| COLTRIMS – Thorsten Weber |  |
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The key note of this research is the investigation of many (or few, e.g. 2, 3, …, 10) particle problems in quantum mechanics. Although the explicit motion of particles in and in-between atoms and molecules is not subject of this very well tested and successful theory (instead a probability density function is introduced which obscures the appreciation), it is the correlated dynamics of electrons which for instance is responsible for the formation of Cooper-pairs, e.g. the elements of the fractionate Hall effect and heavy Fermi-particles.
However, the main idea or focus is to uncover the entangled motion of electrons in unperturbed atoms, molecules, clusters and surfaces. Or in other words to answer the question: What makes the collective motion in bound many particle systems tick ? Since it is not possible to peer inside an atom, molecule or any other many particle system without altering it, it is very difficult to cotton on to this important question.
1.) Motivation
The key note of this research is the investigation of many (or few, e.g. 2, 3, …, 10) particle problems in quantum mechanics. Although the explicit motion of particles in and in-between atoms and molecules is not subject of this very well tested and successful theory (instead a probability density function is introduced which obscures the appreciation), it is the correlated dynamics of electrons which for instance is responsible for the formation of Cooper-pairs, e.g. the elements of the fractionate Hall effect and heavy Fermi-particles.
However, the main idea or focus is to uncover the entangled motion of electrons in unperturbed atoms, molecules, clusters and surfaces. Or in other words to answer the question: What makes the collective motion in bound many particle systems tick ? Since it is not possible to peer inside an atom, molecule or any other many particle system without altering it, it is very difficult to cotton on to this important question.
The fundamental idea is to probe an atom or molecule (or surface) in a manner that preserves most of its internal motion which can be detected in a kinematically complete experiment, e.g. an experiment where all final state momentum vectors of all particles are known with 4π solid angle at best (in most cases without knowledge about the spin).
The key value in the description of the dynamics of these complex systems is the three dimensional momentum vector of each particle. Therefore a device is needed that visualizes the trajectories of all particles in the ‘explosion’ (e.g. ionization or fragmentation) of the target. Once all momentum vectors of the outcome are obtained, highly differential cross sections (triple, quadruple or higher differential in angles and energies) reveal the information of the entangled motion of the particles. The coincident momentum spectroscopy of electrons and recoiling ions, which has been used very successfully in the past and which is known as COLd Target Recoil Ion Momentum Spectroscopy COLTRIMS or reactions microscopy (described below), is able to elucidate what is going on here. Since we would like to extend this technique to time resolved measurements we call it Momentum Imaging for TimE Resolved Studies (MISTERS).
As already mentioned above, peering in a microscopic system like atoms and molecules with particle beams and electromagnetic waves results in exciting and fragmenting or ionizing the target. Once bonds are broken an explosion of the system is triggered which is superimposing the internal motion of the initial state. This is due to Coulomb repulsion or post collision interaction (or post absorption interaction in case of photoionization processes) and is influenced by selection rules as well as momentum and energy conservation laws etc. To track down which structure of the complex emission patterns is due to the initial dynamics of the entangled motion of the particles, the knowledge about final state interactions and their associated highly differential cross sections is indispensable: Ionization mechanisms (such as TS1, TS2, GSC, shake-off for the double ionization), post collision (or post absorption) interaction between the fragments (as well as diffraction and interference), and internal properties of the binding (like symmetry effects, the inter-nuclear separation of a molecule at the time of photo ionization or bond-angles for instance) have to be studied and have to be “subtracted” (resp. isolated) in momentum space in order to see the fingerprint of the dynamics of the initial state.
But investigating these final state interactions and internal properties of the systems as well as ionization mechanisms is much more than just a necessary condition: Once atoms and molecules absorb enough energy by probing the target with particle beams and/or electromagnetic waves, bonds can be broken and ions can form, allowing neutral and charged constituents to react with their neighbors. These processes occur everywhere, from car engines to industrial chemical plants. Ionization processes also occur naturally in earth’s upper atmosphere, driven by the sun’s energy, and are the mechanisms responsible for radiation damage to our bodies. In general, the more energy that is absorbed — either from light or through a collision with another energetic particle — the greater the number of possible reaction pathways and the greater the ‘damage’ done to the system itself and its nearest neighbors. This is a major field of research and momentum spectroscopy can help to answer fundamental questions which play an important role in many branches of physics and chemistry as well as micro biology.
2.) Program
From the passage above we know that for many reasons it is very important to investigate the several ways of breaking-up bound atomic and molecular systems and to find out which mechanisms are behind these ionization processes. There are many ways to fragment such configurations. Besides particle collisions one of the most fundamental is the absorption of light, i.e. pure energy. Dissecting the target with light represents one of the most un-perturbing ways to peer inside an atom or molecule. That is because photons have no charge, and for the present context mostly negligible masses. Photons will mainly just deposit their energy (or parts of it in higher energy regimes) and angular momentum in the system and kick-start the ionization process. So dealing with one photon (with a relatively long wavelength like in the VUV-range) we can speak of a rather soft fragmentation: The initial momenta, the Coulomb repulsion of the charged fragments, and the selection rules will then determine the dynamics in the final state.
This is definitely a function of the photon wave length and the kinetic energy shared between the fragments. While the photo electric effect has been studied widely and successfully, coincident Compton scattering still remains a challenging project for theory and experiment.
Besides the energy the polarization of the incoming light is another important property. Using circular polarized light allows an even more stringent test of quantum mechanics: The loss of a symmetry axis (the alignment of the polarization vector) results in less selection rules, which are restricting the final state. Moreover, with the information about the spin of the photon the experiment becomes complete: The direction of rotation and the propagation vector of the light define a tripod imposing chirality’s to the initial state.
On the other hand using many (30 or more) photons in a short laser pulse we have to expect different results: The intense field perturbs the atomic or molecular potential, other ionization processes occur (sequential, non sequential, re-scattering), and more angular momentum can be transferred leading to many possible final states (especially in molecules). Depending on the development of short laser pulses at usable repetition rates, the investigations in this field have just started.
Depending on the pulse period of the incoming light the intrinsic dynamics of bound many body systems can be mapped more or less directly into the final state. Pulses of single photons from synchrotrons are much longer than the typical times of most inner molecular vibrations. Laser and Free Electron Laser FEL pulses are very short (100 fs and shorter). Femtosecond lasers synchronized to a FEL (such as LCLS in Stanford) will be available soon. With a combination of a laser and FEL pulse(s) the fragmentation time becomes so well resolved that inversions of chemical reaction could be examined: The FEL would start the fragmentation process which would be probed by laser pulses, followed by an investigation of the dynamics of the molecular outcome in momentum space as a function of the time delay between the two pulses. Similar experiments can be done combining high harmonic laser pulses.
On the other hand, effects like the isomerization process (for example in C2H2) or excitation of high vibrational states can be induced by laser photon(s) and then be probed with a synchrotron pulse (fragmentation plus momentum spectroscopy): By doing so, the rearrangement processes respectively the inner nuclear dynamics can be probed time-resolved as a function of the laser-synchrotron-delay.
However, combining pump and probe pulses with momentum spectroscopy represents the ultimate tool to unveil the inner dynamics of atoms and molecules. Up to now, this combination represents the softest way to peer inside atoms and molecules and it comes with a new time-coordinate. These multi coincidence experiments are not simple (high flux, stability and full control for the two different pulses are needed) but highly desirable. They will play an important role in future investigations of many particle dynamics and time resolved spectroscopy of chemical reactions.
3.) Technique
As mentioned above, a reaction microscope is needed which is able to image the final state momentum vectors of an ionization or fragmentation process in order to study the dynamics of the desired target. Instead of using dispersive electron and ion spectrometers, which detect only a small fraction of the entire phase space, a relatively new technique is used to unveil these properties. It is called COLTRIMS, which stands for COLd Target Recoil Ion Momentum Spectroscopy.
This coincident electron and ion imaging technique has been invented and developed over the last 20 years (see [1, 2, 3]). The pioneers in the field include the groups of Prof. Dr. Horst Schmidt Böcking (University of Frankfurt/Germany), Prof. Dr. Lew Cocke (Kansas State University/USA) and Prof. Dr. Joachim Ullrich (Max Planck Institut Heidelberg).
Its idea is pretty simple: The target (gas, cluster, solid) is intersected with the projectile beam (photons, electrons, ions, neutrals) in a weak electric field which is just strong enough to separate the fragments by their charge and guide them to large two dimensional position sensitive detectors. The field is so low (< 50/cm) that no perturbation of the initial bound state occurs. Often a magnetic field in parallel (< 30 Gauss) is used to prevent high energetic electrons from leaving the spectrometer by forcing the light particles gyrating towards the detector. This little trick helps to keep the electric field low which warrants a good resolution (see fig. 1).
Figure 1 (title image): Schematically drawing of the momentum spectrometer: The deuterium molecules, prepared in a supersonic jet going from the bottom to the top, are intersected with the pulsed photon beam in the middle of the copper rings which define a homogenous electrical field. The big coils represent a Helmholtz pair which establishes a homogenous magnetic field in parallel to the spectrometer axis. The nuclei are guided to a position sensitive detector to the left while the electrons get spiraled to the right (due to their low mass in the magnetic field).
Figure 2: Photograph of the position sensitive delay detector from RoentDek . On top of the square aluminum you see the multi channel plate z-stack. Two layers of copper wires a located underneath.
With the knowledge of the dimensions of the spectrometer and the magnitudes of the electromagnetic field the three momentum components of each particle can be deduced from the time of flight to and the position on the specific detector.
The detectors themselves represent the heart of the momentum spectrometer. Their size, resolution and multi-hit capability determine the success of the experiment and the quality of the results. We use fast multi channel plate detectors (40, 80 and 120 mm diameter) to convert and amplify electrons, ions or even photons into a cloud of electrons which hits a two dimensional delay line anode mounted in behind, giving information about the position of impact on the channel plate (see fig. 2 and [4]). These detectors have been developed and permanently enhanced in our group in Frankfurt. In the meantime many other groups all around the world use our technology.
By using one to three detectors in one experiment the electronic read-out and coincident techniques are similar to small set-ups in nuclear science and also that powerful: All data are written in list-mode format which allows to rerun the experiment offline on the computer over and over again by filtering out bad events and doing further calculations. The next improvement will be a very fast and pulse-height sensitive read-out of the detectors (fast transient recorder) which will replace the rather slow NIM and CAMAC standard of nuclear physics used so far. This system was already tested in a first experiment and we expect the multi-hit dead time between the registrations of two hits on the detector to go down significantly, which is very important for the investigation of many particle systems. The power of this list-mode data acquisition technique is essential for the success of such complex experiments at the FEL with its very limited beamtime.
Another important component of the experimental set-up is the preparation of the target. For most of the time gases are used which are prepared in a supersonic expansion formed by high pressure (4 to 40 bar) at a tiny nozzle (50 microns or smaller). The gas beam is then skimmed down by small and sharp apertures. This provides a well localized and cool target (1 mm diameter, 20 °K going down to mK) necessary for a good position-, time and energy resolution. Furthermore a special layout of the momentum spectrometer (a so called Wiley-McLaren geometry) and the use of electric lenses make focusing in time and position possible.
Also surfaces, clusters, dimers and liquids (like water for instance) have been analyzed with this technique (see [5, 6, 7, 8] for instance). A suprafluid and also a spin polarized target are currently set-up in Frankfurt and will be available pretty soon.
On top of this, the flexibility of the momentum spectrometer is very remarkable: The set-up (consisting mainly of copper-rings defining the electric field, ceramic rods and metal meshes) is pretty cheap, easy to simulate numerically and quickly changeable. By now this experimental technique known as COLd Target Recoil Ion Momentum Spectroscopy (COLTRIMS) was able to measure electrons from 0.5 to 200 eV in coincidence with recoiling ions from 5 meV to 15 eV with 4π solid angle in (e,2e), (e,3e), ion-atom, ion-molecule collisions, and photo ionization processes (with synchrotron radiation and strong femtosecond laser pulses). Since it is a spectrometer to track-back the dynamics of fragmentation processes it is also called a ‘reaction microscope’. It is under continual improvement and has proven to be a powerful and considerable tool for the investigation of many particle dynamics.
R. Dörner et al., Physics Reports, 330, (2000), 95-192
R. Dörner…T. Weber et al., Nucl. Instr. Meth. B, 124, (1997), 225
J. Ullrich et al., Rep. Prog. Phys., 66, (2003), 1463-1545
O. Jagutzki et al., Imaging Spectroscopy IV, Proceedings of International Symposium on Optical Science Engineering and Instrumentation, Proc. SPIE Vol 3438, pp 322-333, 1998 Eds. M. R. Descour, S.S. Shen
M. Hattass… Th. Weber et al., Rev. Sci. Inst., 75, (2004), 2373
T. Jahnke et al., Phys. Rev. Lett., 93, (2004), 163401
T. Jalowy et al., International Journal of Mass Spectrometry, 219, (2002), 343-350
T. Jalowy et al., Nuclear Instruments and Methods in Physics Research B, 193, (2002), 762-767
3.) Summary
Before going into the details of some of the ideas mentioned in section 2.) above, the activities of the COLTRIMS research group shall be summarized in the following overview.
General scientific focus of the work group:
| investigation of | the dynamics of simple many particle systems like small atoms and molecules, suprafluids, dimers, surfaces and their ionization mechanisms as well as time resolved inverse chemical reactions (fragmentations) |
| by probing | the momentum phase space, looking for symmetry effects, diffraction and interference, Compton scattering, and internal properties (bond-angle, separation vector etc.) |
| with | single and double photo ionization by single and many photons from synchrotron radiation (low and mid energy range), intense laser fields, and pump-probe experiments |
| using | a powerful and flexible imaging system called COLTRIMS which is capable to measure the square of the wave-function in momentum space of 3 to 5 particles in coincidence |
4.) Idea(s)
In this section some of the ideas mentioned in section 2.) shall be seized and explained in little more detail. Visions and hot topics are rendered briefly:
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.
In terms of time resolved measurements with atoms we plan to perform a two-photon non-linear ionization and double ionization of He. In an initial experiment we will use a very cold helium target with a COLTRIMS set-up to measure the recoil ions. The goal is to set the photon energy below the double ionization limit and monitor the production of double charged He as a function of the power of the x-ray pulse. The goal is to reproduce the “knee” that reflects a transition from a non-sequential to sequential double-ionization. The next step of this experiment is to select two different powers – one below the knee and the other above the knee and compare the momentum that is carried by the ion to try identify any difference due to the electron correlation. We projected and designed a new Ultra-High-Vacuum set-up (see fig. 3), housing a fully equipped reaction microscope, which is able to detect the square of final state wavefunctions of ionization events in momentum space (Momentum Imaging Spectroscopy for TimE Resolved Studies MISTERS). The set-up will be transportable and can serve in various experiments and laser laboratories in building 2 (high harmonics and femto second lasers) as well as at the ALS femtosecond beamline 6. It will also serve as a prototype for a possible COLTRIMS chamber at LCLS.
Figure 3: Sketch of the new UHV setup called Momentum Imaging for TimE Resolved Studies (MISTERS). The supersonic two stage gas jet comes from the top and is intersected with the photon beam inside the momentum spectrometer in the center of the chamber (mounted horizontally). The gas jet is blasted in a two stage jet dump below the main chamber. A pair of Helmholtz coils guides the electron to their detector.
Atoms and Molecules in short Laser Pulses: 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 (e.g. 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 direction of the electron momentum. Thereby a full laser cycle is mapped to a full 360° turn in momentum 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.
Emission from Surfaces: 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 new endstation is in the design phase (called Time Resolved Imaging of Photoelectrons from Surfaces TRIPS). We want to build a unique and also mobile apparatus, which is based on a novel microscopy technique for imaging the dynamics of electrons in solids, and which will be used at the Advanced Light Source (ALS) (slicing beamline 6). This setup can be also employed at the High Harmonics Laser Labs of the Ultrafast X-ray Science Lab as well. The purpose of this apparatus is the coincident measurement of one, two or more electrons emitted from solids and surfaces after the absorption of one (synchrotron), two (High Harmonics and pump & probe) or many (Laser) photons. Utilizing a momentum imaging spectrometer, adapted from gas phase experiments (Reaction Microscope or COLTRIMS), the electronic emission pattern can be studied angle and energy resolved. Applying two short synchronized light pulses with a controllable time delay (pump & probe at the slicing beamline or the High Harmonics lab) the measurements even become time sensitive.