlinear coupling self-trapping on one site of the subdimer is observed. Very large trapping rates lead to an incr easing lifetime of the excitation on both the dimer and the trimer. Th e sink site is then effectively decoupled. We explain this effect usin g an asymptotic theory for strong trapping and demonstrate it by numer ical computation.