Watching Femtosecond Molecular Dynamics using Synchrotrons and X-Ray Free-Electron Lasers

Recording real-time movies of dynamical processes in molecules, as, for example, progressing chemical reactions, has been a driving force for many disciplines in fundamental sciences during the last decades. Comparably new are experimental techniques, that address single molecules in the gas phase and that involve coincident single-particle detection for imaging these dynamics are Coulomb explosion imaging and Photoelectron diffraction imaging. The latter employs the interference pattern of (photo)electrons emitted from molecules to infer the initial molecular geometry, the former uses (for example) ultrashort light pulses to (heavily) fragment the inspected molecules in order to gather such information from the breakup pattern. X-ray free-electron lasers are able to produce ultrashort light pulses with highest intensity, which are perfectly suitable to perform measurements along these lines. In particular, these light sources allow for time-resolved studies in a pump-probe scheme by adding ultrashort UV pulses that are synchronized with the X-ray flashes.

Figure 1: Momentum-space image of a 2-thiouracil molecule fragmented in a Coulomb explosion. Shown are the protons and the oxygen atom.

Since almost five years a dedicated COLTRIMS reaction microscope [1,2] is available at the SQS-instrument of the European X-ray free-electron laser, which was used recently to perform Coulomb explosion imaging (see Fig. 1) and Photoelectron diffraction imaging measurements. Some examples will be presented in the talk. The COLTRIMS technique is a powerful coincidence measurement approach, which allows in addition to examine in some cases molecular dynamics on a femtosecond time scale without the need for ultrashort light pulses. The talk will present some examples of synchrotron-based measurements along this theme, as well.

References
[1] J. Ullrich et al., Rep. Prog. Phys. 66, 1463(2003).

[2] T. Jahnke et al., JESRP 141, 229(2004)