DESIREE storage rings

The DESIREE storage rings have circumferences of 8.6 m and a common section for merging keV- beams of positive ions stored in one ring with keV-beams of negative ions stored in the other ring. In the merging section, both beams travel together and in the same direction through a common, biased, tube-electrode system (see the schematic). The bias voltage can be set to control cation-anion collision energies down to the meV range. This allows studies of interactions between individual pairs of internally cooled ions (in their electronic, vibrational, and sometimes also rotational ground states) at collision energies (temperatures) similar to those in the interstellar medium. The combination of control of internal degrees of freedom, realized by allowing the ions time to relax in the cryogenic environment, and the control of relative motion is a world-unique feature of DESIREE. Equally important is the option to measure distributions of final-state populations (measurements of kinetic energy releases). This opens up for much more detailed studies of ion-ion reaction-mechanism than what has been possible before.

Laser beams can be crossed and/or merged with either or both of the stored ion beams and fluorescence can be observed through optical viewports. The low pressure in DESIREE enables keV-beams of atomic, molecular, or cluster ions to be stored for hours. As mentioned above, the stored ions will in most cases equilibrate thermally (in terms of their internal excitation energies) with the 13 K rings and vacuum chamber on much shorter time scales. In addition, the low temperature allows storage of very loosely bound systems, e.g., anions or dianions, which would be rapidly destroyed by the blackbody radiation field at room temperature. Thus, the DESIREE storage rings offer the possibility to study inherent stabilities of very loosely bound systems including multiply charged molecular anions. Such systems could survive in cold and dark interstellar environments and their reaction cross sections in collisions with cations and neutrals can be expected to be very large (the cross section for electron capture to a cation is expected to be inversely proportional to the electron affinity squared.