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Source of Atomic Hydrogen for Ion Trap Experiments: Review and Basic Properties

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Source of Atomic Hydrogen for Ion Trap Experiments: Review and Basic Properties WDS'15 Proceedings of Contributed Papers — Physics, 155–161, 2015. ISBN 978-80-7378-311-2 © MATFYZPRESS Source of Atomic Hydrogen for Ion Trap Experiments: Review and Basic Properties A. Kovalenko, Š Roučka, S. Rednyk, T. D. Tran, D. Mulin, R. Plašil, J. Glosík Charles University in Prague, Faculty of Mathematics and Physics, Prague, Czech Republic. Abstract. The H-atom source was used to produce atomic hydrogen for study of ion-molecule reactions relevant to astrochemistry at low temperatures. H atoms were cooled and formed into an effusive beam, passing through a 22-pole ion trap. Here we present the basic operating principles of the H-atom source and give a review of this apparatus with some calculations of vacuum conditions and parameters of the produced H-atom beam. Introduction Hydrogen is the most abundant element in the Universe [Field et al., 1966]. It is the main component of stars, giant planets and interstellar clouds. Reactions with atomic or molecular hydrogen are important for understanding the processes which occur in the interstellar medium and during the formation of the new stars. There are three isotopes of hydrogen: protium, deuterium and tritium. The reactions of ions with H atoms have to be studied for better understanding of formation and destruction of more complex ions and molecules observed in interstellar medium. There are regions in space called H I and H II regions where hydrogen is mostly neutral in H I regions, rather than ionized or molecular in the H II regions. The atomic hydrogen plays fundamental role in many astrophysical contexts especially in formation H2 molecules [Habart et al., 2005; Sternberg et al., 2014]. There are only a few numbers of measurements of reaction rate coefficient for reactions with H atoms at temperature lower than 100 K. It makes them unique and demanded for the astrochemistry and plasma physic. In our laboratory we study reactions with atomic hydrogen at temperature lower than 100 K because such kind of temperature (even lower) corresponds to the temperature of nebulas and ion clouds in interstellar space. A by-product of the H atom source is molecular hydrogen, which can penetrate into the trap volume and react with the studied ions via unwanted parasitic reactions. For example, the D+ ions can react with both H atoms and + H2 molecules and produce the same ion, H . Therefore the rate of reaction with H atoms cannot be determined until we know the rate of reaction with molecular hydrogen in the chamber. Calculation of number density of molecular hydrogen can help us to design a more efficient pumping system and to estimate the rate of reaction with H2, which will improve the accuracy of our measurements. Experimental technique Our experimental setup is based on a 22-pole RF ion trap [Gerlich, 1992a; Gerlich et al., 1992b; Gerlich, 1995] and an effusive beam source of atomic hydrogen (AB) [Borodi, 2008] which has been designed and constructed in Chemnitz in laboratory of prof. Gerlich and then it has been moved to Charles University in Prague. Using this apparatus we have studied reactions with atomic hydrogen. For example, there have been studied reactions with hydrogen anion [Gerlich et al., 2012], deuterium anion [Roučka et al., 2015] and CH+ ion [Plašil et al., 2011]. H atoms are produced by disassociation of H2 molecules in an inductively coupled RF discharge (22 MHz, 20 W). The mixture of H atoms and H2 molecules flows through the cooled nozzle in accommodator which is ® covered with a polytetrafluoroethylene (Teflon ) film for minimizing the association of H back to H2 because of exceptionally low atomic recombination coefficient [Collins et al., 1964]. The H-atom beam is formed in the nozzle, whereupon the beam penetrates to 22-pole RF ion trap. The temperature of the H-beam (TACC) and 22-pole trap (T22PT) can be set in the range of 6–300 K and 10–300 K, respectively. The trapped ions are thermalized in collisions with H2 or He buffer gas [Roučka, 2012]. A schematic view of an atomic beam-22 pole trap apparatus (AB-22PT) and a scheme of the three-stages pumping system are shown in Figure 1. The temperatures of neutral particles atoms (TACC) and ions in the trap (T22PT) are different. It is the interaction temperature which defines as TACC+ T22PT T = . (1) kin + 푚1 푚2 where m1 and m2 are masses of neutral particle and ion. 푚1 푚2 In case of hydrogen ion mass ( H ) and neutral mass ( H) of interacting particles + TACC+T22PT 푚 T = 푚 . (2) kin 2 155 KOVALENKO ET AL. SOURCE OF ATOMIC HYDROGEN FOR ION TRAP EXPERIMENTS Figure 1. Schematic view of the AB-22PT apparatus and scheme of the two-stage pumping system. H-atom source part (chamber 1) consists of discharge tube, cold head and accommodator (ACC). After H-atom source there are the chambers 2, 3 (with variable aperture) and 4 (with shutter and reacting chamber (22 Pole Trap)). The chambers are pumped by turbomolecular pumps mTP and oTP (magnetic-levitation and oil vacuum- pumping, respectively). To reduce the ultimate pressure we use the second-stage turbomolecular and turbodrag pumps (TDP). The scroll pumps (SP) and rotary pump (RP) are used as fore-vacuum pumps. The respective calculated values of pressure are also depicted. + The reaction of CO2 ions with hydrogen atoms in the H-atom beam is used to calibrate the number density of the H atoms in the beam [Borodi, 2008]. An advantage of these reactions is that it gives different products in reaction with H and H2: + + CO2 + H → HCO + O (3) + + CO2 + H2 → HCO2 + H (4) Using the shutter we can block the H beam. We measure the rate of HCO+ production with and without H- atom beam and thus we can determine the number density of H atoms. Accuracy of our calibration is limited by + 40% uncertainty of the reaction rate coefficient of CO2 + H [Borodi, 2008]. To improve the accuracy in the planned experiments, we consider to calibrate density of H atoms by using the D+ + H charge exchange reaction. The rate coefficient of this reaction has already been reliably calculated in [Savin, 2002]. Calculation of the number density of H2 inside the chambers In the most cases, molecules of hydrogen are formed by recombination of H atoms on the chamber walls and the mechanism in details has been studied and described in [Borodi, 2008]. The same process occurs in accommodator. In our calculations, we assume that the molecular hydrogen is contained in the chambers and thermalized by collisions with chamber walls. The temperatures of accommodator and ion trap are 300 K. We also suppose that the H-atom beam consists of entirely hydrogen atoms. To calculate appropriate values of number density in the chambers we used parameters of the pumps and some geometrical specification of the chambers which are presented in a Table 1. 156 KOVALENKO ET AL. SOURCE OF ATOMIC HYDROGEN FOR ION TRAP EXPERIMENTS Table 1. The parameters of the pumps and some geometrical specifications. Diameter of the Length of an accommodator Parameters of the pumps orifices, [cm] tube, [cm] Pumping speed for Compression ratio d1 0.2 1.55 H2, [ℓ/s] for H2 d2 0.4 Diameter of an accommodator S 340 CR 2,500 d 0.3 mTP1 mTP1 3 tube, [cm] SmTP2 480 CRmTP2 200,000 Radius of 22PT 0.25 STDP1 3.7 CRTDP1 300 orifice, [cm] SSP1 1.5 — 0.25 Typical pressure of H2 in the SSP2 1.5 — Distance from the chamber 1, [Pa] S 340 CR 2,500 skimmer to 22PT, [cm] mTP3 mTP3 0.001 STP1 48 CRmTP1 140,000 78 The total flow through the orifice between chamber 1 and 2 in molecular regime can be determined as 1 Ф = = ( ), (5) 4 12 1 2 1 2 where Ф1 is flow and Ф2 is backflowФ throughФ − Ф the chambersА 푛 〈1푣 and〉 − 푛2, 〈n푣1〉, n2, are the gas number densities in the respective chambers, A12 is the cross sectional area of the chamber orifice and = 8 / is the average velocity of the particles with the temperature of the chamber walls , where k is the Boltzmann B B wall constant and m is the particle mass. The temperature is the same for all chambers.〈푣〉 �Therefore푘 푇 we휋푚 can wall conclude that the velocity of molecules is also the same. Expressing the gas 푇number density in terms of the wall partial pressure of H2 inside chamber 1 and chamber 2 as 푇p1 and p2, respectively, we obtain 1 = ( ) . (6) 4 B Ф 퐴12〈푣〉 푝1 − 푝2 The total flow through the orifice Ф is equal푘 푇 to , where S is the pumping speed of the pump connected to the chamber 2. Expressing it in terms of the pressure, we finally obtain 푚푚푚1 2 푆 ∙ 푛 = . (7) B2 푚푚푚1 푝 Expression the appropriate pressure p2 fromФ eq.푆 (6) and (7), we obtain 푘 푇 1 12 = 4 . 1 (8) А+ 〈푣〉12 2 4 1 푝 푝 푚푚푚1 Analogous equations can be also written for푆 p2, p3 andА p3,〈 푣p〉4. The typical value of p1 in our experiments is −3 −6 −7 −9 10 Pa. The obtained results for all chambers are p2 = 8.8·10 Pa, p3 = 1.7·10 Pa, p4 = 1.6·10 Pa. For our −6 −7 AB-22PT apparatus the pressure for the chambers are p2exp = 4.0·10 Pa, p3exp = 1.3·10 Pa and for the chamber 4 it is lower than 10–8 Pa. In case of our pumping system we also have to calculate the lowest possible value of pressure (ultimate pressure).
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