Development of a glow-discharge ion-trap instrument for measuring effective radiative-association rate coefficients

TL;DR

A new ion-trap instrument has been developed to directly measure radiative-association rate coefficients of ion-neutral reactions, crucial for understanding processes in cold space environments.

Contribution

The paper introduces a novel glow-discharge ion-trap setup capable of measuring slow radiative-association rates for astrophysically relevant ion-molecule reactions.

Findings

First pressure-dependent study of Ag$^{+}$ + O$_{2}$ reaction.

Lower limit of $1 imes 10^{-15}$ cm$^3$ molecule$^{-1}$ s$^{-1}$ for the rate coefficient.

Instrument can measure diverse ion-neutral radiative-association rates.

Abstract

The ability to directly measure radiative-association rate coefficients for reactions between ions and neutral molecules has long challenged chemical physics laboratories, yet radiative association is one of the most important processes occurring in cold, diffuse regions of space. A reaction kinetics instrument has been developed for the investigation of ion--molecule radiative-association reactions, aimed at measuring slow, effective reaction rate coefficients for species relevant to astrophysical objects. The instrument consists of a glow-discharge ion source for production of bright and stable ion currents, a quadrupole mass filter for mass selection and detection, and a quadrupole ion trap capable of trapping reactants and products for the long times needed to measure slow kinetics. The performance and adaptability of the glow-discharge ion source has been evaluated using several configurations. To assess the feasibility of measuring reaction rate coefficients, the reaction of Ag$^{+}$ and O$_{2}$ was studied under pseudo-first-order conditions in the ion trap at room temperature. We present the first pressure-dependent study of this reaction and extract a lower limit of $1 \times 10^{-15}$ cm$^3$ molecule$^{-1}$ s$^{-1}$ for the Ag$^{+}$ + O$_{2}$ effective radiative-association rate coefficient. Measurements of effective radiative-association rate coefficients are possible for diverse atomic and molecular ions that react with neutral molecules over a range of rates in this versatile new instrument.