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Radicals are prevalent in gas-phase environments such as the atmosphere, combustion systems, and the interstellar medium. To understand the properties of the processes occurring in these environments, it is helpful to study radical reaction systems in isolation—thereby avoiding competing reactions from impurities. There are very few methods for generating a pure beam of gas-phase radicals, and those that do exist involve complex setups. Here, we provide a straightforward and versatile solution. A magnetic radical filter (MRF), composed of four Halbach arrays and two skimming blades, can generate a beam of velocity-selected low-field-seeking hydrogen atoms. As there is no line-of-sight through the device, all species that are unaffected by the magnetic fields are physically blocked; only the target radicals are successfully guided around the skimming blades. The positions of the arrays and blades can be adjusted, enabling the velocity distribution of the beam (and even the target radical species) to be modified. The MRF is employed as a stand-alone device—filtering radicals directly from the source. Our findings open up the prospect of studying a range of radical reaction systems with a high degree of control over the properties of the radical reactants. [ABSTRACT FROM AUTHOR]

Rate coefficients for nuclear spin state-specific recombination of H3+ ions with thermal electrons were measured using FALP and SA techniques at temperatures 77–300 K. For this purpose H2 gas with both thermal and enriched population of the para nuclear spin configuration was used. Measurements have shown that at 77 K para-H3+ exhibits five times higher binary recombination rate coefficient than ortho-H3+: (1.5 ± 0.4) × 10−7 vs. (3 ± 2) × 10−8 cm3s−1.

Detailed measurements employing a combination of a cryogenic flowing afterglow with Langmuir probe (Cryo-FALP II) and a stationary afterglow with near-infrared absorption spectroscopy (SACRDS) show that binary electron recombination of para-H+3 and ortho-H+3 ions occurs with significantly different rate coefficients, pabin and oabin, especially at very lowtemperatures. The measurements cover temperatures from 60Kto 300 K. At the lowest temperature of 60 K, recombination of para-H+3 is at least three times faster than that of ortho-H+3 (pabin = (1.8 ± 0.4) × 10-7 cm³ s-1 vs. oabin = (0+5-0) × 10-8 cm³ s-1). [ABSTRACT FROM AUTHOR]

Recombination of D3+ ions with electrons at low temperatures (80-130 K) was studied using spectroscopic determination of D3+ ions density in afterglow plasmas. The use of cavity ring-down absorption spectroscopy enabled an in situ determination of the abundances of the ions in plasma and the translational and the rotational temperatures of the recombining ions. Two near infrared transitions at (5792.70 ± 0.01) cm-1 and at (5793.90 ± 0.01) cm-1 were used to probe the number densities of the lowest ortho state and of one higher lying rotational state of the vibrational ground state of D3+ ion. The results show that D3+ recombination with electrons consists of the binary and the third-body (helium) assisted process. The obtained binary recombination rate coefficients are in agreement with a recent theoretical prediction for electron-ion plasma in thermodynamic equilibrium with αbin(80 K) = (9.2 ± 2.0) × 10-8 cm3 s-1. The measured helium assisted ternary rate coefficients KHe are in agreement with our previously measured flowing afterglow data giving a value of KHe(80 K) = (1.2 ± 0.3) × 10-25 cm6 s-1. [ABSTRACT FROM AUTHOR]

Measurements in H3+ afterglow plasmas with spectroscopically determined relative abundances of H3+ ions in the para-nuclear and ortho-nuclear spin states provide clear evidence that at low temperatures (77-200 K) para-H3+ ions recombine significantly faster with electrons than ions in the ortho state, in agreement with a recent theoretical prediction. The cavity ring-down absorption spectroscopy used here provides an in situ determination of the para/ortho abundance ratio and yields additional information on the translational and rotational temperatures of the recombining ions. The results show that H3+ recombination with electrons occurs by both binary recombination and third-body (helium) assisted recombination, and that both the two-body and three-body rate coefficients depend on the nuclear spin states. Electron-stabilized (collisional-radiative) recombination appears to make only a small contribution. [ABSTRACT FROM AUTHOR]

We present studies on the thermalisation of H3+ ions in a cold He/Ar/H2 plasma at temperatures 30–70 K. We show that we are able to generate a rotationally thermalised H3+ ensemble with a population of rotational and nuclear spin states corresponding to a particular ion translational temperature. By varying the para-H2 fraction used in the experiment we are able to produce para-H3+ ions with fractional populations higher than those corresponding to thermodynamic values. At 35 K, only the lowest rotational states of para and ortho H3+ are populated. This is the first step towards experimental studies of electron–molecular ion recombination processes with precisely specified quantum states at astrophysically relevant temperatures. [ABSTRACT FROM AUTHOR]

We have applied a continuous-wave near-infrared cavity ring-down spectroscopy method to study the parameters of a H+3-dominated plasma at temperatures in the range 77-200 K. We monitor populations of three rotational states of the ground vibrational state corresponding to para and ortho nuclear spin states in the discharge and the afterglow plasma in time and conclude that abundances of para and ortho states and rotational temperatures are well defined and stable. The non-trivial dependence of a relative population of para-H+3 on a relative population of para-H2 in a source H2 gas is described. The results described in this paper are valuable for studies of state-selective dissociative recombination of H+3 ions with electrons in the afterglow plasma and for the design of sources of H+3 ions in a specific nuclear spin state. [ABSTRACT FROM AUTHOR]

We present studies on the thermalisation of H3+ ions in a cold He/Ar/H2 plasma at temperatures 30-70 K. We show that we are able to generate a rotationally thermalised H3+ ensemble with a population of rotational and nuclear spin states corresponding to a particular ion translational temperature. By varying the para-H2 fraction used in the experiment we are able to produce para-H3+ ions with fractional populations higher than those corresponding to thermodynamic values. At 35 K, only the lowest rotational states of para and ortho H3+ are populated. This is the first step towards experimental studies of electron-molecular ion recombination processes with precisely specified quantum states at astrophysically relevant temperatures.

The results of an experimental study of the H3(+) and D3(+) ions recombination with electrons in afterglow plasmas in the temperature range 50-230 K are presented. A flowing afterglow apparatus equipped with a Langmuir probe was used to measure the evolution of the electron number density in the decaying plasma. The obtained values of the binary recombination rate coefficient are αbinH3(+) = (6.0 ± 1.8) × 10(-8)(300/T)(0.36±0.09) cm(3) s(-1) for H3(+) ions in the temperature range 80-300 K and αbinD3(+) = (3.5 ± 1.1) × 10(-8)(300/T)(0.73±0.09) cm(3) s(-1) for D3(+) ions in the temperature range 50-300 K. This is the first measurement of the binary recombination rate coefficient of H3(+) and D3(+) ions in a plasma experiment down to 50 K.

Measurements in H(3)(+) afterglow plasmas with spectroscopically determined relative abundances of H(3)(+) ions in the para-nuclear and ortho-nuclear spin states provide clear evidence that at low temperatures (77-200 K) para-H(3)(+) ions recombine significantly faster with electrons than ions in the ortho state, in agreement with a recent theoretical prediction. The cavity ring-down absorption spectroscopy used here provides an in situ determination of the para/ortho abundance ratio and yields additional information on the translational and rotational temperatures of the recombining ions. The results show that H(3)(+) recombination with electrons occurs by both binary recombination and third-body (helium) assisted recombination, and that both the two-body and three-body rate coefficients depend on the nuclear spin states. Electron-stabilized (collisional-radiative) recombination appears to make only a small contribution.