Tracking Singlet Fission in Momentum Space

In organic materials with strong exchange interactions, the S = 0 (singlet) exciton might have twice the energy of the S = 1 (triplet) exciton. This allows the spin-conserving fission of the optically prepared singlet to two dark antiparallel triplets. Due to the many-body nature of this process, consensus over the underlying mechanism is still lacking. Here, we use trARPES to resolve the transient states involved in singlet fission in pentacene and provide new insights into the microscopic and ultrafast processes governing it.