Siegel der Universität

Universität zu Köln
Mathematisch-Naturwissenschaftliche Fakultät
Fachgruppe Physik

I. Physikalisches Institut

Research Projects

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  • asvany
  • schlemmer


Protonated methane, CH5+ , is a very floppy molecule for which a coherent theoretical description is missing to date. As a result of this floppiness, its rovibrational C-H-stretching spectrum at 3 μm is chaotic and yet unexplained. In order to stepwise understand this enigmatic molecule we cool it to cryogenic temperatures in our 22-pole ion trap machines (LIRTrap and COLTRAP) and record its C-H stretching vibrational lines in the infrared with high accuracy using a frequency comb. ...more


Laser induced reactions (LIR)
Laser induced reactions (LIR) belong to the family of "action spectroscopy" methods. In the special case of LIR, changes of the rate coefficient of an endothermic ion-molecule reaction serve to detect the excitation of the parent ionic species. This offers not only the possibility of performing very high sensitivity spectroscopy on transient ions (a number of only 1000 ions per trapping period is enough), but LIR can yield information on state-selected reaction rate coefficients and lifetimes of excited states.
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Light Induced Inhibition of Complex Growth (LIICG) and Rotational State-Dependent Attachment of He Atoms
LIICG is a novel action-spectroscopy scheme (see also LIR - Laser Induced Reactions technique) for measuring high-resolution ro-vibrational spectra of gas-phase molecular ions. This method makes use of an inhibition of Helium-attachment to vibrationally excited molecular ions. Furthermore, we also observed a change in the rate of Helium-attachment depending on the rotational state of the cold, stored molecular ions. This effect can be exploited to perform purely rotational action spectroscopy on a wide class of molecular ions. Both methods can, due to the low temperatures needed, only be employed in our two new 4 K 22-pole ion traps COLTRAP and FELion.
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The 22-pole ion trap apparatus LIRTrap is used both for kinetic and spectroscopic characterization of ions.
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COLTRAP and FELion are two new generation 22-pole ion trap instruments developed and built in our laboratory. Both instruments offer unique possibilities to study the kinetics of ion-molecule reactions at low temperatures, and to use highly sensitive methods for spectroscopic studies of molecular ions. Whereas the COLTRAP instrument is located in the Cologne laboratories, FELion has been installed in October 2014 at the FELIX Laboratory (Radboud University Nijmegen, Netherlands).
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Recent Results

The cooling of CH5+ ions and the frequency determination using a frequency comb allows for high accuracy and precision. Up to date, nearly 3000 C-H-stretching transitions between 2900 and 3100 cm-1 have been recorded. The analysis has been published in Science, revealing the ground state structure of CH5+ for the first time.
High-resolution CH-stretching vibrational spectrum of CH5+ measured at nominal temperatures of 10 K and 4 K.
Combination difference spectrum of CH5+ revealing the most prominent ground state combination differences.
Spectrum of a single rovibrational CH5+ line obtained by injecting about 10000 CH5+ ions into a 22-pole ion trap containing a cloud of cold and dense helium and irradiation with a 1.2 W cw laser beam. The trapping/irradiation time is 450 ms. By resonant excitation of CH5+ the number of CH5+He ions, formed by ternary association, is decreased.
Single rovibrational transition of CH5+ detected by inducing the reaction CH5+ + CO2 → CH4 + CO2H+ with an optical parametric oscillator and counting the reaction product CO2H+.

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  • Funding by DFG SFB 956, SCHL 341/6-1 and AS 319/2-2.