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 (microeV) to the dissociation continuum using infrared transitions.

Controlled synthesis and analysis of He–H+3in a 3.7 K ion trap

“…Unfortunately, in most cryogenic traps, the translational temperature of the ions is slightly higher than the He temperature, T He [32,54]. Despite many activities, the forces accelerating the ions have not yet been identified.…”

Direct observation of high-spin states in manganese dimer and trimer cations by x-ray magnetic circular dichroism spectroscopy in an ion trap

“…Under these conditions, vibrations and rotations are thermalized to equilibrium on a time scale of micro to milliseconds. [50][51][52][53][54] The ion trap was continuously filled with parent ions to the space charge limit. Typical storage (halflife) of the parent ions in the ion trap was 1-10 s. This excludes the possibility of trapping metastable configurations.…”

Collisions of FeO+ with H2 and He in a Cryogenic Ion Trap

“…[2]) that, for many ion-molecule reactions, reaction rate coefficients can be simplyd etermined by the long-range attraction between the colliding partners and that more or less sophisticated capturet heories are sufficient for understanding the system. Preparation of cold molecules for applications in chemical processes cannot be based just on lowering the translational motion;t he inter-nal degrees of freedom mustb ef rozen as well.…”

“…In addition more product channels open up, such as the stabilizedr eaction-intermediate complexes H 2 FeO + and He n -FeO + formed by ternary association with He. The rate coefficient for forming Fe + + H 2 Oi ncreases steeply with decreasingt emperature.…”

“…As discussed in Section 2.1.3, FeO + readily binds He atoms, and mass spectra recordeda th igh He densities (saturation limit) indicatet hat complexes with five He atoms are especiallys table at the low temperature of the trap. (1), (2), and (8) are given in the text. The first state- Table 2a nd discussed in the text.…”