REACTIONS OF COLD TRAPPED CH⁺IONS WITH SLOW H ATOMS

Abstract

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.

Influence of the alignment of CH+ ions in reactive collisions with H atoms at very low temperatures

“…(3) is shown in Figure 1 as the full line. This result is practically indistinguishable from the PST result [6], and is clearly in disagreement with the experiment [1] at low temperatures.…”

“…Under these conditions, it would be expected that at low collision energies the reaction would proceed via the formation of a long-lived complex, in which case, the application of standard statistical models (see for example [2][3][4][5] and references therein) should give a reasonable prediction of the reaction rate coefficient. Indeed, one such model based on phase space theory (PST) has been applied [6] to reaction (1) and the resulting rate coefficient at low temperatures approaches a constant Langevin value of k = 1.98 Â 10 À9 cm 3 s À1 , clearly in contrast to the rapid decrease of the measured rate coefficient [1]. Neither quasi classical trajectory (QCT) calculations [6,7] nor quantum mechanical methods [7,8] using the full PES provide a satisfactory explanation of the experimental results.…”

“…First, in Section 2 we briefly recapitulate the standard application of a statistical model to reaction (1). Next, in Section 3 we study the adiabatic rotation states using the angle-dependent long-range interactions and show that the CH + ions in rotational j ¼ 0 and j ¼ 1 states tend to be aligned with the approaching H atoms.…”

Ab initio potential energy surface of CH+2 and reaction dynamics of H + CH+

“…Plasil et al 5,6 have found a similar behaviour. They measured abstraction cross sections which have a strong gradient for temperatures less than 50 K. The rate at 12.2 K, for example, is about 5 · 10 −11 cm 3 s −1 .…”

“…This potential was used by Halvick et al 4 for quasi classical trajectory (QCT) and phase space theory (PST) analysis of the H + CH + reaction. The C + abstraction cross sections are predicted to be monotonically decreasing between 1 K and 1000 K. New measurements of Plasil and Gerlich [5][6][7] show a maximum of the rate coefficient at 60 K and a strong gradient of the thermal rate for lower temperatures, which diverges from theoretical results 3,4 . This discrepancy of experiment and theory and the unexplained abundance of CH + in interstellar environments motivate further research.…”