Experimental Study of Three-Body Recombination Ar + + e + e T. Kotrík, P. Dohnal, R. Plašil, J. Varju, I. Korolov, J. Glosík Charles University, Faculty of Mathematics and Physics, Prague, Czech Republic. Abstract. Binary recombination of atomic ions with electron, unlike fast dissociative recombination of majority of molecular ions, is rather slow with thermal rate constant ≈ 10 -11 cm 3 s -1 . In plasma the recombination process can be enhanced by collision with a third particle carrying away an excess energy thus preventing autoionization. In case of Collisional Radiative Recombination (CRR) the third particle is an electron. Theory pre- dicts very strong inverse temperature dependence of CRR rate coefficient (α CRR ~ T e -4.5 ), making the process a dominant loss process of atomic ions in plasma at temperatures below 100 K. Up to now there were no experimental studies of CRR in afterglow plasma at temperatures below 300 K. Presented is the flowing afterglow (Cryo-FALP) study of CRR in Ar + dominated plasma at temperatures 77–300 K and electron densities up to n e ≈ 10 10 cm -3 . The obtained ternary recombination rate coefficient at 77 K is K CRR = (1.0 ± 0.4) × 10 -17 cm 6 s -1 . The measured temperature dependence of K CRR is in good agreement with theoretical prediction. Introduction Recombination of atomic ions A + with an electron is in general rather slow process comparing to dissociative recombination (DR) of molecular ions like O 2 + etc. When A + meets an electron a highly excited neutral atom is formed. The exceeding energy has to be either transferred into an emitted photon (radiative recombination) or carried away by a third particle (ternary recombination). If the third body is a neutral particle (M) we are talking about neutral assisted ternary recombination with ternary recombination rate coefficient K M . If the third particle is an electron we are talking about Collisional Radiative Recombination (CRR) with ternary recombination rate coefficient K CRR . Stevefelt et al. [1975] derived universal analytical formula for apparent binary rate coefficient for CRR based on earlier works by Bates et al. [1962] and Mansbach and Keck [1969]: , (1) 1 3 37 . 0 e 18 . 2 e 9 63 . 0 e 10 e 5 . 4 e 9 CRR s cm 10 6 10 55 . 1 10 8 . 3 − − − − − − − × + × + × = n T T n T α where T e is electron temperature given in K and n e is electron number density given in cm -3 . At our experimental conditions (T = 77 K, n e ~ 5×10 9 cm -3 ) the first term (recombination through collision with electron) is dominant and the last two can be neglected. We can write α CRR = K CRR n e , where K CRR is the ternary recombination rate coefficient of CRR. Note the strong inverse temperature dependence (α CRR ~ T -4.5 ) and the linear dependence on electron number density. In our experimental study of recombination process in Ar + dominated afterglow plasma in helium buffer gas at temperatures ≈ 100 K the recombination through collisions with electrons (CRR) becomes the dominant deionization process. The loss of charged particles in plasma is governed by recombination and diffusion losses. For describing the recombination losses in helium buffer gas we introduce apparent binary (effective) recombination rate coefficient: [ ] . He He CRR bin eff K n K e + + = α α (2) Here we present the results of collisional radiative recombination study in Ar + dominated plasma at 77–170 K. Previous experimental works by Tsuji et al. [2002], Skrzypkowski et al. [2004] and Veatch and Oskam [1970] made at 300 K show agreement with theoretical predictions. Up to now, to our knowledge, there are no measurements of CRR at temperatures lower than 300 K. Experiment For determination of recombination rate coefficients we use modified FALP apparatus with basic concept originated from D. Smith and described elsewhere [Smith et al., 1975, Larsson et al., 2008]. 70 WDS'10 Proceedings of Contributed Papers, Part II, 70–75, 2010. ISBN 978-80-7378-140-8 © MATFYZPRESS