Jochen Mikosch^1,2, Ulrike Frühling¹, Raphael Berhane¹, Dirk Schwalm²,
Matthias Weidemüller¹, and Roland Wester^1,*

¹ Physikalisches Institut, Universität Freiburg, Hermann-Herder-Str. 3, 79104 Freiburg
² Max-Planck-Institut für Kernphysik, Saupfercheckweg 1, 69117 Heidelberg

The chemistry of molecular ions at low temperatures is pivotal for the reaction networks in interstellar molecular clouds and planetary atmospheres. Ion-molecule reactions furthermore play an important role in solutions, and by comparison with gas phase dynamics allows the goal is to disentangle the intrinsic dynamics from the solvent interaction. Cooling molecular ions into single rovibrational levels offers the opportunity to measure the quantum mechanical state-dependence of the reaction, which allows for tests of high-level quantum scattering calculations.

Controlled cooling of charged molecules and clusters into their lowest rovibrational states by buffer gas thermalization at a few Kelvin benefits significantly from the advent of high-order multipole traps such as the temperature-variable 22-pole trap [1]. Such a trap has been employed to observe the laser-induced reaction of H₃⁺ with argon by vibrational overtone excitation. The analysis of the spectra allowed us to determine the H3+ internal and translational temperature [2]. To study the quantum dynamics of reactive collisions of anions, in particular nucleophilic subsitution reactions [3], we have combined a double time-of-flight mass spectrometer with a 22-pole RF trap. Results on anion reactions inside the trap and ion evaporation from the trap mediated by elastic scattering will be presented. Recently, a new imaging system has been added to the setup with which we will be able to study reactive scattering of anions with the crossed-beam technique.

In the future, embedding reactant ions inside clusters of a few water molecules will enable us to switch-on the solution phase interactions in a clean way, while retaining the means to study and manipulate quantum dynamical properties.

[1] D. Gerlich, Physica Scripta T59, 256 (1995).
[2] J. Mikosch, H. Kreckel, R. Wester, R. Plasil, J. Glosik, D. Gerlich, D. Schwalm, A. Wolf,
J. Chem. Phys. 121, 11030 (2004).
[3] S. Schmatz, ChemPhysChem, 5, 600 (2004).

* Email: roland.wester@physik.uni-freiburg.de