What are molecular ions

Atomic and Molecular Quantum Dynamics

introduction

Our experimental work focuses on the quantum dynamics of elementary ion systems, from atoms to cold molecules and clusters. This research has a direct impact on the field of quantum chemistry and on basic few-particle quantum physics with regard to the dynamics of systems with particles in highly excited or strongly correlated motion. The results provide important experimental reference data for molecular reactions in the cold interstellar medium, for basic atomic and molecular theory, for the chemical physics of ions and for fundamental physics such as quantum electrodynamics. The research is carried out with the ion storage ring TSR (in operation until the end of 2012), the cryogenic ion storage ring CSR, which is about to be completed, with cryogenic high-frequency ion traps and ultra-cold electron beams and covers a wide range of atomic and molecular processes.

 

Dissociative electron-molecule recombination and related processes

Dissociative recombination is an important reaction of ionic molecular compounds with free electrons that has no energy barrier and takes place even at the lowest temperatures. It leads to the destruction and chemical conversion of the molecules and also generates chemically active radical fragments. The prediction of the reaction rates and product channels of these reactions requires detailed knowledge of intramolecular dynamics, which is very actively investigated worldwide, both experimentally and theoretically.

In our experiments, we combine molecular ion beams with very low translational and vibrational energy at the ion storage rings TSR and CSR with continuously improved techniques of event-by-event fragment imaging, which are even suitable for neutral products and for multiple coincidence events of multi-atomic systems, around the inner Uncover mechanisms of molecular dissociation. Stored ion beams open up the unique possibility of bringing the internal molecular degrees of freedom (oscillation and partial rotation) to thermal balance with the storage environment, defined by the black body radiation in the vacuum housing and by the electron beams interacting with the stored ions. New ultra-cold electron beams from cryogenic photocathode sources are used for electron collision studies with record-breaking resolution of the impact energy. The most recent studies deal in particular with polyatomic molecular ions, which are important for ion chemistry as well as in astrophysical and atmospheric processes, and aim to understand the basic mechanisms.

 

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Cold high frequency ion traps and laser spectroscopy

Molecular ions with low internal temperatures are generated and examined in a cryogenic high-frequency ion trap, which is suitable for buffer gas cooling of the ions down to about 10 K. This storage technology was implemented at the TSR to feed in pre-cooled molecular ions, especially H3+, one of the main types of ions that drive chemical reactions in cold thin media, especially in astrophysics; this project triggered the worldwide development of cold ion injectors in numerous storage facilities. The technique is used to carry out dissociative recombination of H3+ in the lowest quantum state of the ortho- or para-nuclear spin variants of H3+ to study. Highly sensitive rotational vibration laser spectroscopy is also used by H3+ carried out in the ion trap itself. With the help of the new methods, H3+-Laser spectroscopy can be performed in dilute media in ion traps. Work is currently underway to extend the techniques to ion beam environments.

 

Coulomb explosion image of small molecular ions - the negative hydrogen molecular ions

The Coulomb explosion of molecular ions with energies around 1 MeV, either from the TSR or directly from the institute's accelerators, can be triggered by sending them through very thin foils, losing their binding electrons in less than a femtosecond. The event-by-event fragment mapping under these conditions provides snapshots of the molecular vibrational movement and allows the determination of important parameters such as e.g. B. the bond length and the oscillation constant, which reflect the molecular potentials. Experiments on negative hydrogen molecules (especially on H.2-) - recently clearly identified short-lived species with lifetimes below a millisecond, which only become relatively stable when they rotate strongly around their axis. In more recent investigations, the sense of chirality of a partially deuterated epoxy sample could be determined directly by means of this molecular imaging method.

 

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Spontaneous and laser-induced fragmentation of highly excited molecular ions and cluster ions

In a plasma, the internal degrees of freedom (rotational, vibrational or electronic F.) of molecular ions and cluster ions are often highly excited by collisions with other particles. The fastest relaxation channels of these states are the emission of an electron or an atomic fragment. For compact matter, these are thermionic electron emission and evaporation. A particularly important aspect of these processes is the energetic coupling of the electronic energy and the vibration energy in these complex systems, which have a large number of vibration modes with a very high density of excited vibration levels. In order to achieve high sensitivity and a low radiation background in these collision studies, beams of molecular and cluster ions are stored in cryogenic storage facilities at high speed. In pilot studies for the development of the cryogenic storage ring (CSR), the Cryogenic Trap for Fast ion beams (CTF) was developed and used in such experiments. The decay of complex negatives like Al4- and SF6- was examined using particle counting and image detectors.
Further information on the pages of the CSR and the CTF.

 

Dielectric recombination and electron impact ionization of astrophysically relevant ions

The resonant - "dielectric" - recombination is the most important recombination mechanism of highly charged ions in hot, dilute plasma. Particularly for photoionized plasma in an astrophysical environment close to strong radiation sources, spectrally broad groups of dielectric resonances define the recombination rates, from which in particular the frequency of charge states result. This also applies accordingly to highly charged ions in hot earth plasmas, for example plasmas in fusion reactors. Important systems that are examined are uploaded systems of iron (e.g. Fe7+ to Fe17+) and from Wolfram (W20+). The prediction of recombination and electron impact ionization rates for these many-electron systems with the help of quantum mechanical calculations requires a large number of approximation methods in order to be treatable within the scope of the possibilities of current theoretical physics. The theoretical predictions used in astrophysical models are subjected to rigorous comparative tests by measurements in the TSR with highly charged ion beams and ultra-cold electron beams.

 

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High resolution dielectric recombination studies for fundamental atomic structure physics

Excitation energies of highly charged ions, measured with high precision, reflect the dynamics of virtual particle pairs in strong Coulomb fields, which are described by quantum electrodynamics. Here, these quantum electrodynamic interactions are examined specifically in systems that are composed of a large number of charged particles (e.g. lithium- and beryllium-like ions). The relevant energy levels of the charged ions can be reached through their resonant - "dielectronic" - recombination with electrons, in investigations with ion beams stored in the TSR, which interact with the ultra-cold electron beam available there.

 

Lifetime measurements of metastable levels in stored atomic ions

Most atoms, even in their high charge states, can exist in energy levels that remain highly excited for long periods of time, up to milliseconds or seconds; their internal symmetry prevents them from decaying, which would normally occur in much smaller fractions of a second due to the emission of electromagnetic radiation. Such metastable excited ions in high charge states are stored in the TSR and their decay times on the millisecond to second time scale are observed and precisely measured. More information…>