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Universität zu Köln
Mathematisch-Naturwissenschaftliche Fakultät
Fachgruppe Physik

I. Physikalisches Institut

Research Projects

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  • bruenken
  • schlemmer


The ammonium ion, NH4+, is related to nitrogen chemistry in the interstellar medium. In cold molecular clouds NH4+, which is formed by subsequent hydrogenation reactions starting from N+ with H2, is assumed to be the gas-phase precursor of the ubiquitous ammonia molecule, NH3. The high deuterium fractionation of ammonia seen in many dark clouds is caused primarily by transfer reactions of ammonia with deuterated forms of H3+, producing deuterated variants of NH4+, followed by dissociative recombination with electrons. Whereas NH4+, a spherical top molecule, is not observable by radio astronomy, its mono- (and doubly-) deuterated forms have comparatively large permanent electric dipole moments and possess rotational transition lines in the mm- and sub-mm wavelength range. The monodeuterated variant NH3D+ was recently tentatively detected towards a massive star-forming region in Orion, and towards a cold prestellar core. The tentative identification was based on a good agreement between the observed spectral lines and predictions of the NH3D+ ground state rotational transition (JK=10-00) from high-resolution infrared data. However, a direct laboratory measurement of the observed rotational ground state transition line was missing. ...more


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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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

We have now succeeded in measuring the rotational ground state transition (JK=10-00) of NH3D+ directly using the method of rotational state-dependent attachment of He atoms to the cold, mass-selected ions. Experiments were performed in the cryogenic 22-pole ion trap instrument FELion. The transition frequency could be determined from the measured spectrum to an accuracy of 5 x 10-8, made possible by the ions low temperature (~10 K) and consequently narrow Doppler linewidth. The obtained experimental transition frequency agrees with the interstellar line observations within the estimated uncertainties. Therefore, from a purely spectroscopic view our laboratory measurements support the assignment of the astronomical lines to NH3D+. Ancillary support for this identification can be obtained by the observation of additional rotational transitions, for which accurate rest frequencies can now be predicted from our spectroscopic analysis of the newly measured rotational ground state transition together with previous infrared ro-vibrational data. An alternative is to search for higher deuterated variants of the ammonium ion, i.e. NH2D2+ and ND3H+. Experimental studies of their ro-vibrational and subsequently rotational spectrum are in preparation.

Measured rotational ground state transition (JK=10-00) of ortho-NH3D+ using the method of rotational state-dependent attachment of He atoms to the cold, mass-selected ions.


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  • Funding by SPP 1573 and SFB 956.