Siegel der Universität

Universität zu Köln
Mathematisch-Naturwissenschaftliche Fakultät
Fachgruppe Physik

I. Physikalisches Institut

Research Projects

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  • asvany
  • schlemmer


The H3+ ion is omnipresent in the ISM. Since the proton affinities of hydrogen and oxygen are very similar, it has been discussed that the proton transfer from the H3+ to molecular oxygen in the near thermoneutral reaction
  • H3+ + O2 ⇌ O2H+ + H2
effectively binds the interstellar oxygen in O2H+. If this is true, the protonated molecule can be taken as a tracer for the neutral molecular oxygen, which cannot easily be observed due to its missing dipole moment. ...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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Collision Dynamics
Experiments are performed in a variable temperature 22-pole ion trap. In the trap, mass selected ions are typically stored over seconds. Upon storage they are accommodated to the ambient trap temperature via He buffer gas cooling. The reactant gas is admitted to the trap at a fixed but variable number density. The trap content is analysed as a function of the trapping time. Rate coefficients are derived from the simulation of the underlying kinetics solving a set of rate equations. This method has been applied to several astrophysically interesting systems.
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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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Recent Results

The proton transfer reaction H3++O2 was characterized in the temperature range of 40-280K. The measured rate coefficients for the proton transfer show an Arrhenius behaviour with an activation energy of EA/k = 113 K. Figure 1 shows a summary of all measured rate coefficients.

For low temperatures only a few rotational states are populated. In Figure 2 we show kinetic measurements for T=129 K and T=49 K, as an example - including our numerical simulations. Using the very sensitive method of laser induced reactions (LIR) it was possible to probe the lowest rotational levels of H3+ at 40 K. Figure 3 shows the two transitions starting from the two lowest rotational levels (1,1) of para-H3+ and the slightly higher lying (1,0) level of ortho-H3+.

Figure 1: Summary of measured rate coefficients for the reaction H3+ + O ⇌ O2H+ + H2. The limiting Langevin collision rate and the Arrhenius fit of our data are also shown.
Figure 2: Kinetic measurements at T=129 K (a) and T=49 K (b). The solid lines are numerical simulations considering only the three lowest levels of H3+ that are labelled with their quantum numbers (J,G). Especially for T=49K the para-H3+ (1,1) reacts much faster than the ortho-H3+. For long storage times the almost non reactive ortho-H3+ dominates the decay of H3+.
Figure 3: LIR spectrum of H3+ in collisions with O2 at 40 K.
Figure 4: First high-resolution spectrum of O2H+, measured in the COLTRAP 22-pole ion trap .


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