Center for Functional Nanomaterials Seminar

The addition of time resolution to photoemission microscopy (PEEM) is a challenging venture, that has so far only been attempted at a few places around the world. It requires the combination of standard PEEM with femtosecond lasers. As the photon energy of the laser pulses is in general not sufficient for photoemission, generation of photoelectrons relies on two photon photoemission (2PPE). This has important ramifications for the 2PPE PEEM contrast, as the photoemission yield is in this case dominated by intermediate energy states that can be excited by the first photon. If Ag islands on Si are imaged in 2PPE PEEM, for instance, dipolar plasmon modes act as the intermediate states and the observed contrast depends on the size and orientation of the particles. In the organic semiconductor anthracene, a singlet exciton at the LUMO Band edge plays a similar role. But time-resolved PEEM really aims to visualize the excitation dynamics of the intermediate state: In a pump-probe experiment, where a first laser pulse is used for the excitation of the intermediate state and a second laser pulse triggers photoemission from the intermediate state, it is possible to determine the characteristic depopulation time of the excited intermediate state or, in the case of the silver islands, watch the electronic perturbation caused by the laser pulse travel across the island as a surface plasmon polariton wave.