J. Varju

Charles University in Prague, Praha, Hlavni mesto Praha, Czech Republic

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Publications (22)39.07 Total impact

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    ABSTRACT: Recombination of D(3) (+) ions with electrons at low temperatures (80-130 K) was studied using spectroscopic determination of D(3) (+) ions density in afterglow plasmas. The use of cavity ring-down absorption spectroscopy enabled an in situ determination of the abundances of the ions in plasma and the translational and the rotational temperatures of the recombining ions. Two near infrared transitions at (5792.70 ± 0.01) cm(-1) and at (5793.90 ± 0.01) cm(-1) were used to probe the number densities of the lowest ortho state and of one higher lying rotational state of the vibrational ground state of D(3) (+) ion. The results show that D(3) (+) recombination with electrons consists of the binary and the third-body (helium) assisted process. The obtained binary recombination rate coefficients are in agreement with a recent theoretical prediction for electron-ion plasma in thermodynamic equilibrium with α(bin)(80 K) = (9.2 ± 2.0) × 10(-8) cm(3) s(-1). The measured helium assisted ternary rate coefficients K(He) are in agreement with our previously measured flowing afterglow data giving a value of K(He)(80 K) = (1.2 ± 0.3) × 10(-25) cm(6) s(-1).
    The Journal of Chemical Physics 11/2012; 137(19):194320. · 3.12 Impact Factor
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    ABSTRACT: Results of an experimental study of binary recombination of para- and ortho- ions with electrons are presented. Near-infrared cavity-ring-down absorption spectroscopy was used to probe the lowest rotational states of ions in the temperature range of 77-200 K in an -dominated afterglow plasma. By changing the para/ortho abundance ratio, we were able to obtain the binary recombination rate coefficients for pure and . The results are in good agreement with previous theoretical predictions.
    Philosophical Transactions of The Royal Society A Mathematical Physical and Engineering Sciences 11/2012; 370(1978):5101-8. · 2.89 Impact Factor
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    ABSTRACT: Measurements in H(3)(+) afterglow plasmas with spectroscopically determined relative abundances of H(3)(+) ions in the para-nuclear and ortho-nuclear spin states provide clear evidence that at low temperatures (77-200 K) para-H(3)(+) ions recombine significantly faster with electrons than ions in the ortho state, in agreement with a recent theoretical prediction. The cavity ring-down absorption spectroscopy used here provides an in situ determination of the para/ortho abundance ratio and yields additional information on the translational and rotational temperatures of the recombining ions. The results show that H(3)(+) recombination with electrons occurs by both binary recombination and third-body (helium) assisted recombination, and that both the two-body and three-body rate coefficients depend on the nuclear spin states. Electron-stabilized (collisional-radiative) recombination appears to make only a small contribution.
    The Journal of Chemical Physics 06/2012; 136(24):244304. · 3.12 Impact Factor
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    ABSTRACT: We have applied a continuous-wave near-infrared cavity ring-down spectroscopy method to study the parameters of a -dominated plasma at temperatures in the range 77–200 K. We monitor populations of three rotational states of the ground vibrational state corresponding to para and ortho nuclear spin states in the discharge and the afterglow plasma in time and conclude that abundances of para and ortho states and rotational temperatures are well defined and stable. The non-trivial dependence of a relative population of para- on a relative population of para-H2 in a source H2 gas is described. The results described in this paper are valuable for studies of state-selective dissociative recombination of ions with electrons in the afterglow plasma and for the design of sources of ions in a specific nuclear spin state.
    Plasma Sources Science and Technology 04/2012; 21(2):024002. · 2.52 Impact Factor
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    ABSTRACT: Results of an experimental study of binary recombination of para- and ortho- ${\rm{H}}_{\rm{3}}^{\rm{ + }}$ ions with electrons are presented. Near-infrared cavity-ring-down absorption spectroscopy was used to probe the lowest rotational states of ${\rm{H}}_{\rm{3}}^{\rm{ + }}$ ions in the temperature range of 77-200 K in an ${\rm{H}}_{\rm{3}}^{\rm{ + }}$ -dominated afterglow plasma. By changing the para/ortho abundance ratio, we were able to obtain the binary recombination rate coefficients for pure para- ${\rm{H}}_{\rm{3}}^{\rm{ + }}$ and ortho- ${\rm{H}}_{\rm{3}}^{\rm{ + }}$ . The results are in good agreement with previous theoretical predictions.
    Philosophical Transactions of The Royal Society A Mathematical Physical and Engineering Sciences 01/2012; 370(1978):5101-5108. · 2.89 Impact Factor
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    ABSTRACT: Recombination of H3+ with electrons is a key process for many plasmatic environments. Recent experiments on storage ring devices used ion sources producing H3+ with enhanced populations of H3+ ions in the para nuclear spin configuration to shed light on the theoretically predicted faster recombination of para states. Although increased recombination rates were observed, no in situ characterization of recombining ions was performed. We present a state selective recombination study of para- and ortho-H3+ ions with electrons at 77 K in afterglow plasma in a He/Ar/H2 gas-mixture. Both spin configurations of H3+ have been observed in situ with a near infrared cavity ring down spectrometer (NIR-CRDS) using the two lowest energy levels of H3+. Using hydrogen with an enhanced population of H2 molecules in para states allowed us to influence the [para-H3+]/[ortho-H3+] ratio in the discharge and in the afterglow. We observed an increase in the measured effective recombination rate coefficients with the increase of the fraction of para-H3+. Measurements with different fractions of para-H3+ at otherwise identical conditions allowed us to determine the binary recombination rate coefficients for pure para-H3+ pαbin(77 K) = (2.0±0.4)×10−7 cm3s−1 and pure ortho-H3+ oαbin(77 K) = (4±3)×10−8 cm3s−1.
    Journal of Physics Conference Series 07/2011; 300(1):012023.
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    ABSTRACT: Utilizing different ratios of para to ortho H₂ in normal and para enriched hydrogen, we varied the population of para-H₃⁺ in an H₃⁺ dominated plasma at 77 K. Absorption spectroscopy was used to measure the densities of the two lowest rotational states of H₃⁺. Monitoring plasma decays at different populations of para-H₃⁺ allowed us to determine the rate coefficients for binary recombination of para-H₃⁺ and ortho-H₃⁺ ions: (p)α(bin)(77 K) = (1.9 ± 0.4) × 10⁻⁷ cm³ s⁻¹ and (o)α(bin)(77 K) = (0.2 ± 0.2) × 10⁻⁷ cm³ s⁻¹.
    Physical Review Letters 05/2011; 106(20):203201. · 7.73 Impact Factor
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    ABSTRACT: Measurements of recombination rate coefficients of binary and ternary recombination of and ions with electrons in a low temperature plasma are described. The experiments were carried out in the afterglow plasma in helium with a small admixture of Ar and parent gas (H2 or D2). For both ions a linear increase of measured apparent binary recombination rate coefficients (αeff) with increasing helium density was observed: αeff = αBIN + KHe[He]. From the measured dependencies, we have obtained for both ions the binary (αBIN) and the ternary (KHe) rate coefficients and their temperature dependence. For the description of observed ternary recombination a mechanism with two subsequent rate determining steps is proposed. In the first step, in + e (or + e) collision, a rotationally excited long-lived Rydberg molecule (or ) is formed. In the following step (or ) collides with a He atom of the buffer gas and this collision prevents autoionization of (or ). Lifetimes of the formed (or ) and corresponding ternary recombination rate coefficients have been calculated. The theoretical and measured binary and ternary recombination rate coefficients obtained for and ions are in good agreement.
    Molecular Physics 09/2010; 108(17):2253-2264. · 1.67 Impact Factor
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    ABSTRACT: We present a study of H3+ recombination performed at 77 K on the two lowest rotational levels of this ion, which belong to its two different nuclear spin states of the studied ion. A near infrared cavity ring-down spectrometer (~1381 nm, CRDS arrangement) has been used to obtain the time evolution of concentration of both states. From the overall ion density decay during the afterglow we obtained the binary recombination rate coefficient αbin (77 K) = 1.2×10−7 cm3s−1. We have also observed ternary helium assisted recombination of both para and ortho H3+. The process is very slow (at 77 K) and the obtained ternary recombination rate coefficient is in contradiction with the theoretical prediction. It is the first time that the binary and ternary H3+ recombination rate coefficient was measured at a known population of para and ortho H3+ ions in decaying plasma.
    Journal of Physics Conference Series 06/2010; 227(1):012026.
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    ABSTRACT: We report on the realization and operation of a fast ion beam trap of the linear electrostatic type employing liquid helium cooling to reach extremely low blackbody radiation temperature and residual gas density and, hence, long storage times of more than 5 min which are unprecedented for keV ion beams. Inside a beam pipe that can be cooled to temperatures <15 K, with 1.8 K reached in some locations, an ion beam pulse can be stored at kinetic energies of 2-20 keV between two electrostatic mirrors. Along with an overview of the cryogenic trap design, we present a measurement of the residual gas density inside the trap resulting in only 2 x 10(3) cm(-3), which for a room temperature environment corresponds to a pressure in the 10(-14) mbar range. The device, called the cryogenic trap for fast ion beams, is now being used to investigate molecules and clusters at low temperatures, but has also served as a design prototype for the cryogenic heavy-ion storage ring currently under construction at the Max-Planck Institute for Nuclear Physics.
    The Review of scientific instruments 05/2010; 81(5):055105. · 1.52 Impact Factor
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    ABSTRACT: We report on an energy-sensitive imaging detector for studying the fragmentation of polyatomic molecules in the dissociative recombination of fast molecular ions with electrons. The system is based on a large area (10 cm x 10 cm) position-sensitive, double-sided Si-strip detector with 128 horizontal and 128 vertical strips, whose pulse height information is read out individually. The setup allows to uniquely identify fragment masses and is thus capable of measuring branching ratios between different fragmentation channels, kinetic energy releases, as well as breakup geometries, as a function of the relative ion-electron energy. The properties of the detection system, which has been installed at the TSR storage ring facility of the Max-Planck Institute for Nuclear Physics in Heidelberg, is illustrated by an investigation of the dissociative recombination of the deuterated triatomic hydrogen cation D2H+. A huge isotope effect is observed when comparing the relative branching ratio between the D2+H and the HD+D channel; the ratio 2B(D2+H)/B(HD+D), which is measured to be 1.27 +/- 0.05 at relative electron-ion energies around 0 eV, is found to increase to 3.7 +/- 0.5 at ~5 eV. Comment: 11 pages, 12 figures, submitted to Physical Review A
    Physical Review A 03/2010; · 3.04 Impact Factor
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    ABSTRACT: Electron energy distribution functions (EEDF) have been measured in decaying plasma in Flowing Afterglow Langmuir Probe (FALP) experiment. The measurements have been carried out in diffusion and recombination governed plasmas used for studies of recombination of KrD+ and H3+ ions.
    Journal of Physics Conference Series 10/2009; 192(1):012023.
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    ABSTRACT: Reported is flowing afterglow (FALP) study of recombination of KrH+ and KrD+ ions with electrons at 250 K in mixtures of He/Kr/H2 and He/Kr/D2, respectively. The influence of fast recombining cluster ions formation on apparent effective recombination rate coefficients (αeff) was measured and used in data analysis. The obtained binary rate coefficients for recombination of KrH+ and KrD+ are αKrH+ = 2×10−8 cm3s−1 and αKrD+ = 1×10−8 cm3s−1.
    Journal of Physics Conference Series 10/2009; 192(1):012018.
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    ABSTRACT: An experimental study is reported about the recombination of D3+ ions with electrons in a low-temperature plasma (200–300 K) consisting of He with a small admixture of D2. At several temperatures, the pressure dependence of the apparent binary recombination rate coefficient (αeff) was measured over a broad range of helium pressures (200–2000 Pa). The binary and ternary recombination rate coefficients were obtained from measured pressure dependences of αeff. The binary recombination rate coefficient obtained αbin(300 K)=(2.7±0.9)×10−8 cm3 s−1 is in agreement with recent theory. The ternary recombination rate coefficient obtained is KHe(300 K)=(1.8±0.6)×10−25 cm6 s−1. In analogy with the recently described process of helium-assisted ternary recombination of H3+ ions, it is suggested that the ternary helium-assisted recombination of D3+ ions proceeds through the formation of a neutral long-lived highly excited Rydberg molecule D3 followed by a collision with a He atom.
    Physical Review A 10/2009; 80(4). · 3.04 Impact Factor
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    ABSTRACT: We study binary and the recently discovered process of ternary He-assisted recombination of H3+ ions with electrons in a low-temperature afterglow plasma. The experiments are carried out over a broad range of pressures and temperatures of an afterglow plasma in a helium buffer gas. Binary and He-assisted ternary recombination are observed and the corresponding recombination rate coefficients are extracted for temperatures from 77 to 330 K. We describe the observed ternary recombination as a two-step mechanism: first, a rotationally excited long-lived neutral molecule H3∗ is formed in electron-H3+ collisions. Second, the H3∗ molecule collides with a helium atom that leads to the formation of a very long-lived Rydberg state with high orbital momentum. We present calculations of the lifetimes of H3∗ and of the ternary recombination rate coefficients for para- and ortho-H3+. The calculations show a large difference between the ternary recombination rate coefficients of ortho- and para-H3+ at temperatures below 300 K. The measured binary and ternary rate coefficients are in reasonable agreement with the calculated values.
    Physical Review A 05/2009; 79(5). · 3.04 Impact Factor
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    ABSTRACT: The design of the electrostatic Cryogenic Storage Ring (CSR) currently being built in Heidelberg, Germany will be presented. The Cryogenic Trap for Fast ion beams (CTF), which was used to develop the concepts and technology required to build the CSR, has demonstrated cryogenic ion beam storage at temperatures down to 2 K for 2-8 keV cations and anions at residual gas densities equivalent to pressures on the order of 10􀀀13 mbar at room temperature. Some of the recent results in storing ion beams at cryogenic temperatures will be presented, including the first betatron frequency measurement in an ion beam trap and the application of the knock-out mass selection technique.
    PAC09, Vancouver, BC, Canada; 01/2009
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    ABSTRACT: New advanced data analysis method for calculation of recombination rate coefficients from the decay of electron density during the afterglow (along the flow tube) in a Flowing Afterglow Langmuir Probe (FALP) experiment is presented. The method is advantageous for measurements of recombination rate coefficients of slowly recombining ions in conditions where formation of the studied ions is slow, decaying plasma contains several types of ions and the ion formation and recombination proceed simultaneously. The new method is demonstrated on the analysis of the data obtained from plasma decay in He/Kr/H2 and He/Kr/D2 gas mixtures. In a limit for [Kr]/[D2] ≫ 1, the recombination rate coefficients for KrD+ ions (KrD+, 250 K) = 1 × 108cm3s–1 was obtained. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
    Beiträge aus der Plasmaphysik 06/2008; 48(5‐7):521 - 526.
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    ABSTRACT: Recombination of H+3 with electrons is studied in a low-temperature plasma containing He, H2 and Ar at different He and H2 densities. The effective plasma recombination rate is driven by binary, H+3 + e-, and ternary, H+3 + e-+ He, processes with the rate coefficients 7.5 × 10-8 cm3 s-1 and 2.8 × 10-25 cm6 s-1 respectively at 260 K. We suggest that the ternary recombination involves formation of neutral highly excited Rydberg H3 followed by an l-changing collision with He. The difference between recombination of para- and ortho-H+3 is discussed.
    Journal of Physics B Atomic Molecular and Optical Physics 01/2008; 41(19). · 2.03 Impact Factor
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    ABSTRACT: Recombination of H3+ with electrons was studied in a low temperature plasma in helium. The plasma recombination rate is driven by two body, H3+ + e, and three-body, H3+ + e + He, processes with the rate coefficients 7.5x10^{-8}cm3/s and 2.8x10^{-25}cm6/s correspondingly at 260K. The two-body rate coefficient is in excellent agreement with results from storage ring experiments and theoretical calculations. We suggest that the three-body recombination involves formation of highly excited Rydberg neutral H3 followed by an l- or m- changing collision with He. Plasma electron spectroscopy indicates the presence of H3.
    Journal of Physics B Atomic Molecular and Optical Physics 01/2008; 41(19). · 2.03 Impact Factor
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    ABSTRACT: At the Max-Planck-Institut für Kernphysik (MPIK) in Heidelberg a next generation electrostatic storage ring at cryogenic temperatures is under development. The main perspective of this unique Cryogenic Storage Ring (CSR) is the research on ions, molecules and clusters up to biomolecules in the energy range of 20-300 keV at low temperatures down to 2 K. The achievement of such temperatures for all wall materials seen by the ions in the storage ring not only causes a strong reduction of black body radiation incident onto the stored particles, but also acts as a large cryopump, expected to achieve a vacuum of better than 10-15 mbar (corresponding to 10-13 mbar at room temperature). The low temperature and the extremely high vacuum (XHV) will allow novel experiments to be performed, such as rotational and vibrational state control of molecular ions during their interaction with ultra-low energy electrons and laser radiation. A 2 K/21 W refrigerator was successfully commissioned. The connection with a fully assembled cryogenic prototype ion trap is finished and results of first cooling tests will be presented. In this paper we describe the concept and the status of the CSR.
    Proceedings of the European Particle Acclerator Conference (EPAC) 2008; 01/2008