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Universität zu Köln
Mathematisch-Naturwissenschaftliche Fakultät

I. Physikalisches Institut

Sonderforschungsbereich 494

Project Section E4 (Proposal)

Laser Induced Reactions (LIR): spectroscopy and dynamics of astrophysically relevant ions

Ions play a key role in molecule formation in space. However, they are hard to detect since their fractional abundance is much below that of common neutral species. Therefore ions make up only a very small fraction of the detected molecules. Nevertheless many ions have been detected in various environments, H3+ being the most abundant. Very similar restrictions hold for the laboratory work. Ions are only transient species which are hard to be formed in high densities or with substantial optical depth to make them available for absorption spectroscopy. Therefore only very abundant ions like SO+ or H3+ have been studied in greater detail. This challenge calls for specific methods which can circumvent this problem.

Traps have been used for many years to store ions and to study ion-molecule collisions in great details. Thanks to the development of higher order multipole traps, mainly developed in the group of Dieter Gerlich, it became possible to study these collisions at energies comparable to those of cold environments in space in particular dense molecular clouds. In recent years we combined the trapping technique with IR and FIR radiation sources. As it turns out many reactions of astrophysical interest are substantially enhanced due to internal excitation of the ion. This fact is used in the method of laser induced reactions to (i) obtain spectra of molecular ions. The method is so sensitive that only ~1000 parent ions are necessary. (ii) State specific knowledge of the ion molecule reaction is obtained from these measurements as well. Current results on C2H2+ show that one is even able to determine radiative lifetimes of the excited species as well as reaction rate coefficients for the underlying collision.

In this project section LIR shall be further explored and exploited. Spectra of the astrophysically relevant species CH5+ and C3H+ shall be recorded at high-resolution thanks to the new IR laser system which shall be purchased by the current funding. For the H3+ system it is planned to study the endothermic reaction H2D+ + H2. From ro-vibrational spectra of the H2D+ ion the rotational population shall be inferred. Most interesting in that respect is the question how large the fraction of ortho-H2D+ is. On the one hand side this number is closely related to the ortho to para ratio of H2. On the other side deuterium fractionation, which is widely believed to be driven by H3++HD, will be delicately depend on this number. This example nicely demonstrates the capabilities of LIR. The work in the laboratory will be accompanied by ortho and para H2D+ observations which are planned in project section A6. Combining all our knowledge in modelling appropriate chemical network systems will help to unravel the mystery of the very large deuterium fractionation in H2D+ and D2H+.

In the second apparatus emission spectra of polycyclic aromatic hydrocarbon cations (PAH+) shall be taken. These species are thought to be the carriers of the unidentified infrared bands (UIR), which despite their ubiquitous presence have not been assigned to any particular species. This project involves a very sensitive IR spectrometer, SPIRES (Single Photon Infrared Emission Spectrometer), which will be made available to the cologne spectroscopy group by Prof. R.J. Saykally at Berkeley. A beam of PAH+ will be produced and put in the viewing region of the spectrometer. It is a great experimental challenge to produce, guide, mass select and potentially trap these ions in the 4 K environment of the spectrometer. It will be crucial to image the light of some 106 ions. Guiding of slow ions will be used in one setup. Alternatively the spectrometer will be adapted to an electrostatic trap at cryogenic temperatures which will become available soon at the Max-Planck-Institute for nuclear physics in Heidelberg. Large ion currents suitable for the planned emission experiments make this a promising approach in close cooperation with the Heidelberg group. First emission spectra of mass selected PAH+ are the major goal of this project. It is the aim to either find a decent match with observations or to guide the search for viable candidates or to discard the PAH+ hypothesis as carriers of the UIR bands.


See the home page of the Laboratory Spectroscopy Group for more information.