Merged-beams Apparatus Research Articles

Measurement of the Charge Exchange Cross Section for N7+, O7+ Ions in Collision with Atomic H

The absolute total cross sections for the charge exchange between highly charged ions 15N7+, O7+, and atomic H have been measured with the ion-atom merged-beams apparatus at Oak Ridge National Laboratory. The collision energy range is from 1224 down to 2 eV u−1, which covers outflowing hot components of astrophysical charge exchange plasmas like stellar-wind and supernova remnants. Good agreement with the previous measurements and theory is found for the collision energies above 100 eV u−1, while below 100 eV u−1 limited agreement is achieved with the available calculations. These cross-section data are useful for modeling X-ray emission resulting from the charge exchange at the interface of hot plasma interacting with ambient neutral gas.

Ion-Molecule Reactions below 1K: Strong Enhancement of the Reaction Rate of the Ion-Dipole Reaction He^{+}+CH_{3}F.

The reaction between He^{+} and CH_{3}F forming predominantly CH_{2}^{+} and CHF^{+} has been studied at collision energies E_{coll} between 0 and k_{B}·10 K in a merged-beam apparatus. To avoid heating of the ions by stray electric fields, the reaction was observed within the orbit of a highly excited Rydberg electron. Supersonic beams of CH_{3}F and He(n) Rydberg atoms with principal quantum number n=30 and 35 were merged and their relative velocity tuned using a Rydberg-Stark decelerator and deflector, allowing an energy resolution of 150mK. A strong enhancement of the reaction rate was observed below E_{coll}/k_{B}=1 K. The experimental results are interpreted with an adiabatic capture model that accounts for the state-dependent orientation of the polar CH_{3}F molecules by the Stark effect as they approach the He^{+} ion. The enhancement of the reaction rate at low collision energies is primarily attributed to para-CH_{3}F molecules in the J=1, KM=1 high-field-seeking states, which represent about 8% of the population at the 6K rotational temperature of the supersonic beam.

Mutual Neutralization in Li+−D− Collisions: A Combined Experimental and Theoretical Study

Abstract We present a combined experimental and theoretical study of the mutual neutralization (MN) process in collisions of lithium ions (Li+) with deuterium anions (D−) at collision energies below 1 eV. We employ a merged-beam apparatus to determine total and state-to-state MN cross sections. We perform nuclear dynamics calculations using the multichannel Landau–Zener model based on accurate ab initio molecular data. We obtain an excellent agreement between the experimental and theoretical results over the energy range covered in this work. We show that the basis sets used in the ab initio calculations have a limited influence on the total cross section, but strongly impacts the results obtained for the partial cross sections or the reaction branching ratios. This demonstrates the important role of high-precision measurements to validate the theoretical approaches used to study gas-phase reactive processes. Finally, we compute MN rate coefficients for Li+ + H− and Li+ + D−, and discuss their significance for astrochemistry models.

Magnetic control of a reaction path.

Magnetic or electric fields have long been used to align or orient atomic or molecular species in a molecular beam. Now, experiments in a merged beam apparatus show that an external magnetic field can be used to favour one particular reaction path.

Merged neutral beams

A detailed description of a merged beam apparatus for the study of low energy molecular scattering is given. This review is intended to guide any scientist who plans to construct a similar experiment, and to provide some inspiration in describing the approach we chose to our goal. In our experiment a supersonic expansion of paramagnetic particles is merged with one of polar molecules. A magnetic and an electric multipole guide are used to bend the two beams onto the same axis. We here describe in detail how the apparatus is designed, characterised, and operated.

State-resolved measurements of mutual neutralization at subthermal collision energies

SynopsisWe have measured the total cross sections and kinetic energy distributions of the neutral fragments for the mutual neutralization of C− and O− with C+, N+ and O+ by using a merged beam apparatus. A fine resolution in the kinetic energy spectra of the product neutral atoms has allowed us to uniquely identify the atomic states which are participating in the mutual neutralization process. A detailed analysis of the branching ratios sheds new light on these charge transfer processes.

Laboratory studies into the cosmic origins of organic chemistry

We have constructed a novel merged-beams apparatus to study the cosmic origins of organic chemistry. Here we report rate coefficients measurements for reactions of atomic C with H+3. These data are important for astrochemical models.

Laboratory studies of the cosmic origins of organic chemistry

We are constructing a novel merged-beams apparatus to study the cosmic origins of organic chemistry. With this, we plan to measure reaction rate coefficients of atomic C with molecular ions. Such chemical data is important for astrochemical models and observations of cosmic objects.

Associative detachment of H−+ H→H2+e−

Using a merged-beams apparatus, we have measured the associative detachment (AD) reaction of ${\mathrm{H}}^{\ensuremath{-}}+\mathrm{H}\ensuremath{\rightarrow}{\mathrm{H}}_{2}+{e}^{\ensuremath{-}}$ for relative collision energies up to $E$${}_{\mathrm{r}}\ensuremath{\le}4.83$ eV. These data extend above the 1-eV limit of our earlier results. We have also updated our previous theoretical work to account for AD via the repulsive ${}^{2}{\ensuremath{\Sigma}}_{\mathrm{g}}^{+}$ H${}_{2}$${}^{\ensuremath{-}}$ potential energy surface and for the effects at ${E}_{\mathrm{r}}\ensuremath{\ge}0.76$ eV on the experimental results due to the formation of long-lived H${}_{2}$ resonances lying above the $\mathrm{H}+\mathrm{H}$ separated atoms limit. Merging both experimental data sets, our results are in good agreement with our new theoretical calculations and confirm the prediction that this reaction essentially turns off for ${E}_{\mathrm{r}}\ensuremath{\gtrsim}2$ eV. Similar behavior has been predicted for the formation of protonium from collisions of antiprotons and hydrogen atoms.

Absolute energy-resolved measurements of theH−+H→H2+e−associative detachment reaction using a merged-beam apparatus

Using a merged-beam configuration, we have performed absolute measurements for the associative detachment reaction ${\mathrm{H}}^{\ensuremath{-}}+\mathrm{H}\ensuremath{\rightarrow}{\mathrm{H}}_{2}+{e}^{\ensuremath{-}}$. Our energy-resolved measurements for this process remove a long-standing discrepancy between theory and experiment for this fundamental reaction. In particular, we find excellent agreement with theoretical results which previously seemed to be ruled out by earlier experiments performed using a flowing afterglow technique.

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.

A novel merged beam apparatus to study the cosmic origins of organic chemistry

Reactions of atomic carbon with molecular ions play a critical role for gas phase molecular formation in interstellar clouds. These interactions are the first links in the chain of chemical reactions leading to the synthesis of complex organic species. Much of our knowledge of this process is through spectroscopic observations and theoretical models. However, our understanding of this chemistry is constrained by uncertainties in the underlying reaction rate coefficients. Data from quantum calculations are limited to reactions involving three or fewer atoms. Meanwhile, previous experimental studies have been hampered by the difficulty in generating a sufficiently intense and well characterized neutral carbon beam. To address these issues and to study these reactions experimentally, we are building a novel laboratory device which does not suffer from such limitations.

Investigation of charge transfer in low energy D2+ + H collisions using merged beams

(H2-H)+ is the most fundamental ion-molecule two-electron system. This temporary complex which is formed during charge transfer collisions of H2+ + H proceeds through dynamically coupled electronic, vibrational, and rotational degrees of freedom. Using the upgraded ion-atom merged-beams apparatus at Oak Ridge National Laboratory, absolute direct charge transfer cross sections for D2+ + H are measured from keV/u collision energies where the collision is considered "ro-vibrationally frozen" to meV/u energies where collision times are long enough to sample vibrational and rotational modes. The measurements presented here are the first to benchmark high energy theory and vibrationally specific adiabatic theory.

Investigation of charge transfer in low energy D2+ + H collisions using merged beams

The hydrogen – hydrogen (deuterium) molecular ion is the most fundamental ion-molecule two-electron system. Charge transfer proceeds through dynamically coupled electronic, vibrational and rotational degrees of freedom. Using the ion-atom merged-beams apparatus at Oak Ridge National Laboratory absolute charge transfer cross sections for D2+ + H are measured from keV/u collision energies where the collision is considered "ro-vibrationally frozen" to meV/u energies where collision times are long enough to sample vibrational and rotational modes. The measurements benchmark high energy theory and vibrationally specific adiabatic theory.

Production of multiply charged ions from the 4d photoionization of Cs+

Using an ion-photon merged-beam apparatus, photoion yield spectra of Cs2+ and Cs3+ from the 4d photoionization of Cs+ have been measured as a function of photon energy in the range 80-140 eV. Above 90 eV, the 4d giant resonance peak is observed clearly for Cs3+ spectrum. Below 90 eV, some prominent discrete peaks are observed for both Cs2+ and Cs3+. These discrete peaks can be attributed to the 4d-np and 4d-nf transitions.

Low-energy electron capture byNe2+ions from H(D)

Using the Oak Ridge National Laboratory (ORNL) ion-atom merged-beams apparatus, the absolute, total single-electron-capture cross section has been measured for collisions of ${\mathrm{Ne}}^{2+}$ with deuterium (D) at center-of-mass (c.m.) collision energies of $59--949\phantom{\rule{0.3em}{0ex}}\mathrm{eV}∕\mathrm{u}$. With the high-velocity ion beams now available at the ORNL Multicharged Ion Research Facility, we have extended our previous merged-beams measurement to lower c.m. collision energies. The data are compared to all four previously published measurements for ${\mathrm{Ne}}^{2+}+\mathrm{H}(\mathrm{D})$ which differ considerably from one another at energies $\ensuremath{\lesssim}600\phantom{\rule{0.3em}{0ex}}\mathrm{eV}∕\mathrm{u}$. We are unaware of any published theoretical cross-section data for ${\mathrm{Ne}}^{2+}+\mathrm{H}(\mathrm{D})$ at the energies studied. Early quantal rate coefficient calculations for ${\mathrm{Ne}}^{2+}+\mathrm{H}$ at eV/u energies suggest a cross section many orders of magnitude below previous measurements of the cross section at $40\phantom{\rule{0.3em}{0ex}}\mathrm{eV}∕\mathrm{u}$ which is the lowest collision energy for which experimental results have been published. Here we compare our measurements to recent theoretical electron-capture results for ${\mathrm{He}}^{2+}+\mathrm{H}$. Both the experimental and theoretical results show a decreasing cross section with decreasing energy.

Ion–atom merged-beams measurements

The ion–atom merged-beams apparatus at Oak Ridge National Laboratory measures benchmark total charge-transfer cross sections for a variety of multicharged ions with atomic H from keV/u down to meV/u center-of-mass collision energies. The apparatus has recently been upgraded and moved to the new high voltage platform where a permanent magnet ECR ion source provides intense high-velocity ion beams. Upgrades to the merged-beams apparatus include more accurate beam profiles and a shortened merge path to increase the angular acceptance of the apparatus. An observed factor of four decrease in the angular divergence of the ion beam translates into access to lower collision energies with higher resolution. The higher velocity ion beams will enable charge transfer measurements with heavier ions, and measurements with both H and D at eV/u energies and below to directly observe the isotope effect.

Charge transfer in low-energy collisions ofHe2+with atomic hydrogen

Using the Oak Ridge National Laboratory ion-atom merged-beams apparatus, absolute total charge-transfer cross sections have been measured for collisions of He{sup 2+}+H over a range of energies from 380 to 2620 eV/u. The experimental results are compared to previous measurements using different experimental techniques. A hidden-crossing coupled-channel (HCCC) calculation is performed in the collision energy range of 10 to 3000 eV/u and is compared with the measured data as well as with other theories. The HCCC calculation, which is deemed accurate below 1000 eV/u, is also used to determine differential cross sections. The resultant angular scattering information is used to correct the merged-beams data, which is characterized by a large but limited angular acceptance at these collision energies. Our combined experimental and theoretical study provides an improved benchmark for this fundamental system.

Electron capture byNe4+ions from atomic hydrogen

Using the Oak Ridge National Laboratory ion-atom merged-beams apparatus, the absolute total electron-capture cross section has been measured for collisions of ${\mathrm{Ne}}^{4+}$ with hydrogen and deuterium at relative energies in the center-of-mass frame between 0.10 and 1006 eV/u. Comparison with previous measurements shows large discrepancies between 80 and 600 eV/u. For energies below $\ensuremath{\sim}1\phantom{\rule{0.3em}{0ex}}\mathrm{eV}∕\mathrm{u}$, a sharply increasing cross section is attributed to the ion-induced dipole attraction between the reactants. Multichannel Landau-Zener calculations are performed between 0.01 and 5000 eV/u and compare well to the measured total cross sections. Below $\ensuremath{\sim}5\phantom{\rule{0.3em}{0ex}}\mathrm{eV}∕\mathrm{u}$, the present total cross section calculations show a significant target isotope effect. At 0.01 eV/u, the H:D total cross section ratio is predicted to be $\ensuremath{\sim}1.4$ where capture is dominated by transitions into the ${\mathrm{Ne}}^{3+}$ $(2{s}^{2}2{p}^{2}3d)$ configuration.

Vibrational dependence in the dissociative recombination of O2+

We report data on a study into the vibrational dependence in the dissociative recombination of O+2 (X2Πg). The experiment has been performed with a merged-beam apparatus in the heavy-ion storage ring, CRYRING. We present the total rate coefficients as function of collision energy for five different vibrational populations of the ion beam and the partial rate coefficients and branching fractions near 0 eV collision energy for the vibrational levels v = 0, 1 and 2. Furthermore, we report on the control over and characterization of vibrational populations. The total rate coefficient is found to be weakly dependent on the vibrational excitation. The rate decreases upon increasing vibrational excitation, though only within an order of magnitude. The partial rate coefficients are strongly dependent on the vibrational level; with the v = 0 oxygen ions recombining the fastest. The partial branching fractions are also found to be strongly dependent on the vibrational level, specifically the O(1S) quantum yield which increases strongly upon increasing vibrational level.