Article pubs.acs.org/JPCA
Ternary Recombination of H3+ and D3+ with Electrons in He−H2 (D2) Plasmas at Temperatures from 50 to 300 K R. Johnsen,*,† P. Rubovič,‡ P. Dohnal,‡ M. Hejduk,‡ R. Plašil,‡ and J. Glosík‡ †
Department of Physics and Astronomy, University of Pittsburgh, Pittsburgh, Pennsylvania 15260, United States Faculty of Mathematics and Physics, Department of Surface and Plasma Science, Charles University, Prague, Czech Republic
‡
ABSTRACT: 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 thirdbody-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.
densities from 4.4 × 1017 to 8.6 × 1017 cm−3 at T = 300 K, and from 4.1 × 1017 to 7.8 × 1017 cm−3 at 205 K. They found that the recombination rates were independent (within about 5%) of density and that they were independent of the experimental electron densities from 5 × 109 to about 1/20 of that density. Over the limited range of temperatures the measured recombination coefficients varied with temperature as T−1/2, exactly what was expected for binary recombination, and hence the authors were confident that they had observed binary recombination of H3+, but as we will discuss later, they probably measured a “saturated” three-body reaction that only gives the appearance of binary recombination. A crude estimate of the three-body rate coefficient at 300 K would be given by the ratio (2.4 × 10−7 − 0.6 × 10−7)/ 4.4 × 1017 = 4 × 10−25 cm6/s, a rather large value that cannot be ascribed to known three-body mechanism15 such as collisional radiative recombination in which atoms act as third bodies. We emphasize that the large body of data for ions other than H3+ collected by the microwave technique was confirmed later and that three-body effects are not at all common. The gas phase recombination of H3+, the simplest triatomic ion, clearly has some unusual features! Systematic measurements of the neutral-assisted recombination have been carried out in the Prague laboratory over a period of about four years and have recently been extended to unprecedented low temperatures (50 K). Both stationary and
1. INTRODUCTION The recombination of H3+ ions and their deuterated analogs with thermal electrons has been studied for more than four decades, motivated largely by its pivotal role in the chemistry and physics of astrophysical clouds and the atmospheres of the outer planets, applications to man-made discharges, and basic interest. However, a true reconciliation of often discrepant experimental data, and a convergence of theory and experiment have not been achieved, as has been pointed out in recent reviews of the subject (see, e.g., Johnsen and Guberman1). This article focuses on three-body recombination of H3+ ions, i.e., the enhancement of recombination by collisions with ambient plasma particles, such as electrons, ions, and neutrals. Theoretical2−5 and experimental6−8 work on purely binary recombination of H3+ has made enormous progress since the advent of storage rings, advanced afterglow techniques, and the modern Jahn−Teller type theories.2 At this time, the theory reproduces the experimental values of the thermal (Maxwellian) rate coefficients very well, but discrepancies still exist between the calculated resonances and the structures seen in high-resolution storage-ring data, as has been discussed in great detail by Petrignani et al.9 Some of these resonances may also play a role in the three-body effects discussed here (see section 5). It is important to realize that third-body assisted recombination does not always lead to experimentally detectable dependences on ambient gas density. We briefly illustrate this point by revisiting the early experiment of Leu et al.10,11 Their microwave-afterglow measurements yielded an H3+ recombination coefficient (2.4 × 10−7 cm3/s at 300 K) that is far larger (by a factor of nearly 4) than that obtained later (about 0.6 × 10−7 cm3/s) in low-pressure afterglows12−14 and storage-ring experiments.6−8 Leu et al. employed helium as buffer gas, at © 2013 American Chemical Society
Special Issue: Oka Festschrift: Celebrating 45 Years of Astrochemistry Received: December 5, 2012 Revised: February 6, 2013 Published: February 6, 2013 9477
dx.doi.org/10.1021/jp311978n | J. Phys. Chem. A 2013, 117, 9477−9485
The Journal of Physical Chemistry A
Article
flowing afterglow apparatus were employed, always in conjunction with mass spectrometric identification of the recombining ions. In some measurements the traditional Langmuir probe technique was used to determine electron densities. Although accurate recombination coefficients can be obtained by this method, the internal state of the ions remains unspecified. Hence, optical absorption (cavity-ring-down-spectroscopy, CRDS) was added to observe the decay of H3+ (D3+) ions in known vibrational/rotational states and to measure the dependence of the recombination on the nuclear spin modification (para, ortho, meta). Complete descriptions of the earlier experiments can be found in Glosiḱ et al.,16−19,22 Varju et al.,20,23 Kotriḱ et al.,21 Plašil et al.,29 Rubovič et al.,24 and Dohnal et al.25−27 The earliest such measurements,28 carried out over a small range of helium densities, were consistent with a lack of density dependence. However, as the accuracy of the data improved, and the density range was increased, a dependence of the recombination on neutral (helium) density was invariably observed. The measurements revealed that the three-body coefficients vary with temperature and that they also depend on the nuclear spin states of the H3+ (D3+) ions (see Varju et al.,20,23 Plašil et al.,29 and Dohnal et al.25,27). We limit the discussion to plasmas in which helium is the dominant neutral gas and ignore possible effects of the minority gases that are often present in afterglow experiments, typically argon and hydrogen. Tests showed that small additions of argon have no significant effect. The hydrogen density is kept small (
