  Harnessing the optoelectronic response of organic semiconductors requires a
thorough understanding of the fundamental light-matter interaction that is
dominated by the excitation of correlated electron-hole pairs, i.e. excitons.
The nature of these excitons would be fully captured by knowing the
quantum-mechanical wavefunction, which, however, is difficult to access both
theoretically and experimentally. Here, we use femtosecond photoemission
orbital tomography in combination with many-body perturbation theory to gain
access to exciton wavefunctions in organic semiconductors. We find that the
coherent sum of multiple electron-hole pair contributions that typically make
up a single exciton can be experimentally evidenced by photoelectron
spectroscopy. For the prototypical organic semiconductor buckminsterfullerene
(C$_{60}$), we show how to disentangle such multiorbital contributions and
thereby access key properties of the exciton wavefunctions including
localization, charge-transfer character, and ultrafast exciton formation and
relaxation dynamics.
