Prof.dr. Claus Ropers: Ultrafast low-energy electron diffraction using nanotip photocathodes

ABSTRACT

The investigation of atomic-scale dynamics with high spatio-temporal resolution yields unparalleled insights into ultrafast structural reorganizations associated with energy transfer or phase transitions. Over the past years, tremendous progress has been made in establishing methods for the time-resolved structural analysis of bulk media using pulsed x-ray or electron beams. In contrast, the structural dynamics at surfaces and ultrathin films remain largely elusive. This limits our ability to study quasi-two-dimensional systems exhibiting a host of unique phase transitions and topologically controlled ordering, as well as the dynamics of surface reconstructions and complex adsorbate superstructures.

Our research group has recently implemented ultrafast low-energy electron diffraction (ULEED) to study structural changes with high temporal resolution and ultimate surface sensitivity. We utilize nanoscopic needle emitters in an electrostatic lens geometry as high-brightness sources of pulsed electrons. The spatial confinement of the source is realized by the local enhancement of nonlinear photoemission at the tip apex.

This talk will introduce photoelectron emission from nanoscopic emitters over a broad range of physical parameters, and a first ULEED study with a temporal resolution of few picoseconds will be presented. Specifically, we study the ultrafast structural dynamics of a polymer adsorbate superstructure on freestanding graphene in transmission. We identify sequential relaxation processes associated with superstructure melting and the appearance of an expanded amorphous phase.