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Universität zu Köln
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Mathematisch-Naturwissenschaftliche Fakultät
Fachgruppe Physik

I. Physikalisches Institut

COLTRAP - Cologne 4K Ion Trap Apparatus

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COLTRAP is a new generation 22-pole ion trap apparatus developed and built in our group.
 

The key features of COLTRAP are:

  • Cryogenic 22-pole ion trap: storage of mass-selected molecular ions at temperatures as low as 4 K
  • Tandem mass spectrometer for mass-selection of primary and product ions
  • Single ion detection
  • Two optical ports for two-color spectroscopy, e.g. IR/THz double resonance
  • Proximity of optical port to trap: easy coupling of THz radiation
  • Optional molecular beam source providing neutral reaction partner in LIR or kinetics experiments

Cross-section of the COLTRAP ion trap apparatus

Primary ions are generated by electron impact ionization from suited precursor gases in the ion storage source, where they are stored for typically 100-1000 ms while they undergo further reactions resulting in a variety of secondary ions.
 
After opening the storage source the ions of interest are selected from the generated slow ion pulse by the first quadrupole mass filter and guided by an electrostatic quadrupole bender into the 22-pole ion trap, which is mounted on a closed-cycle 4 K Helium cryostat.
The ions are confined in radial direction by opposite-phase RF voltages applied to the 2 x 11 rods of the trap, and in longitudinal direction by applying a DC voltage to the entrance and exit electrodes. Short pulsing of these DC voltages is used during the filling of the trap and for extracting the ions after a defined storage time. The ions are cooled to the ambient trap temperature (4 – 300 K) by collisions with Helium atoms which are admitted to the trap in a fast, intense pulse shortly before the ions enter the trap volume.
The cold, mass selected ions in the trap can then undergo reactions with neutral gas molecules that are admitted to the trap volume continuously or in a pulsed fashion. The ionic products of these reactions are then extracted, analysed by a second quadrupole mass filter, and counted by a single ion detector. The temperature dependence of rate coefficients of specific ion-molecule reactions are obtained by varying the storage, i.e. reaction, time, and the temperature of the ion trap. With COLTRAP it is also possible to record high-resolution infrared and far-infrared spectra of cold molecular ions by the very sensitive method of laser induced reactions (LIR).
The 22-pole ion trap in its copper housing mounted on the 4K coldhead.

Experimental Scheme