The Ultrafast Science Laboratory was started in 2007 within the newly appointed South African Research Chair for Photonics: Ultrafast and Ultra-intense Laser Science (H Schwoerer). We currently pursue two major projects: the investigation of ultrafast molecular dynamics with conventional femtosecond (light) spectroscopy, and the investigation of ultrafast structural dynamics with femtosecond electron diffraction. Where the first applies well established spectroscopic methods and primarily focuses on the investigated specimen, the latter is worldwide a fairly new method combining femtosecond lasers and electron diffraction. The experimental details of the ultrafast electron diffraction are being optimized in parallel with first applications to reasonably uncomplicated sample targets.

The group:

The following figure gives an overview of our experimental equipment. We start, on the left, with our amplified femtosecond laser, split the beam into four beams. We simultaneously drive two optical parametric amplifiers for tuneable ultrashort laser pulses, a femtosecond white light continuum source and a femtosecond electron gun. All signals are intrinsically synchronized within better than 100 fs. We have at the moment two sample target areas, one for molecules in solution for spectroscopy and one in vacuum for the electron diffraction. A fifth beam-line for two-dimensional spectroscopy is in the process of being included..

Ultrafast Molecular Spectroscopy

Currently we are investigating photo-induced reaction dynamics such as energy and charge transfer or  configuration changes in in a variety of organic molecules and metallo-organic complexes. We for example investigate  ultrafast photo-induced isomerization and proton transfer reaction of metal-dithizonate complexes, which are strongly photochromic substances and therefore of potential interest in molecular electronics and sensing applications. However, due to the isomerization at a C=N double bond they are also of fundamental chemical interest in the context of conical intersections.

This work is done in a close cooperation with Karel van Eschwege (Free State University in Bloemfontein).

Ultrafast Electron Diffraction

Femtosecond electron diffraction in principle allows for the observation of the initial dynamics of photo-induced processes in molecules and condensed phase with atomic spatial and temporal resolution. The method is based on the classical pump-probe spectroscopy with fs laser pulses, but the difference being that the laser probe pulse is replaced by an ultrashort electron pulse, which is diffracted off the target. Ultrafast electron diffraction therefore offers a direct view onto the dynamics of the electron density distribution and atomic position in a crystalline specimen, whereas classical fs spectroscopy detects electronic excitations. The required temporal resolution is achieved by using one fs laser pulse to initiate a photo-induced process and a second, optically delayed fs laser pulse to photo-electrically generate a femtosecond electron pulse. The electron pulse is then accelerated with a static electric field with a particularly compact 40 kV electron gun.

We have built a short pulsed electron gun and have fully characterized the electron beam, see figure 2. We have developed an extremely compact, phototriggered electron streak camera with an unprecedented resolution of 250 fs, which allows us to measure electron pulse duration down to 300 fs. These pulses can however only that contain very few (<10000) electrons due to Coulomb repulsion during pulse propagation.