1. Experiments with Synchrotron Radiation

2. Pump and Probe
3. Atoms and Molecules in Short Laser Pulses
4. Photo Double ionization with High Energetic Photons
5. Electron Emission from Surfaces
6. Electron Attachment

In this section some of the ideas mentioned above shall be seized and explained in little more detail. Visions and hot topics are rendered briefly; technical problems and their possible solutions will be mentioned:

I.) Experiments with Synchrotron Radiation

In the past the momentum spectroscopy method was used to investigate single photon interaction with small and fundamental atomic and molecular targets (He, H₂, N₂, CO, C₂H₂…) in order to study ionization mechanism, Auger electron emission, internal properties and many body problems and so on. There are still many open questions, thus these very successful experiments will be continued in the future; but it is important to extend the studies to bigger and life sustaining molecules or clusters. The first step would represent the fragmentation of water molecules. In the long run it is the dream to ionize a K-shell electron of one atom of a chemical bond (like a DNA molecule) which triggers a confirmation change which can be studied time resolved (see also next paragraph).

In order to study a real three body problem the triple ionization of Lithium with synchrotron radiation would be very interesting: Since a single photon couples to only one of the target electrons three body interactions of the final state as well as correlation effects could be studied very well. The ionization mechanisms and correlation effects as well as the three body dynamics are sensitive to the energy of the incoming light. Thus experiments with a broad range in photon wavelengths are highly desirable. However, the cross section for triple ionization is very small and the (multihit-)detection efficiency is not very high either. Therefore a dense target has to be set up. A promising remedy is the combination of a Magnetical Optical Trap (MOT) with a momentum spectrometer. This has been done before in other work groups (Denmark, Michigan and Kansas) but only recoiling ions have been measured so far.

Also the fragmentation of heteronuclear molecules is very interesting since CPT violations could be studied nicely using circular polarized light.

II.) Pump and Probe

The next step in the investigation of dynamical processes is the combination of momentum spectroscopy with double pulses of light. The first pulse of these pairs spawns the conformation change, triggers the excitation or fragmentation of the target which then is probed by time resolved photoelectron diffraction using the second light pulse. The direction and energy of the Coulomb-exploding fragments, given by momentum spectroscopy, reveal the geometry of the molecule, while a controllable delay between the two light pulses introduces the essential time coordinate for the scanning of the dynamical process (comparable to stroboscopy). Highly differential cross sections demand long data acquisition times, respectively stable and well synchronized light beams with high flux. According to the target properties and the expected time of inner atomic processes the pulses have to be short and energetic enough. A perfect combination represents the combination of FEL and femto second laser pulses. Alternatively higher harmonics of visible laser light, and sometimes synchrotron radiation as well, can be used instead of novel FEL pulses. Their advantage lies in the availability and the stabile beam conditions. In this regard a cooperation with an experienced laser group and a permanent setup is highly desirable.

Short and intense laser pulses represent a very useful tool to investigate ionization mechanisms, final state interaction and dynamical processes. There are many experiments with atoms and molecules thinkable. On hot topic is the investigation of Laser Induced Electron Diffraction (LIED) in molecules. In a femto second laser pulse an electron is rescattered and once it has enough energy (approx. 100 eV which means an intensity of 7·10¹⁴ W/cm²) it undergoes diffraction in the two dwell Coulomb potential. These processes are expected to be very scarce (10^-4 in reference of the primary channel). The reason for this is probably mainly due to the scattering geometry that complicates the situation considerably. LIED in a tangential scattering geometry (i.e. the rescattered electron and the molecular axis are aligned in parallel) is deemed to fail. The perpendicular scattering geometry seems much more promising but will require two laser pulses: For example a pump-probe experiment on H₂ with two sub 10 fs pulses has to be performed, with the pump pulse removing the first electron and inducing the dissociation. The probe pulse would have to be perpendicularly polarized with respect to the pump pulse in order to have the second electron wavepacket moving transverse to the molecular axis. By adjusting the delay between the two pulses one could even choose the width of the molecular double slit. The potential of LIED as a tool for time resolved structural imaging still lies largely uncovered and proof of principle experiments are strongly required.

On the other hand, molecular clock experiments are thinkable, which would employ only one pulse and circularly polarized light to map the absolute phase of the laser electric field on the spatial di-rection of the electron momentum. Thereby a full laser cycle is mapped to a full 360° turn in mo-mentum space. Thus, different electron ejection angles in the laboratory frame correspond to different ejection times. Together with the correlated Kinetic Energy Release (KER) of the Coulomb exploding molecules an unambiguous clock running from 0-8 fs with a few 100 as resolution can be envisioned.

At photon energies of 5 keV and higher Compton scattering becomes the dominant double ionization mechanism. Here kinematically complete experiments yielding electron angular and momentum distributions and measuring the two-electron-Compton-profile are highly desirable. Investigating Compton scattering processes means probing the momentum distribution of bound electrons. Once a simple two electron-system like a Helium atom is chosen as a target, the correlated initial electron wave function could be studied in a double ionization process. Since ionization mechanisms via electron-electron interaction are not very likely, due to the very unequal energy sharing, initial state correlation effects play an important role and can be studied nicely.

Investigating the double ionization of simple molecules at high photon energies is even more in-sightful: In a Compton scattering experiment, where the molecular fragments and the Compton-scattered electron are measured in coincidence, a Compton-profile for a molecule (such as H₂ or even N₂) along and perpendicular to the molecular axis would be accessible. This would constitute a far reaching textbook experiment for the mapping of the molecular wave-function.

The combination of the electron imaging technique with solid-state physics opens doors for the understanding of semiconductors, magnetic surfaces (for data storage media), and especially superconductors. With this method it is in reach to map the correlated electron motion from bound states of solids and surfaces for the first time: While the correlated electron motion in these targets shows up only very diffusively, it can be visualized directly in the continuum.

As for 'conventional superconductors' the mechanism of superconductivity is much elaborated (BCS theory + Cooper-pairs) there is no clear picture how correlated electron pairs are generated in high temperature superconductors. With the direct (resp. coincident) investigation of the correlated electron pairs we hope to give more insight into this phenomenon. The experiments are challenging from the perspective of detectors and target preparation, they are however doable with very low photon flux. Therefore they can be done at almost any beamline. For instance a high temperature superconductor (like Bi₂Sr₂CaCu₂O₈) can be cooled down to 40°K and the break-up of Cooper-pairs can be studied and compared to the emission of electron pairs at room temperature from the same surface. The fraction of emitted Cooper-pairs is small (about 0.1 ‰ of all emitted photo electrons) but it can be resolved in energy (a resolution of 0.15 eV is needed) and furthermore isolated in kinematics (the electrons share the same energy). A test measurement in cooperation with Prof. J. Capuzano (University of Illinois) at the ALS was done recently - this agenda should be pushed forward.

Dealing with low energetic electrons in terms of a projectile beam is very demanding: Any electric and magnetic fringe field will deflect the light particles very easily. A main challenge is given by the beam preparation and its control: The electron beam has to be confined in a magnetic field which is established by a pair of Helmholtz coils. A conventional electron gun will not do the job (the energy resolution is just not good enough): Instead the laser assisted emission of low energetic electrons from semiconductor surfaces is considered to be a promising approach for creating short pulses with well defined kinetic energy. Possible targets are NO and CO and also H₂O for instance.

For more information please contact: Thorsten Weber