In AMO physics, the fundamental processes in the interaction of light with atoms, molecules, and complex systems are investigated or even controlled. The primary response of the system that is being studied to the incident laser pulse is electron excitation that, in general, takes place on ultrafast (femtosecond or even down to attosecond) time scales. Initial electron excitations are then coupled to other electronic states and to nuclear motion. The evolution of system dynamics is probed by a second laser pulse after a certain delay. By changing the delay between the two pulses, a complete picture can be obtained of the dynamical processes in the system under study.
Depending on the wavelength of incident light, there are different types of excitations. Strong visible/near infrared spectrometer (NIR) fields liberate electrons from valence or higher lying states by tunneling or multi-photon ionization. On the other hand, VUV/XUV photons ionize electrons by single photon ionization from deeper lying localized levels. X-rays excite core levels, typically followed by Auger decay. They also provide chemical sensitivity.
The system under study can have different complexities. In the simplest case of single atoms, details of the ionization dynamics such as the contribution from different orbitals and the dynamics of Auger decay can be revealed. In molecules, coupling of electron and nuclear dynamics and details of molecular explosion can be investigated. In particular, charge transfer and coupling between electrons and nuclei are of great importance in more complex molecules and biomolecules. Studying even more complex systems can involve examining clusters with sizes from a few nanometers to micrometers, where collective electron phenomena and a manifold of ionization and explosion pathways are present.
The ELI Beamlines facility will provide experimental tools for studying the above-mentioned phenomena in laser-matter interaction. The following capabilities will be provided:
- A range of light pulses with wavelengths covering X-rays, UV, visible, IR, and THz regimes to excite or probe different electronic levels
- Femtosecond pulse duration to investigate ultrafast electron dynamics
- Well-synchronized beams for pump-probe experiments with high temporal resolution.
- High photon flux in VUV/XUV/X-ray regimes to obtain sufficient signals even from diluted samples
- A variety of target systems to be investigated, including atomic or molecular gases, injected bioparticles, nanoparticles, and clusters.
Initial experiments will provide characterization of the photon source by terahertz streaking and can be further extended to investigate ultrafast electron dynamics in complex systems.
Terahertz (THz) streaking
THz streaking will first be used to measure the pulse duration of the XUV pulse and then to investigate details of the electron ionization in more complex systems.
In a THz streaking experiment, atoms are ionized by single photon ionization using the XUV pulse with durations of around 10 fs. A THz pulse synchronized with the XUV pulse then accelerates or decelerates the ionized photoelectron depending on the phase of the field. When the duration of the XUV pulse is considerably shorter than the period of the THz field (which is on the order of 100 fs), the energy of the electron is modulated on a sub-cycle time scale. From measured photoelectron spectra in the presence of a THz field one can obtain detailed information (both amplitude and phase) of the XUV pulse . This experiment will, then, provide a complete characterization of the XUV source.
With respect to more complex systems, a streaking experiment can be performed, such as on clusters. The experimental setup is the same for this, and the XUV pulse (now with known duration and phase) ionizes the electrons while the THz pulse provides the streaking field. In this case, a manifold of the ionization pathways are present and the measured photoelectron spectra contain information about multiple photoemission channels that can, in principle, be revealed from the measured streaking trace.
 Nature Photonics3, 523 – 528 (2009)