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Universität zu Köln
Mathematisch-Naturwissenschaftliche Fakultät
Fachgruppe Physik

I. Physikalisches Institut

Research Projects

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  • asvany
  • schlemmer


The hydroxyl anion OH is one of the simplest molecular anions. Its presence in the interstellar medium remains unclear, especially considering the fact that the electron affinity of OH is quite low (1.8 eV), compared to other molecules creating anions (>3.0 eV), that have already been confirmed. The rovibrational and pure roatational spectra have been measured previously (see Rosenbaum and Matshushima respectively). The resolution of rotational spectra measured by Matshusima (~300 kHz) is good enough for radio-astronomy identification. We are presenting a different approach of acquiring a transition wavelength with accuracy down to a kHz range. The two photon laser induced reaction (LIR) scheme is used. It relies on a endothermic reaction:
  • OH(ν,J) + H2 → [H3O]* → H + H2O,
proceeding only when OH is sufficiently excited (endothermicity of reaction is around 0.36 eV). ...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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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

In this experiment rovibrational and pure rotational spectroscopy have been carried out. The rovibrational transitions observed previously by Rosenbaum have been remeasured using a standard LIR scheme. At this stage, the nominal trap temperature was kept a bit higher, around 16 K, in order to populate higher rotational levels. Transitions from levels up to J = 3 could be observed. One particular transition (depletion of stored reactant OH as well as increase of a product H) can be seen in figure 1.
Figure 1. Rovibrational transition (1,1) ← (0, 0) of OH recorded using laser induced reaction (LIR) scheme. Upper panel represents the decrease of the primary anion OH, while the lower panel represents the increase in H − the product. The discrepancy between the reactants and products is attributed to the mass discrimination of the quadrupole between masses 17 and 1 u.
In the second experiment a novel two photon scheme is used to influence the rotational level distribution in order to record a pure rotational spectra. This is the first system, where the reaction endothermicity is higher than energy available from rotational excitation (see H2D???), thus two photon are used. First being a THz photon changing responsible for the rotation and a second NIR photon, responsible for the rovibrational transition. The fundamental rotational transition of OH can be seen in figure 2. The nominal trap temperature was kept around 10 K, in order to avoid H2 freezing. The low temperature allows us to keep the doppler width of a line reasonably low, moreover multiple measurements can be used to determine the center position with accuracy down to 1 ppb. Such high accuracy is not needed for radio-astronomy observations, however it may find its place in fundamental physics.
Figure 2. Pure rotational transition (0,1) ← (0, 0) of OH recorded using the two photon LIR scheme. The nominal trap temperature is 10 K. The resonance is very narrow, the doppler broadening indicates the temperature of 15 K.


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  • This work has been financially supported by the Deutsche Forschungsgemeinschaft (DFG) via SCHL 341/6-1.