The destruction of CH + ions in collisions with H atoms has been studied in a temperature-variable 22 pole ion trap (22PT) combined with a cold effusive H-atom beam. The stored ions are relaxed to temperatures of T 22PT 12 K. The hydrogen atoms, produced in a radio frequency discharge, are slowed down to various temperatures of T ACC 7 K. They are formed into an effusive beam. The effective density of the hydrogen atoms in the trap as well as the H 2 background are determined in situ using chemical probing with CO 2 +. The experimental arrangement allows us not only to measure thermal rate coefficients (T 22PT = T ACC), but also to extract state-specific rate coefficients k(J,T t) at selected translational temperatures T t and for the CH + rotational states J = 0, 1, and 2. The measured thermal rate coefficients have a maximum at 60 K, k = (1.2 ± 0.5)×10 −9 cm 3 s −1. Toward higher temperatures, they fall off in accordance with previous measurements and the trend predicted by phase space theory. Toward lower temperatures, the rate coefficients decrease significantly, especially if the rotation of the ions is cooled. At the coldest conditions achieved (beam: 7.3 K; trap: 12.2 K), a value as low as (5 ± 4) × 10 −11 cm 3 s −1 has been measured. This leads to the conclusion that non-rotating CH + is protected against attacks of H atoms. This surprising result is not yet understood. It is most probably due to quantum-dynamical effects already occurring at large distances.

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.

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.

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Flowing and stationary afterglow experiments were performed to study the recombination of D(3)(+) ions with electrons at temperatures from 77 to 300 K. A linear dependence of apparent (effective) binary recombination rate coefficients on the pressure of the helium buffer gas was observed. Binary (D(3)(+)+e(-)) and ternary (D(3)(+)+e(-)+He) recombination rate coefficients were derived. The obtained binary rate coefficient agrees with recent theoretical values for dissociative recombination of D(3)(+). We describe the observed ternary process by a mechanism with two rate determining steps. In the first step, a rotationally excited long-lived neutral D(3)* is formed in D(3)(+)-e(-) collisions. As the second step, the D(3)* collides with a helium atom that prevents autoionization of D(3)*. We calculate lifetimes of D(3)* formed from ortho-, para-, or metastates of D(3)(+) and use the lifetimes to calculate ternary recombination rate coefficients.