Ternary Recombination in Hydrogen Plasma

The recombination of H3 ions with electrons was studied using FALP apparatus. Reported is observation of dependence of overall recombination process on He pressure and hydrogen partial pressure at 260 K. The effective plasma recombination rate is driven by binary H3 + e and ternary H3 + e + He processes with the rate coefficients αbin = 7.5×10 cms and KHe = 2.8×10 cms respectively. The difference between recombination of paraand orthoH3 is discussed. Scheme of overall recombination process is proposed and discussed. Introduction Recombination of H3 ions with electrons, as a key process playing role in plasma physics, technology, astrophysics and theory, has been studied for nearly 50 years. Despite a great effort, results of the recombination studies of H3 remained very controversial over these years [Johnsen et al., 2005; Smith et al., 1993]. The discrepancy between theoretical calculations, storage ring experiments and plasma experiments caused the problems with interpretation of astronomical observations of H3 [McCall et al., 2003; Oka, 2000]. The “enigma” of H3 seemed to be solved by the inclusion of the non-Born-Oppenheimer Jahn-Teller coupling into the theory, which gives a high rate of dissociative recombination of this ion [Kokooline et al., 2001; Kokooline et al., 2003]. The qualitative agreement of theory and storage ring experiments using rotationally and vibrationally cooled H3 ions has been achieved [McCall et al., 2004; Kreckel et al., 2005]. Now the theory predicts that at temperatures below 100 K the recombination of H3 becomes nuclear spin state dependent as para(p)H3 will recombine substantially faster than ortho(o)H3 [Santos et al., 2007]. The main remaining problem is the lack of reliable interpretation of many plasma experiments which give very different values of recombination rate coefficient in comparison with theoretical and storage ring studies (see compilation in refs. [Johnsen et al., 2005; Smith et al., 1993; Plasil et al., 2002]). In recent years we have studied the recombination of H3 ions using stationary afterglow experiment AISA. We observed the dependence of the recombination rate coefficients of H3 on partial pressure of hydrogen [Poterya et al., 2002; Plasil et al., 2002; Glosik et al., 2005]. Nevertheless, in AISA experiment the problem with identification of the internal state of recombining ion H3 arose. To identify the recombining ions we build second stationary afterglow experiment, Test Discharge Tube (TDT). In TDT the density of recombining ions (H3(v=0)) was measured by NIR cavity ring-down absorption spectroscopy (CRDS) [Macko et al., 2004]. The TDT-CRDS studies showed a very good agreement with the AISA results. The dependence of recombination rate coefficient of H3 on a partial pressure of H2 observed in the AISA gives a sufficient reason to believe that the recombination process is not pure binary dissociative recombination but a multicollisional process. Despite the fact, that we were missing any theoretical prediction of a lifetime of a neutral H3* formed in the collision of H3 with electron, it became obvious that the recombination rate coefficient measured in AISA is an effective recombination rate coefficient and not the rate coefficient corresponding to the binary dissociative recombination (DR) measured in colliding beams experiments and calculated by the theory of DR. As in the stationary afterglow technique the discharge is ignited in a mixture of the gasses He, Ar and H2, the long-living excited species and radicals, influencing the process of plasma decay and hence the determination of apparent recombination rate coefficients, can be produced. To solve the problem, the flowing afterglow technique (Flowing Afterglow with Langmuir Probe FALP), where the discharge is ignited in pure helium, was used. In FALP the formation of H3 ions proceeds via sequential introduction of gases to already cold afterglow plasma. 31 WDS'08 Proceedings of Contributed Papers, Part II, 31–36, 2008. ISBN 978-80-7378-066-1 © MATFYZPRESS