Mass-selected ion–molecule reactions at very low temperatures. The CRESUS apparatus

Abstract

An improvement of the CRESU technique is presented, consisting of a device for injecting mass-selected ions into the core of a uniform supersonic flow of very low temperature, pertaining to those in dense interstellar clouds. This version of the experiment has been called CRESUS, standing for ‘Cinétique de Réactions en Ecoulement Supersonique Uniforme avec Sélection’(Kinetics of Reactions in Uniform Supersonic Flow with Selection). This allows ion-molecule reaction rate coefficients to be determined as in the well known SIFT apparatus, i.e. by working under true thermal conditions in a reaction zone where only the primary ion and the neutral of interest are present. After mass-selection the ions are injected into the flow through an exit hole at a given location on the nozzle wall. They are drifted radially in the nozzle by using an electric fields. Because of the combination of drift vicinity of the nozzle exit plane. The present data have been obtained at 20 and 70 K in helium buffer gas. They concern both binary reactions of Ar⁺, He⁺, N⁺, N⁺₂, and O⁺₂ ions, and ternary reactions of C⁺ and CH⁺₃ ions. For binary reactions the results are compared when possible to those collected in previous CRESU studies. A good agreement is found, which establishes the validity of the improved technique. The perspectives for the near future are briefly discussed.