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

TL;DR

The paper addresses the challenge of directly measuring effective radiative-association rate coefficients for ion–molecule reactions in astrophysically relevant conditions. It introduces the glow-discharge ion-trap (GDIT), a modular instrument that combines a bright, continuous ion source, a dual-pass quadrupole mass filter, and a linear ion trap to study slow reaction kinetics and identify products. Using Ag$^+$ + O$_2$ as a proof-of-principle, the authors extract a pseudo-first-order rate constant and deconvolute radiative from three-body stabilization contributions via pressure dependence, reporting a lower limit for the effective radiative rate $k_r^{\mathrm{eff}} \ge 1\times10^{-15}\ \mathrm{cm^{3}\,molecule^{-1}\,s^{-1}}$ and a small three-body term $k_3^{\mathrm{eff}}$. The results validate GDIT as a capable platform for direct radiative-association measurements and set the stage for extended measurements across a broader range of ions, temperatures, and reaction partners, informing astrochemical models and the fate of charged species in space.

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.

Figures (7)

• Figure 1: Right: The GDIT instrument schematic view: (1) glow-discharge cathode, (2) exit lens serving as the anode, (3) gate-valve aperture, (4) quadrupole ion guide with asymptotic rods, (5) focusing ion lenses, (6) electron-multiplier detector 1, (7) quadrupole deflector 1, (8) focusing ion lenses, (9) quadrupole mass filter, (10) quadrupole deflector 2, (11) electron-multiplier detector 2, (12) ion-trap entrance lens, (13) ion trap with asymptotic rods, (14) ion-trap exit lens. Top left: A photo of the glow-discharge cathode pictured with an unused silver cathode. The cylindrical metal sample is inserted into the glow-discharge wand and fixed by three set screws. Bottom left: Timing sequence for operating the GDIT instrument. The top trace controls the first deflector electrode pair switch, which governs the trap-fill process. The second trace controls the ion-trap entrance opening time for ion extraction. The bottom trace defines the data collection window. An example ion packet obtained each duty cycle is shown in the inset window (blue trace). Not to scale.
• Figure 2: Ion trajectories for a) mass spectrum collection; b) kinetics experiments. In both configurations after generation in the glow-discharge chamber the reactant ions travel following the red trace through the ion guide, bend 90° via deflector 1 and are a) mass resolved or b) mass selected in the quadrupole mass filter. While in a) ions culminate at electron-multiplier detector 2, in b) ions bend 90° by deflector 2 towards the ion trap. In b), after a pre-selected time interval both reactant and product ions exit the trap and travel following the yellow trace through deflector 2 towards the quadrupole mass filter to be mass resolved and detected by electron-multiplier detector 1.
• Figure 3: Mass spectra of produced ions using (top) nickel, (middle) silver, and (bottom) xenon precursors.
• Figure 4: Example mass spectrum resulting from reaction between Ag+ and O2 for identification of reactant and product ions. The mass spectrum is collected for a 2% mixture of O2 diluted in He at room temperature and 17 mTorr in the trap with a trapping time of 0.5 seconds. The peak centered at m/z = 107 is assigned to the Ag+ reactant and m/z = 139 corresponds to the AgO2+ product.
• Figure 5: Kinetic trace of Ag+ depicting reactant decay in the presence of O2. Ion packet peak voltages are accumulated and averaged over 20 samples with error bars showing uncertainty due to random fluctuations. Measurements obtained for a 0.35% mixture of O2 diluted in He to a trap pressure of 18 mTorr at room temperature. The solid line is the result of an exponential fit to the data yielding $k' = (0.93\pm0.13)$ s$^{-1}$.