Buffer gas cooling of polyatomic ions in rf multi-electrode traps

Abstract

Cooling all degrees of freedom of a molecule, a cluster, or even a nanoparticle which is suspended in a vacuum, is an experimental challenge. Without suitable schemes, cold or ultracold chemical reactions are not feasible. Methods such as laser based preparation of very slow atoms, decelerating molecules to low velocities with electric fields or freezing molecular ions into Coulomb crystals, are generally not suitable to cool the vibrational or rotational motion of molecules. This contribution describes a new method in which a beam of slow atoms or molecules (H, He, H₂, or D₂) is used for cooling charged particles confined in a multi-electrode rf trap. For reaching sub-K temperatures, the fast part of a cold effusive beam is removed with a shutter before the slow remaining neutrals interact with the ion cloud. The development of a pulsed cold beam source is discussed as well as suitable methods for determining the ion temperature. A challenging application is to prepare internally cold CH₅⁺ for spectroscopy or chemistry. New experimental results for hydrogen abstraction in collisions with slow H atoms are reported at energies of a few meV. For evaluating these measurements and for predicting effective rate coefficients at lower energies, the kinematic conditions of the slow neutral beam–ion trap arrangement have been analyzed in detail. The potential of cooling ions such as protonated methane or H₃⁺ with slow energy selected H atoms is briefly mentioned. An interesting process is the formation of weakly bound ions such as H₄⁺ or CH₆⁺via radiative or ternary association. Such ions are ideal candidates for preparing the corresponding collision complexes very close (µeV) to the dissociation continuum using infrared transitions.