  We present a detailed investigation of ultrafast (subpicosecond) exciton
dynamics in the lattice model of a donor/acceptor heterojunction. Exciton
generation by means of a photoexcitation, exciton dissociation, and further
charge separation are treated on equal footing. The experimentally observed
presence of space-separated charges at $\lesssim 100$ fs after the
photoexcitation is usually attributed to ultrafast transitions from excitons in
the donor to charge transfer and charge separated states. Here, we show,
however, that the space-separated charges appearing on $\lesssim 100$-fs time
scales are predominantly directly optically generated. Our theoretical insights
into the ultrafast pump-probe spectroscopy challenge usual interpretations of
pump-probe spectra in terms of ultrafast population transfer from donor
excitons to space-separated charges.
