  Singlet fission may boost photovoltaic efficiency [by transforming a singlet
exciton into two triplet excitons and thereby doubling the number of excited
charge carriers. The primary step of singlet fission is the ultrafast creation
of the correlated triplet pair. While several mechanisms have been proposed to
explain this step, none has emerged as a consensus. The challenge lies in
tracking the transient excitonic states. Here we use time- and angle-resolved
photoemission spectroscopy to observe the primary step of singlet fission in
crystalline pentacene. Our results suggest a charge-transfer mediated mechanism
with a hybridization of Frenkel and charge-transfer states in the lowest bright
singlet exciton. We gained intimate knowledge about the localization and the
orbital character of the exciton wave functions recorded in momentum maps. This
allowed us to directly compare the localization of singlet and bitriplet
excitons and decompose energetically overlapping states based on their orbital
character. Orbital- and localization- resolved many-body dynamics promise deep
insights into the mechanics governing molecular systems and topological
materials.
