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Universität zu Köln
Mathematisch-Naturwissenschaftliche Fakultät
Fachgruppe Physik

I. Physikalisches Institut

Research Projects

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  • schlemmer


The fluoromethylidynium cation, CF+, has been detected in a variety of interstellar environments, including photon-dominated-regions (PDRs), diffuse clouds, the envelope of a high-mass protostar, and even an extragalactic source. The main interest in CF+ observations lies in its formation pathway, which proceeds by an exchange of the fluorine atom of hydrogen fluoride (HF) in a bimolecular reaction with ionized atomic carbon (C+). Thus CF+ can serve as a molecular tracer for HF and C+, both keystones in astrochemistry, but not observable from the ground. CF+ (with a rotational constant of around 52.3 GHz) has many accessible transitions at mm-wavelengths, and its rotational spectrum is well studied up to 1.6 THz by absorption spectroscopic techniques. The CF+ rotational ground state transition, however, had not been observed in the laboratory so far. In fact, the highest resolution measurement of this line so far came from its astronomical observation in the Horsehead PDR. Interestingly, the astronomical line showed a double-peaked line-profile, which was originally attributed to kinematic effects. Shortly thereafter, quantum chemical calculations suggested that the line profile could be explained spectroscopically by hyperfine splitting (hfs) caused by the non-zero nuclear spin (I=1/2) of the fluorine nucleus. An experimental verification of these calculations, however, was lacking. ...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

Using the method of rotational state-dependent attachment of He atoms to cold, mass-selected CF+ ions we have now succeeded in measuring for the first time the rotational ground state transition (J=1-0) of this closed-shell diatomic ion. Experiments were performed in the cryogenic 22-pole ion trap instrument FELion. Owing to the low ion temperatures, and thereby narrow Doppler widths achievable in our trap experiments, we were able to resolve the two hyperfine components of the CF+ rotational ground state transition and extract their transition frequencies to within 3-4 kHz, i.e. to a relative accuracy better than 10-7. From this we accurately determined the spin-rotation interaction constant to CI=227(3) kHz, which is in excellent agreement with the theoretical value of 229.2 kHz that was used to explain the observed double-peaked line structure of this transition towards the Horsehead PDR. Thus our spectroscopic experiments confirm the intrinsic nature of the astronomically observed line structure, ruling out that it stems from kinematic effects.

Measured rotational ground state transition (J=1-0) of CF+ with resolved hyperfine structure 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.