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We study binary and the recently discovered process of ternary He-assisted recombination of H 3 + ions with electrons in a low-temperature afterglow plasma. The experiments are carried out over a broad range of pressures and temperatures of an afterglow plasma in a helium buffer gas. Binary and He-assisted ternary recombination are observed and the corresponding recombination rate coefficients are extracted for temperatures from 77 to 330 K. We describe the observed ternary recombination as a two-step mechanism: first, a rotationally excited long-lived neutral molecule H 3 ‫ء‬ is formed in electron-H 3 + collisions. Second, the H 3 ‫ء‬ molecule collides with a helium atom that leads to the formation of a very long-lived Rydberg state with high orbital momentum. We present calculations of the lifetimes of H 3 ‫ء‬ and of the ternary recombination rate coefficients for para-and ortho-H 3 + . The calculations show a large difference between the ternary recombination rate coefficients of ortho-and para-H 3 + at temperatures below 300 K. The measured binary and ternary rate coefficients are in reasonable agreement with the calculated values.

This contribution summarizes a variety of results and ongoing activities, which contribute to our understanding of inelastic and reactive collisions involving hydrogen ions. In an overview of our present theoretical knowledge of various HmD+ collision systems (m + n < or = 5), it is emphasized that although the required potential energy surfaces are well characterized, no detailed treatments of the collision dynamics are available to date, especially at the low energies required for astrochemistry. Instead of treating state-to-state dynamics with state of the art methods, predictions are still based on: (i) simple thermodynamical arguments, (ii) crude reaction models such as H atom exchange or proton jump, or (iii) statistical considerations used for describing processes proceeding via long-lived or strongly interacting collision complexes. A central problem is to properly account for the consequences of the fact that H and D are fermions and bosons, respectively. In the experimental and results sections, it is emphasized that although a variety of innovative techniques are available and have been used for measuring rate coefficients, cross-sections or state-to-state transition probabilities, the definitive experiments are still pending. In the centre of this contribution are our activities on various m + n = 5 systems. We report a few selected additional results for collisions of hydrogen ions with p-H2, o-H2, HD, D2 or well-defined mixtures of these neutrals. Most of the recent experiments are based on temperature variable multipole ion traps and their combination with pulsed gas inlets, molecular beams, laser probing or electron beams. Based on the state-specific model calculations, it is concluded that for completely understanding the gas phase formation and destruction of HmDn+ in a trap, an in situ characterization of all the experimental parameters is required with unprecedented accuracy. Finally, the need to understand the hydrogen chemistry relevant for dense pre-stellar cores is discussed.

Recombination of H + 3 with electrons was studied in a low temperature plasma in helium. The plasma recombination rate is driven by two body, H + 3 + e − , and three-body, H + 3 + e − + He, processes with the rate coefficients 7.5 × 10 −8 cm 3 s −1 and 2.8 × 10 −25 cm 6 s −1 correspondingly at 260 K. The two-body rate coefficient is in excellent agreement with results from storage ring experiments and theoretical calculations. We suggest that the three-body recombination involves formation of highly excited Rydberg neutral H3 followed by an l-or m-changing collision with He. Plasma electron spectroscopy indicates the presence of H3.

Abstract. The column density distribution of trapped OH − ions in a 22-pole ion trap is measured for different trap parameters. The density is obtained from position-dependent photodetachment rate measurements. Overall, agreement is found with the effective potential of an ideal 22-pole. However, in addition we observe 10 distinct minima in the trapping potential, which indicate a breaking of the 22-fold symmetry. Numerical simulations show that a displacement of a subset of the radiofrequency electrodes can serve as an explanation for this symmetry breaking. ‡ present address: