Ion trap studies of ternary and radiative association processes

Francesco Antonio Gianturco, Alexander Dörfler, Stefan Willitsch +3 more

By employing ab initio computed intermolecular potential energy surfaces we calculate the radiative association probabilities and rates for two different associative mechanisms involving trapped molecular ions N2+(2Σg) interacting either directly with ultracold Rb atoms or undergoing charge-exchange (CE) processes leading to the formation of complexes of the strongly exothermic products N2(X1Σg) plus Rb+(1S0). The two processes are expected to provide possible paths to ion losses in the trap within the timescale of experiments. The present calculations suggest that the associative rates for the 'vibrational' direct process are too small to be of any significant importance at the millikelvin temperatures considered in the experiments, while the 'vibronic' path into radiatively associating the CE products has a probability of occurring which is several orders of magnitude larger. However the reaction rate constants attributed to non-adiabatic CE [F. H. J. Hall and S. Willist, Phys. Rev. Lett., 2012, 109, 233202] are in turn several orders of magnitude larger than the radiative ones calculated here, thereby making the primary experimental process substantially unaffected by the radiative losses channel.

Arnab Sen, V. G. McEwan, M. J. Anicich

Jennifer S. Brodbelt

D. Gerlich, Gheorge Borodi

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.

D. Gerlich

This contribution concentrates on the use of an ion trapping technique which has been developed for studying collision processes and chemical reactions at low temperatures and which utilizes inhomogeneous radio frequency fields. Special emphasis is put onto recent progress achieved with a cryo cooled 22-pole ion trap. Experimental results, measured at a nominal temperature of 10 K, are presented for radiative association of CH 3 + and C 2 H 2 + with H 2 . Another group of results discussed in this paper deals with the dynamics of the growth of hydrogen clusters starting from H 3 + . In all cases, the influence of H 2 rotation was examined by utilizing p-H 2 and n-H 2 . Also the process of isotope fractionation in the presence of HD and D 2 is briefly mentioned.

K. Ravi