Trapping and lasercooling of (molecular) ions: toward ultrafast imaging of structures and dynamic processes

Dr. T. Schätz, Experimental Physics, MPQ

For more detailed information on these studies, please contact Dr. Tobias Schätz
(Tel.: +49 +89 32905 199, e-mail: tschaetz@mpq.mpg.de)

Imaging of the structure and/or femtosecond dynamics of a single particle

The group of Tobias Schätz focuses on trapping and cooling of (molecular) ions. We plan to investigate the possibilities to exploit the un-precedent time resolution and intensity of photon and photon-generated electron pulses on targets of the size down to one single particle.

In the framework of the IMPRS-APS, we want to contribute to the following aspect:

On the time scale of femtoseconds and beneath the dynamics of molecules and atoms are almost frozen. Due to their smaller mass electrons are still able to follow the applied forces mediated by the laser fields causing a corresponding displacement of their charge. Thus, the fast dynamic of the electrons initiate the subsequent slower chemical processes.

Different methods of diffraction allow to access complementary information directly. X-ray diffraction is suited for the investigation of the structure of the particle of interest, while electron diffraction provides insight into the non-equilibrium dynamics of its chemical evolution.

Due to the recent developments towards higher laser intensities and shorter pulse duration (<10 femtoseconds and mediumterm sub-femtosecond) we will be provided with tools that should enable us to perform "single molecule imaging".

But these new imaging tools require new methods of sample preparation. The ideal scenario would be realized by a single particle of interest fixed at a precisely defined position in space, isolated against external disturbances and that can be reliably prepared in a desired initial state. Electrically trapped ions offer possibilities close to this ideal scenario. It is possible to store (molecular) ions for durations of hours. Sympathetic cooling the molecular ions via the Coulomb interaction with a cold bath, e.g. of laser cooled ancilla ions, finally leads to a phase transition of the ensemble - a crystallisation where neighbouring ions can't exchange their position anymore. Even though the molecular ions will not resonantly emit radiation and therefore will stay dark, the fluorescing ancilla ions will be detectable and therefore, the dark molecular ions will show up as "missing/dark spots" in the crystalline structure. An individually addressable "dark spot" indicates the position of the molecular ion as the target for the stroboscopic illumination via the femtosecond photon and electron pulse, respectively.

In a first step, we want to build up an apparatus to setup laser cooling and diagnostics on the ancilla (Mg+) ions. In parallel, we will work on the preparation of electron pulses generated by femtosecond laser pulses. The simple formation of molecular ions, like Magnesium hydride will provide us with a test target for first diffraction measurements of electron pulses, with an average electron number down to one and a time resolution shorter than a picosecond. To fully exploit the advantages of the method, we will adapt the apparatus for trapping, cooling and state preparation of more complex (bio) molecules (featuring an evolution, e.g. during a chemical or biological transition, that challenges this time resolution). By reducing the electron number per pulse down to one and simultaneously increasing the repetition rate of the pulses, we are aiming for "single molecule imaging" with a time resolution better than 50 femtoseconds.

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