Orbital-resolved Observation of Singlet Fission

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 and show that it occurs in a charge-transfer mediated mechanism. We gained intimate knowledge about the localization and the orbital character of the exciton wavefunctions 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.