  The exciton dissociation and charge separation occurring on subpicosecond
time scales following the photoexcitation are studied in a model donor/acceptor
heterojunction using a fully quantum approach. Higher-than-LUMO acceptor
orbitals which are energetically aligned with the donor LUMO orbital
participate in the ultrafast interfacial dynamics by creating photon-absorbing
charge-bridging states in which charges are spatially separated and which can
be directly photoexcited. Along with the states brought about by
single-particle resonances, the two-particle (exciton) mixing gives rise to
bridge states in which charges are delocalized. Bridge states open up a number
of photophysical pathways that indirectly connect the initial donor states with
states of spatially separated charges and compete with the efficient
progressive deexcitation within the manifold of donor states. The diversity and
efficiency of these photophysical pathways depend on a number of factors, such
as the precise energy alignment of exciton states, the central frequency of the
excitation, and the strength of carrier-phonon interaction.
