H.A. Sakaue

L. Pichl

T. Kusakabe

National Institute for Fusion Science (NIFS-Repository)

Cross Sections of Charge Transfer bySlow Helium Ions in Collisions withWater MoleculesKusakabe, T. (Dept. Phys. Kinki Univ.)Pichl, L. (ICU)Sakaue, H.A.Water molecule (H2O) is main components of cometsand one of the typical impurities in almost all laboratoryplasmas. Recent observations of X-ray emission fromcomets have been modeled using the charge transferprocesses. The fully and partially striped solar wind ionscapture electrons from cometary gases into the excited state,and emit photons.1)In the large thermonuclear fusiondevices, adsorbed H2O molecules are released from theinner wall of the vacuum vessel and some electrodes. It isconsidered to have affected the character of plasma throughvarious collision processes. On the other hand, a heliumatom is one of the common elements in the solar windplasmas and the fusion product in the next day large fusiondevices. It is important, therefore, to measure the crosssections for charge transfer of slow He+and He2+ionscolliding with H2O molecules. But the previous crosssection measurements in these collision partners are scarce.To obtain precise and reliable cross section data and adetailed understanding of the collision dynamics, therefore,we have carried out a joint experimental and theoreticalstudy for charge transfer of He+and He2+ions collidingwith H2O molecules at low keV energies.In the present experiment, He+and He2+ions wereextracted from an electron impact ion source andintroduced into a 4 cm long collision cell filled with highpurity water molecules. After collisions, fast ions andproduct fast neutral particles were detected with a positionsensitive micro-channel plate detector. The cross sectionsof charge transfer were determined by an initial growth ratemethod. In order to extract high pure ground state He+ions, they were produced from4He atoms by 30.5 eVelectron impact. On the other hand, He2+ions weregenerated by 270 eV electron impact from the isotope3Heatoms to prevent mixing of the H2+ions. The introducingsystem of water vapor with water reservoir was constituted,which can be cooled by liquid nitrogen in order to removethe air absorbed into water.As a first step of this study, the cross sections forcharge transfer by H+ions colliding with H2O moleculeshave been measured in the collision energy range between0.2 and 4.0 keV. As shown in Fig. 1, the present crosssection data gradually decrease with increasing collisionenergy, and are found to be in good agreement withprevious experimental data of Greenwood et al.,1)Lindsayet al.,2)and Dagnac et al.3)Recent theoretical calculationsof Mada et al.4)are in fairly good agreement with theexperimental data in the energy range of above 1 keV,while they are apart from the measured cross sectionsat energies below 1 keV.The present cross sections for charge transfer by He+ions colliding with H2O molecules have been measured inthe collision energy range between 0.2 and 4.0 keV. Thepresent data increase with increasing collision energy andcan be connected smoothly with the data of Rudd et al.measured at higher energy region.5)The previousexperimental data of Greenwood et al. measured in theenergy range of 1.2 to 6.6 keV1)are almost constant, andare larger by a factor of two at 1.2 keV in energy than thepresent results. Recent theoretical calculations ofCabrera-Trujillo et al.6)are found to be in good agreementwith the present data.In He2++ H2O collisions, only two data sets areavailable till now. Rudd et al. measured both single- anddouble-charge transfer cross sections (�21, �20) at above 5keV/u in energy.5)Greenwood et al. have presented twoarticles concerning the cross section data at energies below7 keV/u.1,7)The present measurements for He2+ionscolliding with H2O molecules have been carried out in thecollision energy range between 0.6 and 8.0 keV. Thepresent �21 results are in excellent agreement with theprevious data of Greenwood et al.1,7)However, thepresent �20 data are found to gradually increase withincreasing collision energy, while the previous data ofGreenwood et al.7)decrease. Theoretical calculations ofCabrera-Trujillo et al.6)are supporting the present results.The theoretical calculations using the molecular-orbitalclose-coupling method (MOCC) will be applied to thesespecific processes considered with the molecularvibrational effects.Fig. 1. Charge-transfer cross sections for H+ions incollisions with H2O molecules.1) Greenwood, J.B. et al.: Astrophys. J., 529 (2000) 605.2) Lindsay, B.G. et al.: Phys. Rev. A, 55 (1997) 3945.3) Dagnac, R. et al.: Compt. Rend., Ser. B, 268 (1969) 676.4) Mada, S. et al.: Phys. Rev. A, 75 (2007) 022706.5) Rudd, M. et al.: Phys. Rev. A, 32 (1985) 2128.6) Cabrera, R. et al.: Phys. Rev. A, 75 (2007) 052702.7) Greenwood, J.B. et al.: Phys. Scripta, T110 (2004) 358.10-110010110-1610-1510-14Theory:Mada et al. (2007)Present workGreenwood et al. (2000)Lindsay et al. (1997)Baribaud et al. (1971)Dagnac et al. (1970)H++ H2OCrosssection(cm2)Collision energy (keV/u)Berkner et al. (1970):D+Coplan and Ogilvie (1970)Koopman (1968)Chambers (1965)427§6. Cross Sections f Charge Transfer by Slow Helium Ions in Collisions with Water MoleculesKusakabe, T. (Dept. Phys. Kinki Univ.),  Pichl, L. (ICU),  Sakaue, .A.

Cross Sections of Charge Transfer by Slow Helium Ions in Collisions with Water Molecules

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