Development of an Ultrafast Low-Energy Electron Diffraction Setup

Ultrafast Probing and Coherent Vibrational Control of a Surface Structural Phase Transition

“…The local field enhancement also has positive effects on the electron pulse duration: In the vicinity of the apex, the electric field of the negatively-biased tip resembles the field of a point charge 8 , leading to a rapid acceleration of photoemitted electrons almost to their final energies within the first few nm of propagation [217]. This is in stark contrast to planar photocathodes, where electrons are accelerated in a homogeneous electric field over significantly larger distances.…”

“…In this context, analytical calculations by Refs. [217,218] show that for a fixed distance from the electron source, acceleration in the spatially inhomogeneous field of a needle emitter results in significantly shorter electron pulse durations (50 − 100 times shorter) compared to a planar emitter geometry.…”

“…The following chapter gives an overview of the capabilities of ULEED. From proofof-principle experiments [217] to the observation and control of surface-specific structural dynamics [53], the technique has demonstrated its potential in various fields of surface science, including polymer and molecular dynamics at surfaces [217,219], surface reconstructions on the µm scale [231], as well as ultrafast structural transitions and phase ordering kinetics in CDW systems [18,53,55]. Of crucial importance for all these experiments is the high temporal and momentum resolution of the two ultrafast electron guns, which will be reviewed first for this reason.…”

“…Regarding indium on silicon, time-resolved surface crystallography would be the ideal tool to identify potential differences between the metastable and the equilibrium (4×1) structure. ULEED with ps temporal resolution was first demonstrated in a transmission geometry, revealing ultrafast polymer dynamics on graphene after optical excitation [17,217]. While this approach was without alternative at the time due to the relatively large electron source, today the combination of this concept with the functionalities of miniaturised electron guns opens up new perspectives for ULEED, as will be discussed below.…”