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Universität zu Köln
Mathematisch-Naturwissenschaftliche Fakultät
Fachgruppe Physik

I. Physikalisches Institut

Research Projects

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The C3H+ ion has been suspect to exist in space since 2012. Several emission lines towards the horsehead PDR and absorption lines in cold, diffuse gas clouds around Sgr B2(N) at mm-wavelengths have been observed. Due to a lack of experimental rotational spectra this hypothesis could never been proven. Therefore this hypothesis was questioned, and even the C3H- anion was proposed to explain these features based on high-accuracy coupled cluster ab initio calculations. Using pure-rotational LIICG spectroscopy, it was possible to obtain the first experimental rotational spectrum of the bare C3H+ cation, proving the existence of C3H+ in space. ...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

Four rotational transitions (J=1-2 up to J=4-5) of the linear C3H+ cation could be measured using the novel action spectroscopic LIICG technique. A fit of these rotational lines to a closed-shell linear rotor Hamiltonian to quartic order using the program Pickett yielded the rotational constant B = 11244.95000(216) MHz and the centrifugal distortion constant D = 7685(69) MHz. The measured frequencies are, within their error bars, in good accordance with the transitions observed by McGuire et al. and Pety et al., thereby identifying l-C3H+ as the ion responsible for the emission lines.
Spectra of the J = 1-2, J = 2-3, J = 3-4 and J = 4-5 rotational transitions of linear C3H+ obtained using the pure-rotational LIICG technique.
Rotational energy scheme of l-C3H+ showing the rotational constants J and the energy in terms of the rotational constant B. The arrows indicate the measured rotational transitions.


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  • Funding: DFG SPP-PAH; IMM, Radboud University Nijmegen