Effective Recombination Rate Coefficients Research Articles

Stationary afterglow apparatus with CRDS for study of processes in plasmas from 300 K down to 30 K.

A cryogenic stationary afterglow apparatus equipped with a near-infrared cavity-ring-down-spectrometer (Cryo-SA-CRDS) for studies of electron-ion recombination processes in the plasma at temperatures 30-300 K has been designed, constructed, tested, and put into operation. The plasma is generated in a sapphire discharge tube that is contained in a microwave cavity. The cavity and the tube are attached to the second stage of the cold head of the cryocooler system, and they are inserted to an UHV chamber with mirrors for CRDS and vacuum windows on both ends of the tube. The temperature of the discharge tube can be made as low as 25 K. In initial test measurements, the discharge was ignited in He/Ar/H2 or He/H2 gas mixtures and the density of H3+ ions and their kinetic and rotational temperatures were measured during the discharge and afterglow. From the measured decrease in the ion density, during the afterglow, effective recombination rate coefficients were determined. Plasma relaxation was studied in He/Ar gas mixtures by monitoring the presence of highly excited argon atoms. The spectroscopic measurements demonstrated that the kinetic temperature of the ions is equal to the gas temperature and that it can be varied from 300 K down to 30 K.

Electron-ion recombination in low temperature hydrogen/deuterium plasma

The stationary afterglow with cavity ring down spectrometer (SA-CRDS) was used to study the recombination of H3 + , H2 D+ , HD2 + and D3 + ions with electrons in low temperature (77–300 K) plasmas in He/Ar/H2 /D2 gas mixtures. By measuring effective recombination rate coefficients (α eff ) in plasma with mixtures of ions and their dependences on temperature and partial densities of He, H2 and D2 , α eff (T , [He],[H2 ],[D2 ]), we determined binary (α binH3 , α binH2D , α binHD2 , α binD3 ) and ternary (K H3 , K H2D , K HD2 , K D3 ) recombination rate coefficients for H3 + , H2 D+ , HD2 + and D3 + ions. For all four ions we observed very efficient He assisted ternary recombination which is comparable with binary recombination already at [He] =1 × 1017 cm−3 . The removal of excited particles in afterglow plasma was monitored to obtain the plasma thermalisation rate at given experimental conditions. The inferred deexcitation rates for reaction of helium metastable atoms with D2 are k D2 (300 K)=(2.1 ± 0.3) × 10−10 cm3 s−1 and k D2 (140 K)=(1.3 ± 0.3) × 10−10 cm3 s−1 .

Stationary afterglow measurements of the temperature dependence of the electron–ion recombination rate coefficients of and in He/Ar/H2/D2 gas mixtures at T = 80–145 K

We report measurements of the binary and ternary recombination rate coefficients of deuterated isotopologues of . A cavity ring-down absorption spectrometer was used to monitor the fractional abundances of , , and during the decay of a plasma in He/Ar// mixtures. A dependence of the measured effective recombination rate coefficients on the helium buffer gas density was observed and hence both the binary and the ternary recombination rate coefficients for and were obtained in the temperature range 80–145 K.

Monitoring the removal of excited particles in He/Ar/H2low temperature afterglow plasma at 80–300 K

Stationary afterglow (SA) experiments with cavity ring down absorption spectrometer (cw-CRDS) have been used to study recombination of H 3 + ions with electrons. To characterize the plasma during the afterglow we monitored the time evolution of density of He 2 excited dimers ( a 3 Ʃ u + ) in plasmas in pure helium and in helium with small admixture of Ar and H 2 . By monitoring the plasma decay and its dependence on [H 2 ] and [Ar] in the afterglow in pure He and in He/Ar/H 2 mixture we estimated the rate of plasma thermalization in the temperature range of 80–300 K. The inferred deexcitation rate coefficients for reaction of helium metastable atoms with H 2 were (0.9 ± 0.3) × 10 −10 cm 3 s −1 , (1.9 ± 0.2) × 10 −10 cm 3 s −1 and (2.7 ± 0.2) × 10 −10 cm 3 s −1 at 80 K, 140 K and 300 K, respectively. The effective recombination rate coefficients for H 3 + were evaluated from the decay of the electron number density. We propose the lower estimate for the saturation of the effective recombination rate coefficient in H 3 + and D 3 + dominated plasma.

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.

Concentration-Dependent Hole Mobility and Recombination Coefficient in Bulk Heterojunctions Determined from Transient Absorption Spectroscopy

A simple analytical function based on the multiple trapping model, is used to describe the biomolecular recombination of charge carriers in a bulk heterojunction (BHJ) film in the presence of an exponential energetic tail of localized hole states. The function is used to fit charge carrier decay data from an unannealed P3HT/PCBM film measured by transient absorption. The analysis assumes that only free holes participate in recombination and transport. This implies an effective recombination rate coefficient which varies with the ratio of free to trapped holes. The fit parameters yield a bimolecular recombination constant for free holes with free electrons (k(o) = 3.4 x 10(-12) cm(3) s(-1)) and information about the distribution of trap states (trap distribution parameter beta = 0.29) Assuming the Langevin recombination limit, the analysis yields a concentration dependent effective hole mobility saturating at mu(o) approximate to 7 x 10(-2) cm(2) V-1 s(-1). This approach should be useful to compare BHJs in a consistent and meaningful manner.

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 (αeff) was measured and used in data analysis. The obtained binary rate coefficients for recombination of KrH+ and KrD+ are αKrH+ = 2×10−8 cm3s−1 and αKrD+ = 1×10−8 cm3s−1.

Recombination studies in a He-Ar-H2 plasma

The recombination of H+3 ions with electrons has been studied in afterglow plasma in three different experiments. In two experiments, using the Variable Temperature Stationary Afterglow (VT-AISA) and the Variable Temperature Flowing Afterglow (VT-FALP) techniques, a decay of the electron number density was measured by an electrostatic Langmuir probe to determine the recombination rate coefficient. In the third experiment a near infrared Cavity Ring-Down Absorption Spectrometer (CRDS) was used to monitor the decay of the H+3 (v = 0) ion density during the afterglow. Measurements were carried out in helium buffer gas with small admixtures of argon and hydrogen at total pressures ranging from 150 up to 1200 Pa and at buffer gas temperatures ranging from 100 up to 330 K. In the experiments the partial number density of hydrogen was varied from 5 × 1010 up to 1 × 1016 cm−3 and for this broad range of hydrogen number densities effective recombination rate coefficients were obtained, which varied over three orders of magnitude from 2 × 10−9 cm3s−1 at [H2] = 5 × 1010 cm−3 up to 3 × 10−6 cm3s−1 at [H2] = 1 × 1016 cm−3. Using our experimental results we discuss possible mechanisms of recombination in hydrogen plasma in a very broad range of several parameters: buffer gas pressure, temperature, electron number density, hydrogen number density and internal excitation of recombining ions.

The recombination of spectroscopically identified H3+ (v=0) ions with electrons

We report a study of the recombination of H3+ (v = 0) ions with thermal electrons at 330 and 100 K. A near infrared cavity ring-down absorption spectrometer (CRDS) working on the v2 = 3 ← 0 transition of H3+ (λ = 1382 nm) has been used to monitor the H3+ (v = 0) ion number density in decaying afterglow plasma. The plasma was created in helium gas with small admixtures of argon and hydrogen by pulses of microwaves. The measurements were carried out for hydrogen number densities ranging from 1013 up to 1016 cm−3. The total pressure in the discharge tube was 4–10 mbar. The temperature of the recombining ions was determined from the Doppler broadening of absorption lines. The obtained effective recombination rate coefficients are α (H3+ (v = 0)) = (0.8 ± 0.3) × 10−7 cm3s−1 and α(H3+ (v = 0)) = (2.3 ± 1.1) × 10−7 cm3s−1 at 330 and 100 K, respectively. The influence of the formation of H5+ at higher pressures and lower temperature is also discussed.

The recombination of and ions with electrons in hydrogen plasma: dependence on temperature and on pressure of H2

The recombination of and ions with electrons has been studied in a low-temperature, high-pressure flowing afterglow in a mixture of He–Ar–H2. At high H2 number densities and lower temperatures, ions are formed and the electron decay is controlled by their recombination with electrons (rate coefficient ). At lower H2 number densities, ions dominate the plasma and the decay is controlled by these ions (rate coefficient ). In the intermediate pressure regime the decay of the afterglow plasma depends on the ratio and both rate coefficients. In the experiment the overall effective recombination rate coefficient, αeff, as a function of the H2 number density was measured. Recombination coefficients and and the equilibrium constant, KC(T), were determined at several temperatures. The observed pressure and temperature dependences are in good agreement with thermodynamical data.

Recombination Spectra of Helium‐like Ions

We calculate the recombination spectra of the He-like ions He I, C V, N VI, O VII, Ne IX, Mg XI, Si XIII, S XV, Ar XVII, Ca XIX, and Fe XXV. We include the following physical processes: radiative recombination, dielectronic recombination, three-body recombination, electron impact ionization, and collisional excitation by electrons, protons, and α-particles. The calculations also account for the effects of lowering of the continuum at high densities and high-density corrections to dielectronic recombination. From the populations of all levels in the recombined ions, we construct models for He-like ions for fast computation of their spectra. Every model includes 29 bound levels up to n = 5, a pair of superlevels that account for radiative and collisional cascades from highly excited levels, six doubly excited levels that account for the most important satellite lines, and a level that represents the hydrogenic recombining ion. The models are constructed in a way that allows for the correct approach to LTE under appropriate conditions. These models can simultaneously solve for the H/He-like ionization balance in photoionized or collisionally ionized plasmas and compute emission spectra, including the combined effects of radiative and dielectronic recombination, collisional excitation, photoionization from excited levels, fluorescence, and line trapping. The models can be used for any temperature between 100 and 109 K and electron densities of up to 1018 cm-3. The models can be used easily within spectral modeling codes or as stand-alone tools for spectral analysis. We present comparisons between the results of the present models and previous work. Significant differences are found between the present effective recombination rate coefficients to the n = 2 and those of previous estimates. Later, we study various emission-line ratio diagnostics under collisional ionization and photoionized conditions.

Dielectronic recombination rate coefficients to the excited states of C II from C III

The dielectronic recombination rate coefficients to the excited states for n = 2–6 are calculated including 1s22l2l'nl'' (n = 2–6, l'' ⩽ (n − 1)). The values for the excited states higher than n = 6 are extrapolated and the total dielectronic recombination rate coefficients are derived. The rate coefficients to the excited states are fitted to an analytical formula and the fit parameters are given. The effective recombination rate coefficient for different electron densities are also derived.

Density dependence of dielectronic recombination in selenium

Dielectronic recombination has been found to be the dominant recombination process in the determination of the ionization balance of selenium near the Ne-like sequence under conditions relevant to the exploding-foil EUV laser plasmas. The dielectronic recombination process tends to populate excited levels, and these levels in turn are more susceptible to subsequent excitation and ionization than are the ground-state ions. If one defines an effective recombination rate which includes, in addition to the primary recombination, the subsequent excitation and ionization of the additional excited-state population due to the primary recombination, then this effective recombination rate can be density-sensitive at relatively low electron density. We present results for this effective dielectronic recombination rate at an electron density of 3\ifmmode\times\else\texttimes\fi{}${10}^{20}$ electrons/${\mathrm{cm}}^{3}$ for recombination from Ne-like to Na-like selenium and from F-like to Ne-like selenium. In the former case, the effective recombination rate coefficient is found to be 1.8\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}11}$ ${\mathrm{cm}}^{3}$/sec at 1.0 keV, which is to be compared with the zero-density value of 2.8\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}11}$ ${\mathrm{cm}}^{3}$/sec. In the latter case (F-like to Ne-like), the effective recombination rate coefficient is found to be 1.3\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}11}$ ${\mathrm{cm}}^{3}$/sec, which is substantially reduced from the zero-density result of 3.3\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}11}$ ${\mathrm{cm}}^{3}$/sec. We have examined the effects of dielectronic recombination on the laser gain of the dominant Ne-like 3p-3s transitions and have compared our results with those presented by Whitten et al. [Phys. Rev. A 33, 2171 (1986)]. The results obtained for the gain of the J=2 transitions at 206.9 and 209.8 A\r{} disagree by a factor of 2, and the calculated F-like to Ne-like isoelectronic sequence ratio differs by a factor of 4. These discrepancies are investigated and are found to be due to three factors: inclusion of post-recombination collision effects, inclusion of Rydberg levels, and choice of line shape.