Ionization of molecular excitons detected by electron emission: coronene and α-perylene

Biphotonic emission of electrons has been observed when a crystal of aromatic hydrocarbon, coronene or α-perylene, is excited at 3.49 eV (λ=355 nm). Measurements using a pair of pulses with variable intervals show that an intermediate state is involved which has a lifetime in the nanosecond range. The lifetime, however, is shorter than the fluorescence lifetime, indicating that excitons in the surface region can have a shorter lifetime compared to the bulk excitons. Depending on the material and also on the excitation energy, mutual annihilation or photoionization of surface excitons is responsible for the electron emission.