Ultrafast electron crystallography of graphite

Abstract

In the pursuit of measuring structural dynamics on the atomic spatial (0.1 nm) and temporal (100 fs) scales, a pump-probe ultrafast electron diffraction (UED) setup has been developed in the group CQT. The setup involves photoemission of an electron bunch from a copper cathode using a fs laser pulse as well as electron beam optics to tackle the issue of the Coulomb repulsion. In this way, bunches containing up to 1 pC are created, allowing for single shot recording of diffraction patterns, whilst retaining the desired spatial and temporal resolution. Dynamics, induced in the sample by pumping with a fs pump laser pulse give rise to changes in the sample structure. These changes are monitored by recording diffraction patterns with the electron bunch.This report gives an overview of the improvements implemented in the setup. The effect of fluctuations in the charge per electron bunch was reduced by calibrating and monitoring the photoemission process. The electron beam optics were carefully aligned to guarantee the quality of diffraction patterns. Several diagnostic techniques were implemented to monitor the parameters relevant for pump-probe UED measurements. It is demonstrated that all these improvements together have led to a stability of order 1% in the recording of diffraction patterns. As these fluctuations are much smaller than the expected effects of the dynamic processes under investigation, it is concluded that pump-probe measurements should now in principle be possible with this setup.To test the pump-probe capabilities, the heating mechanism of graphite was investigated. For this, graphite samples as thin as 30 nm were produced through mechanical exfoliation. The structure of these samples was studied by electron crystallography. Pump-probe experiments were performed on these sample, but have, for reasons as yet unknown, not been successful to date.

Date of Award	31 Aug 2013
Original language	English
Supervisor	P.L.E.M. Pasmans (Supervisor 1) & O.J. (Jom) Luiten (Supervisor 2)