22nd International Symposium on July 5-10, 2015; Antwerp, Belgium

An -electron recombination in hydrogen/helium plasma at low temperatures

R. Plašil, P. Dohnal, Á. Kálosi, P. Rubovič, M. Hejduk and J. Glosík

Charles University in Prague, Faculty of Mathematics and Physics, Department of Surface and Plasma Science, V Holešovičkách 2, CZ-18000 Prague, Czech Republic

Abstract: We present a study of ion-electron recombination in low temperature hydrogen/helium plasma. A near infrared cavity ring-down spectrometer has been used to + measure decay of H3 in afterglow plasma. The main aim of the study was to understand ternary processes that govern an evolution of hydrogen/helium plasma composition at higher pressures.

Keywords: ion-electron recombination, hydrogen plasma, absorption

1. Introduction ternary recombination further below 300 K to understand Hydrogen plasma is present in many technological this extremely fast process. applications and also in many astrophysical environments. It may be found in hydrogen dominated atmospheres of 2. Experiment Jovian planets and in cold interstellar clouds. The Measurements took place in stationary afterglow with + important constituent of typical hydrogen plasma is H3 time-resolved cavity ring-down spectrometer [6]. Pulsed + ion. H3 is the most abundantly produced molecular ion in microwave discharge was periodically ignited in a interstellar space and it stands on the beginnings of many mixture of He, Ar and H2 gas. Helium was used to ensure reaction chains. As proton affinity of H2 molecule is low, thermalization of ions and electrons and to reduce + H3 starts a tree of ion-molecular reactions leading to diffusion of plasma. Trace of argon was added to remove formation of other astrophysically significant molecules metastable helium atoms and to enhance production of + [1]. H3 . Details about chemical kinetics were given in [7]. One of its important destruction mechanisms is Fast switch allowed us to turn off the incident microwave recombination with electrons. We have been studying the power within 30 µs. The kinetic temperature was + recombination of H3 with electrons for several years and measured from the Doppler broadening of absorption we found agreement between results of plasma and beam lines of neutral molecules and ions and it was found to be experiments. We explained discrepancies among most of close to the temperature of the wall of the apparatus the other plasma afterglow experiments by unexpectedly within few kelvins. fast helium-assisted ternary recombination channel in To measure decay curves of the lowest rotational + hydrogen/helium plasma [2]. At 300 K, the rate energy levels of H3 we used a near infrared cavity ring- coefficients of helium-assisted ternary recombination down spectrometer with synchronous detection overtop by more than two orders of magnitude expected capabilities. For each ring-down event the time of data value of such processes [3]. acquisition start was recorded, relative to the discharge cycle. More detailed description of the used apparatus can H + + e– → H # (1) 3 3 be found in [6,8] and in references therein. # H3 + M → H2 + H (or 3 H) + M (2) For this measurement the second overtone transitions originating from the ground vibrational level of H + were In this scheme highly excited neutral molecule H # in 3 3 used. The lowest rotational levels (1,1) (para, transition Rydberg state is formed and it is consequently stabilized 1 0 3v2 (2,1)←0v2 (1,1)) and (1,0) (ortho, transition by collision with neutral molecule M. In our recent 3v 1(2,0)←0v 0(1,0)) of the vibrational ground state were publication we identified also ternary recombination 2 2 monitored. For evaluation of rotational temperature we channel enhanced by H2 at 300 K [4] with ternary rate –23 6 –1 also probed another state (3,3) (ortho, transition coefficient 9×10 cm s . We observed a saturation of 1 0 3v (4,3)←0v (3,3)). the ternary recombination process due to finite rate of 2 2 Measured data show good kinetic and rotational Equation (1). At higher number densities of H2 the overall thermalization of H + ion in the discharge and in the rate coefficient is driven only by the formation of the 3 # afterglow because at used conditions prior recombination highly excited neutral molecule H . + 3 with electron, H undergoes many collisions with The independence of measured recombination rate 3 hydrogen and helium. Electrons are hot during the coefficient in saturated region may clarify data measured discharge, but they are thermalized within few by Amano [5] that presented three times higher binary + microseconds in the afterglow. We found that electron recombination rate of H3 than the most of other temperature also corresponds to neutral gas temperature at experiments. We want to study the hydrogen-assisted similar conditions [9].

P-I-2-56 1 From time resolved measurement of spectral lines we effective binary recombination rate coefficients at 273 K + derived decays of H3 number density n. If the are plotted in Figure 2. recombination process is dominant we can write the The plotted statistical errors of the rate coefficients are following equations. given by used fit procedure. A systematic error of our experimental setup consists mainly in an uncertainty of = (3) the dimension of the plasma column in the resonator and d푛 2 it is less than 10 %. eff d푡= 훼 푛+ , (4) Measured αeff consists of binary recombination rate ( ) –8 3 –1 1 1 coefficient 6×10 cm s and comparable addition of eff –8 3 –1 where αeff푛 denotes푛 푡=0 measured훼 푡 binary rate coefficient of helium-assisted ternary recombination (~5×10 cm s recombination. Examples of measured decays are plotted between 900 Pa and 1800 Pa) with rate coefficient in the in Figure 1. Reciprocal value of ion number density is order of 10–25 cm6 s–1. The increase of measured effective used for clarity. recombination rate coefficient on the left side of Figure 2 16 –3 for [H2] < 10 cm is caused by hydrogen-assisted 8 ternary recombination characterized by rate coefficient 273 K 9×10–23 cm6 s–1. 7 α –7 3 –1 eff = 3.0×10 cm s The obvious difference between data measured at + 300 K and 273 K is brought out by a formation of H5 ) 6 3 and its relatively fast recombination with electrons. The 5

cm pressure dependence at 273 K is induced by smaller +

–11 proportion of H5 in equilibrium at lower pressure. The

4 losses of ion number density caused by recombination of + (10 H5 are comparable to all other channels even at 273 K. n 3

1/ 2 4.0 273 K 1 300 K 3.5 α = 2.1×10–7 cm3 s–1 1800 Pa eff 900 Pa 0 ) 0 100 200 300 –1 3.0 s 3 time (µs) 2.5 cm

Fig. 1. Examples of measured plasma decays in afterglow –7 plasma plotted in reciprocal graph at two temperatures. 2.0 (10

These plots illustrate the dominance of recombination eff 16 –3 300 K process at used conditions, [H2] = 3×10 cm and α 1.5 1800 Pa overall pressure 1800 Pa. 900 Pa 1.0 To obtain more accurate values of recombination rate 0 1 2 3 4 5 6 16 –3 coefficient we included also diffusion losses in the [H ] (10 cm ) 2 evaluation, for details see reference [10]. Fig. 2. Measured binary recombination rate coefficients at 3. Results and conclusions 273 K at two pressures. The comparison with rate In our experiment we decreased temperature of the wall coefficients measured at 300 K [4] shows substantial + of the apparatus to 273 K. At high number densities we difference caused by the formation of H5 . observed increase of measured binary recombination rate + coefficient due to formation of H5 ions. At high number To determine dependence of hydrogen-assisted ternary + densities of H2 and sufficiently low temperature H5 recombination rate coefficient we need to separate + cluster may be formed and it keeps chemical equilibrium influence of H5 clusters from hydrogen-assisted ternary described by ratio [H +]/[H +] that depends on recombination. Therefore we will need to study formation 5 3 + + + temperature. of H5 / H3 equilibrium and the influence of H5 + + recombination with electrons at temperatures below H3 + H2 + He → H5 + He (5) 300 K. + + H5 + He → H3 + H2 + He (6) + – H5 + e → neutral products (7) At our conditions, model based on [7,11,12] shows that + + ratio between H5 and H3 is very close to equilibrium taking the Equations (5 – 7) into account. Measured

2 P-I-2-56 Acknowledgements This work was partly supported by Grant Agency of Czech Republic GACR P209/12/0233 and GACR 14- 14649P and GAUK 692214.

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