Research on correlated quantum dynamics represents one of the great challenges in contemporary science. Researchers at the MPIK explore quantum dynamics at its very fundamental level, starting from a limited number of few interacting particles in atoms and molecules, with first exciting results on more complex finite quantum systems like clusters or even large biological systems.

Collisions of charged particles (electrons, ions) with atoms, molecules, clusters and surfaces are a key method for the study of quantum systems. Novel multi-coincident imaging techniques developed at MPIK provide most comprehensive information about few-body quantum dynamics. For example, single and double ionization by electron and positron impact, the most fundamental 3- or 4-particle reactions, are studied covering the full solid angle for electron emission providing the ultimate test of theoretical calculations.

Detailed studies of reaction dynamics require good instrumental resolution as well as cold targets and projectiles. Otherwise, thermal motion would blur the images. Here, a new setup combines the cooling techniques from a storage ring (TSR) and a magneto-optical trap.

On the molecular level, quantum physics manifests itself in numerous facets and determines the outcome of all types of chemical conversions. Researchers at the MPIK apply various complementary methods to study molecular systems and their interactions under strictly controlled conditions. A powerful method is fast-beam fragmentation spectroscopy using stored and cooled molecular ion beams (e. g. in the TSR), where molecular break-up is induced by electrons and the fragments are observed and imaged on an event-by-event basis.

One puzzling question is the formation of organic compounds in interstellar clouds. This complex chemistry is driven by reactions with ions and radicals which are created in collisions with photons and cold electrons. The break-up of molecules after electron capture (“dissociative recombination”) is studied in detail in the TSR at MPIK. Here, the H₃⁺ molecule plays a key role. Another example is the dissociative recombination of hydronium leading to hot water molecules of low kinetic energy in a cold environment. Here, the reaction energy is transferred to vibration and rotation of the molecules. This explains the observed infrared radiation from hot water molecules on comets.

Collisions of extracted highly charged ions from an EBIT with neutral atoms leading to charge exchange are investigated in a reaction microscope. Intense, low-energy electron beams collide in the TSR with highly charged ions at the conditions of an astrophysical plasma heated by strong X-ray sources such as active galactic nuclei.

Division Ullrich    Division Blaum    Group Fischer

Atoms under Bombardment: From Tumor Therapy with Fast Ions to Ion-Atom-Collisions (pdf)
The TSR Heavy-Ion Storage Ring and the Accelerator Facilities (pdf)
Stellar Furnace in the Freezer Box: Highly Charged Ions at 100 Millionen Degrees (pdf)

Reaction microscopes (REMI) – “the bubble chambers of atomic and molecular physics” – have been developed and are continuously improved at MPIK. Ultra-short intense laser pulses or a particle beam break up simple molecules. The fragment ions and electrons are caught by means of electric and magnetic fields and recorded by large-area time- and position-sensitive detectors. Their complete momentum vectors can be determined from the reconstructed trajectories of the fragments ("kinematically complete experiments"). Recently, a novel REMI incorporating ultra-cold lithium atoms as a target in a magneto-optical trap (MOT), has been integrated in the TSR. Instruments are deployed in-house and at external light sources such as free-electron lasers (FELs). Further REMIs are dedicated to experiments with single attosecond pulses at MPIK and other institutes.

Both, neutral and charged fragments from fast molecular ion beams interacting with energetic photons are detected in coincidence with imaging detectors in the TIFF setup.

The CFEL-ASG Multi-Purpose (CAMP) instrument represents the latest step in technological development. Its unique feature is the simultaneous momentum imaging of ejected ions, electrons and photons. Therefore, large-area photon cameras, pnCCDs, have been implemented. Its development and operation are a collaborative endeavour of the Max Planck Advanced Study Group (MP-ASG, lead by MPIK) at the Center for Free Electron Laser Science (CFEL) in Hamburg.

Further, a multistrip silicon detector identifies neutral fragmentation products of fast molecular ion beams circulating in the TSR and measures their velocities.

Division Ullrich    Group Fischer

Atoms under Bombardment: From Tumor Therapy with Fast Ions to Ion-Atom-Collisions (pdf)

From the early days of quantum mechanics until today it has remained a dream of physicists to watch the correlated motion of electrons in atoms or molecules. This is a difficult task as a simple estimate shows that electrons move on timescales of a few tens of attoseconds (1 as = 10^–18 s) in the outer atomic or molecular shells.

The basis for time-resolved experiments are ultrashort intense laser pulses which are used to steer the atomic or molecular dynamics with extremely high precision. Electrons, released from an atom by a strong laser field are driven back and forth and revisit its parent ion while probing its structure. The wave nature of the electron leads to interference effects like in a holographic image which can be analysed to resolve the time-dependent interaction with the residual electrons of the atom on an attosecond time scale.

In many cases, a ‘pump-probe’ scenario is applied, where the first ‘pump’ laser pulse prepares the system in the desired way and starts the time evolution which is then probed by the second laser pulse. Molecular motion like vibration and rotation can be traced and movies of these quantum systems can be generated, visualising the dynamics. Observing chemical reactions in real time at femtosecond resolution is one of the most promising research areas envisioned at free-electron lasers (FEL). In combination with the REMI technology even the ultrashort time span needed for a so-called isomerization reaction – a rearrangement of atoms within a molecule – which is essential in eye-vision or photosynthesis could be observed.

In addition, the high intensities of FEL sources open the field of multiphoton processes in atoms or even nuclei in the ultraviolet and X-ray regime.

Division Ullrich    Group Pfeifer

Ultra-Short Laser Pulses: "Chemistry" in Slow Motion (pdf)
Free-Electronen Lasers: Physics with Ultra-Short and Super Brilliant X-Ray Pulses (pdf)