Dynamics of the isotope exchange reaction of D with H3 +, H2D+, and D2H. J Chem Phys 2021;154:084307. [PMID: 33639774 DOI: 10.1063/5.0038434]

We have measured the merged-beams rate coefficient for the titular isotope exchange reactions as a function of the relative collision energy in the range of ∼3 meV-10 eV. The results appear to scale with the number of available sites for deuteration. We have performed extensive theoretical calculations to characterize the zero-point energy corrected reaction path. Vibrationally adiabatic minimum energy paths were obtained using a combination of unrestricted quadratic configuration interaction of single and double excitations and internally contracted multireference configuration interaction calculations. The resulting barrier height, ranging from 68 meV to 89 meV, together with the various asymptotes that may be reached in the collision, was used in a classical over-the-barrier model. All competing endoergic reaction channels were taken into account using a flux reduction factor. This model reproduces all three experimental sets quite satisfactorily. In order to generate thermal rate coefficients down to 10 K, the internal excitation energy distribution of each H₃ ⁺ isotopologue is evaluated level by level using available line lists and accurate spectroscopic parameters. Tunneling is accounted for by a direct inclusion of the exact quantum tunneling probability in the evaluation of the cross section. We derive a thermal rate coefficient of <1×10^-12 cm³ s^-1 for temperatures below 44 K, 86 K, and 139 K for the reaction of D with H₃ ⁺, H₂D⁺, and D₂H⁺, respectively, with tunneling effects included. The derived thermal rate coefficients exceed the ring polymer molecular dynamics prediction of Bulut et al. [J. Phys. Chem. A 123, 8766 (2019)] at all temperatures.

Measurements of the effective electron density in an electron beam ion trap using extreme ultraviolet spectra and optical imaging

In an electron beam ion trap (EBIT), the ions are not confined to the electron beam, but rather oscillate in and out of the beam. As a result, the ions do not continuously experience the full density of the electron beam. To determine the effective electron density, n _e,eff, experienced by the ions, the electron beam size, the nominal electron density n _e, and the ion distribution around the beam, i.e., the so-called ion cloud, must be measured. We use imaging techniques in the extreme ultraviolet (EUV) and optical to determine these. The electron beam width is measured using 3d → 3p emission from Fe xii and xiii between 185 and 205 Å. These transitions are fast and the EUV emission occurs only within the electron beam. The measured spatial emission profile and variable electron current yield a nominal electron density range of n _e ∼ 10¹¹-10¹³ cm^-3. We determine the size of the ion cloud using optical emission from metastable levels of ions with radiative lifetimes longer than the ion orbital periods. The resulting emission maps out the spatial distribution of the ion cloud. We find a typical electron beam radius of ∼60 μm and an ion cloud radius of ∼300 μm. These yield a spatially averaged effective electron density, n _e,eff, experienced by the ions in EBIT spanning ∼ 5 × 10⁹-5 × 10¹¹ cm^-3.

Generation of neutral atomic beams utilizing photodetachment by high power diode laser stacks

We demonstrate the use of high power diode laser stacks to photodetach fast hydrogen and carbon anions and produce ground term neutral atomic beams. We achieve photodetachment efficiencies of ∼7.4% for H(-) at a beam energy of 10 keV and ∼3.7% for C(-) at 28 keV. The diode laser systems used here operate at 975 nm and 808 nm, respectively, and provide high continuous power levels of up to 2 kW, without the need of additional enhancements like optical cavities. The alignment of the beams is straightforward and operation at constant power levels is very stable, while maintenance is minimal. We present a dedicated photodetachment setup that is suitable to efficiently neutralize the majority of stable negative ions in the periodic table.

The impact of recent advances in laboratory astrophysics on our understanding of the cosmos

An emerging theme in modern astrophysics is the connection between astronomical observations and the underlying physical phenomena that drive our cosmos. Both the mechanisms responsible for the observed astrophysical phenomena and the tools used to probe such phenomena-the radiation and particle spectra we observe-have their roots in atomic, molecular, condensed matter, plasma, nuclear and particle physics. Chemistry is implicitly included in both molecular and condensed matter physics. This connection is the theme of the present report, which provides a broad, though non-exhaustive, overview of progress in our understanding of the cosmos resulting from recent theoretical and experimental advances in what is commonly called laboratory astrophysics. This work, carried out by a diverse community of laboratory astrophysicists, is increasingly important as astrophysics transitions into an era of precise measurement and high fidelity modeling.

A simple double-focusing electrostatic ion beam deflector

We have developed an electrostatic, double-focusing 90 degrees deflector for fast ion beams consisting of concentric cylindrical plates of differing heights. In contrast to standard cylindrical deflectors, our design allows for focusing of an incoming parallel beam not only in the plane of deflection but also in the orthogonal direction. The optical properties of our design resemble those of a spherical capacitor deflector while it is much easier and more cost effective to manufacture.

A novel merged beams apparatus to study anion-neutral reactions

We have developed a novel laboratory instrument for studying gas phase, anion-neutral chemistry. To the best of our knowledge, this is the first such apparatus which uses fast merged beams to investigate anion-neutral chemical reactions. As proof-of-principle we have detected the associative detachment reaction H(-)+H-->H(2)+e(-). Here we describe the apparatus in detail and discuss related technical and experimental issues.