Exciton sensitive microscopy of anthracene monolayers on Si(1 1 1)

Thin anthracene films were studied with photoemission electron microscopy, using both a Hg discharge lamp and a femtosecond Ti:Sapphire laser for generation of the photoelectrons. In threshold photoemission, under illumination of the surface with the Hg lamp, anthracene islands appear dark due to the high work function of anthracene. Under illumination with frequency doubled femtosecond laser pulses (λ=400nm) two photon photoemission takes place, and the anthracene islands each exhibit a characteristic brightness. We find that the brightness of the islands in two photon photoemission (2PPE) is modulated by the S1S1-exciton in anthracene that serves as the intermediate state in 2PPE. Whenever the b→-axis of the unit cell within a particular anthracene island is aligned with the in-plane component of the electric field of the laser pulses, the island exhibits a maximal two photon photoemission yield. Based on exciton sensitive microscopy we find that the orientation of molecules in each island is different and that the anthracene film as a whole is textured.