Michal Hejduk

Binary and ternary recombination of para-H3+ and ortho-H3+ with electrons: State selective study at 77–200 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.

Binary and ternary recombination of and ions with electrons in low temperature plasma

Measurements of recombination rate coefficients of binary and ternary recombination of and ions with electrons in a low temperature plasma are described. The experiments were carried out in the afterglow plasma in helium with a small admixture of Ar and parent gas (H2 or D2). For both ions a linear increase of measured apparent binary recombination rate coefficients (αeff) with increasing helium density was observed: αeff = αBIN + K He[He]. From the measured dependencies, we have obtained for both ions the binary (αBIN) and the ternary (K He) rate coefficients and their temperature dependence. For the description of observed ternary recombination a mechanism with two subsequent rate determining steps is proposed. In the first step, in + e− (or + e−) collision, a rotationally excited long-lived Rydberg molecule (or ) is formed. In the following step (or ) collides with a He atom of the buffer gas and this collision prevents autoionization of (or ). Lifetimes of the formed (or ) and corresponding ternary recombination rate coefficients have been calculated. The theoretical and measured binary and ternary recombination rate coefficients obtained for and ions are in good agreement.

State specific stabilization of H+ + H2(j) collision complexes.

Stabilization of H3(+) collision complexes has been studied at nominal temperatures between 11 and 33 K using a 22-pole radio frequency (rf) ion trap. Apparent binary rate coefficients, k(*) = kr + k3[H2], have been measured for para- and normal-hydrogen at number densities between some 10(11) and 10(14) cm(-3). The state specific rate coefficients extracted for radiative stabilization, kr(T;j), are all below 2 × 10(-16) cm(3) s(-1). There is a slight tendency to decrease with increasing temperature. In contrast to simple expectations, kr(11 K;j) is for j = 0 a factor of 2 smaller than for j = 1. The ternary rate coefficients for p-H2 show a rather steep T-dependence; however, they are increasing with temperature. The state specific ternary rate coefficients, k3(T;j), measured for j = 0 and derived for j = 1 from measurements with n-H2, differ by an order of magnitude. Most of these surprising observations are in disagreement with predictions from standard association models, which are based on statistical assumptions and the separation of complex formation and competition between stabilization and decay. Most probably, the unexpected collision dynamics are due to the fact that, at the low translational energies of the present experiment, only a small number of partial waves participate. This should make exact quantum mechanical calculations of kr feasible. More complex is three-body stabilization, because it occurs on the H5(+) potential energy surface.

Binary recombination of para- and ortho-H3+ with electrons at low temperatures

Results of an experimental study of binary recombination of para- and ortho- ions with electrons are presented. Near-infrared cavity-ring-down absorption spectroscopy was used to probe the lowest rotational states of ions in the temperature range of 77–200 K in an -dominated afterglow plasma. By changing the para/ortho abundance ratio, we were able to obtain the binary recombination rate coefficients for pure and . The results are in good agreement with previous theoretical predictions.

Ternary recombination of H3(+) and D3(+) with electrons in He-H2 (D2) plasmas at temperatures from 50 to 300 k.

We present results of plasma afterglow experiments on ternary electron-ion recombination rate coefficients of H3(+) and D3(+) ions at temperatures from 50 to 300 K and compare them to possible three-body reaction mechanisms. Resonant electron capture into H3* Rydberg states is likely to be the first step in the ternary recombination, rather than third-body-assisted capture. Subsequent interactions of the Rydberg molecules with ambient neutral and charged particles provide the rate-limiting step that completes the recombination. A semiquantitative model is proposed that reconciles several previously discrepant experimental observations. A rigorous treatment of the problem will require additional theoretical work and experimental investigations.

Binary recombination of H3(+) and D3(+) ions with electrons in plasma at 50-230 K.

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.

Flowing-afterglow study of electron-ion recombination of para–H3+ and ortho–H3+ ions at temperatures from 60 K to 300 K

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 (SA-CRDS) show that binary electron recombination of para-H3(+) and ortho-H3(+) ions occurs with significantly different rate coefficients, (p)αbin and (o)αbin, especially at very low temperatures. The measurements cover temperatures from 60 K to 300 K. At the lowest temperature of 60 K, recombination of para-H3(+) is at least three times faster than that of ortho-H3(+) ((p)αbin=(1.8±0.4)×10(-7) cm(3) s(-1) vs. (o)αbin=(0-0 (+5))×10(-8) cm(3) s(-1)).

Binary and ternary recombination of D3+ ions at 80–130 K: Application of laser absorption spectroscopy

Recombination of D(3)(+) ions with electrons at low temperatures (80-130 K) was studied using spectroscopic determination of D(3)(+) 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 D(3)(+) ion. The results show that D(3)(+) 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 α(bin)(80 K) = (9.2 ± 2.0) × 10(-8) cm(3) s(-1). The measured helium assisted ternary rate coefficients K(He) are in agreement with our previously measured flowing afterglow data giving a value of K(He)(80 K) = (1.2 ± 0.3) × 10(-25) cm(6) s(-1).

Experimental study of para- and ortho-H3+ recombination

Recombination of H3+ with electrons is a key process for many plasmatic environments. Recent experiments on storage ring devices used ion sources producing H3+ with enhanced populations of H3+ ions in the para nuclear spin configuration to shed light on the theoretically predicted faster recombination of para states. Although increased recombination rates were observed, no in situ characterization of recombining ions was performed. We present a state selective recombination study of para- and ortho-H3+ ions with electrons at 77 K in afterglow plasma in a He/Ar/H2 gas-mixture. Both spin configurations of H3+ have been observed in situ with a near infrared cavity ring down spectrometer (NIR-CRDS) using the two lowest energy levels of H3+. Using hydrogen with an enhanced population of H2 molecules in para states allowed us to influence the [para-H3+]/[ortho-H3+] ratio in the discharge and in the afterglow. We observed an increase in the measured effective recombination rate coefficients with the increase of the fraction of para-H3+. Measurements with different fractions of para-H3+ at otherwise identical conditions allowed us to determine the binary recombination rate coefficients for pure para-H3+ pαbin(77 K) = (2.0±0.4)×10−7 cm3s−1 and pure ortho-H3+ oαbin(77 K) = (4±3)×10−8 cm3s−1.

Recombination of KrD+ and KrH+ ions in afterglow plasma

Reported is flowing afterglow (FALP) study of recombination of KrH+ and KrD+ ions with electrons at 250 K in mixtures of He/Kr/H2 and He/Kr/D2, respectively. The influence of fast recombining cluster ions formation on apparent effective recombination rate coefficients (?eff) was measured and used in data analysis. The obtained binary rate coefficients for recombination of KrH+ and KrD+ are ?KrH+ = 2?10?8 cm3s?1 and ?KrD+ = 1?10?8 cm3s?1.