Experimental investigations of ion–molecule reactions relevant to interstellar chemistry

Abstract

Based on the method to confine charged particles in inhomogeneous radiofrequency fields, two experimental set-ups have been developed for the study of ion–molecule reactions approximating interstellar conditions. The central part of the first apparatus is a variable-temperature ion trap (10–300 K), ring-electrode or 22-pole geometry). Long trapping and interaction times lead to high sensitivity, and allow one to determine very small rate coefficients, e.g. for radiative association. In the second apparatus a slow guided ion beam is superimposed coaxially with a supersonic neutral beam, combining the advantages of internally and translationally cold neutrals (also for condensable gases) with the kinematic compression resulting from the merged beam geometry. This technique enables one to determine absolute integral cross-sections over a wide range of collision energies, especially at energies of astrophysical interest. The internal temperature of the primary ions can be varied between 15 and 300 K. Both machines have been used to study a variety of ion–molecule reactions of interstellar interest. In this contribution new results are presented for the fine structure, rotational and isotope effects on the reaction N⁺+ H₂→ NH⁺+ H. Rate coefficients have been determined at temperatures between 15 and 300 K using the ion trap, while state-specific cross-sections have been measured in the beam apparatus at collision energies between 1 and 500 meV. The experiments corroborate earlier conclusions that 17 meV are required for NH⁺ formation from N⁺(³P₀)+ H₂(j= 0), but the measured activation energy for the deuteriated analogue raises anew the question as to whether the energy represents a barrier or endoergicity. Other examples include the formation of weakly bound products such as HeN⁺ and isotopic scrambling in D⁺₃+ H₂ collisions. Finally, very recently measured ternary and radiative association rate coefficients C₂H⁺₂+ H₂→ C₂H⁺₄ at 15 K are reported with special emphasis on the role of ortho- and para-H₂. The perspectives are briefly discussed.