Measurements of kinetic energies of highly charged argon ions produced by inner-shell photoionization and by ion-beam impact have been made using time-of-flight techniques. High-charge-state recoil ions produced by beams of approx..5 - 1 MeV/u Cl/sup +5/ are found to have energies one to two orders of magnitude higher than ions of the same charge produced by vacancy cascades following inner-shell photoionization by synchrotron radiation. The results may have application to the development of a very-cold ion source useful for angle-resolved atomic collision studies

Cederquist, H.

Church, D. A.

Huldt, S.

Johansson, S.-E.

Levin, J. C.

Liljeby, L.

Nilsson, E.

O, C.-S.

Sellin, I. A.

Short, R. T.

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PRODUCTION OF VERY COLD, HIGHLY CHARGED IONS BY SYNCHROTRONRADIATION: COMPARISONS OF THE "SCALPEL" AND "HAMMER" METHODS*J . C. LEVIN, H. CEDERQUIST, C . - S . 0 , R. T. SHORT, AND I . A. SELLINU n i v e r s i t y of Tennessee , K n o x v i l l e , TN 37996 andOak Ridge Na t iona l Labora to ry , Oak Ridge, TN 37831, USA C O N F - 8 7 0 3 1 1 6 — 2L. LILJEBY DE87 009123I n s t i t u t e of Microwave Technology, Stockholm, SwedenS. HULDT, S.-E. JOHANSSON, and E. NILSSON %ZZJ% 1***"*°"—ACOS 640R21400.Fysiska I n s t i t u t i o n e n , Lund Univers i ty , Lund, Sweden tov^[!|L5f^ ^ *Teon«riwtion. orSTSST G - m_ _ „. tor US. GonrnmantD. A. CHURCH P*P«»"Texas A&M Univ. , College S t a t i on , TX 778H3, USAMeasurements of k ine t i c energies of highly charged argon ions „produced by inne r - she l l photoionizat ion and by ion-beam impact havebeen made using t ime-of - f l igh t techniques . High-charge-s ta te r e c o i lions produced by beams of - .5 - 1 MeV/u C l + 5 a re found t o haveenergies one to two orders of magnitude higher than ions of the samecharge produced by vacancy cascades following i n n e r - s h e l lphotoionizat ion by synchrotron r a d i a t i o n . The r e s u l t s may haveapp l i ca t i on t o the development of a very-cold ion source useful forangle-resolved atomic collision studies.Recently we have engaged in an i n i t i a l study [1] concerning theproduction of very-low-energy highly charged ions by synchrotronradiat ion. Our purpose wai to measure ion production ra tes and toverify the cold recoil ion temperatures anticipated, with theAAASTE R D1ST"T1BBpossibility in mind of uti l izing the method as a lower emittance sourceof recoil ions than is available using charged-particle impact as thedirect ionizing agent (the so-called "hammer" method [2]) .In this study, white and monochromatic x rays from a StanfordSynchrotron Radiation Laboratory wiggler l ine were used to createsingle vacancies primarily in the K shells of Ne and Ar, and L shellsof Kr and Xe. The subsequent vacancy cascades produced highly chargedions whose recoil energies were found to be so small that they l i e wellwithin the range of characteristic energies of the Maxwell-Boltzmanndistribution of the room temperature target gas. The ion temperaturesobserved are > 6 orders of magnitude lower than those characteristic oflaboratory and s te l la r plasmas in which ions of corresponding ioniza-tion states have comparable abundances. These temperatures are also -2 orders of magnitude smaller than those characteristic of recoil ionsproduced using the "hammer" method.Our "scalpel" technique involves use of a time-of-flight (TQF)analyzer to permi-t joint examination of ion photoproduction rates andenergies through study of the yields and widths of the correspondingTOF peaks. These peaks arise from the arrival of ions at a dualchannelplate detector £ 1 ys subsequent to their creation by asynchrotron x-ray burst in a dilute gas sample (-0.2 mT) maintained inthe extraction region at the front end of the analyzer.X rays from an 8-pole wiggler operated at 15 kG were focussed by atoroidal mirror and collimated to a 1-mm diameter x-ray beam at theposition of the gas target . The cr i t ica l energy of the radiation was 4keV (corresponding to the 2-GeV electron-beam energy) and wasattenuated below 3 keV by Be windows. This target was viewed by thevertically mounted TOF analyzer through a 3-mm long s l i t located justabove the x-ray beam. X-ray intensity was monitored by ion chamberspositioned both up- and downstream of the uhv system housing theanalyzer. Typical flux through the target was - 1012 photons mnr2sec~ l , and total ion counting rates in the TOF detector were - 1~5kHz. Detector backgrounds from stray x rays were found to benegligible (̂  3 Hz). The x-ray intensity was sufficient to permit theuse of a Si(111) double-crystal monochromator (bandpass 1 x 10-") totune radiation above, below, and interleaving the hlt L2, L3 edges ofXe to explore effects of tuning the x-rays through an absorption edge.The timing resolution achieved sufficed not only to make the desired"temperature measurements but also to achieve adjacent aass isotopicresolution of the charge state (q) spectra of Ne, Ar, Kr, and Xe, asin seen in Fig. 1 . Measurements of mean charge s ta te q are presentedin Table I . The mean charges l isted can be compared with pioneeringmass-spectroscopic measurements carried out long ago using x-ray gunsand f i l t e r s by Carlson et a l . [4 ] , and (for the ca3e of Xe) very recentmeasurements by Tonuma et a l . t5] using monochromatic x-rays from thePhoton Factory. In general, the agreement between our results and themeasurements of Tonuma et a l . i s seen to be excellent, resolving adiscrepancy we observed [1] in the mean charge s ta te increment measuredfor Xe as the L, edge i3 crossed. Consideration of Coster-Kronigyields [63 for transfer of an L, vacancy to the L2 3 subshells and ofrelative photoionization cross sections of the Lt 2 > J levels [7] leadsto an estimated shift in mean charge state on the order of that seen byour group [1] and by Tonuraa et al. [5].Recently we have applied similar TOF techniques to measure theenergies of recoil ions produced using the "hammer" technique (produc-tion in a dilute gas sample by fast beams of highly charged heavyions). Beams of - .5 - 1 Mev/u Cl+S produced by the ORNL EN Tandemhave been used to create argon and neon recoil ions which have beenobserved both in "singles" mode (averaged over all scattered ion chargestates), and in coincidence with individual scattered ion charge statesranging from 6 to 8 [8],Mean recoil energies were determined through study of the widthsof the corresponding TOF peaks, using procedures similar to thosedescribed earlier [1]. For each charge state r the mean initial energyUo was deduced from a fit of the peak widths at FWHM to the formAt2 - a + 8/(r<) + Y/(rO2.The constants a, 8, 1 represent contributions to measured flight timefrom, respectively, timing uncertainty; flight time variations due tofield fringing; and recoil-ion energy [1,9]. The electric-fieldscaling parameter (K) represents six different choices ofproportionately scaled [1,9] electric field values in the TOFanalyzer. The essential correctness of the Ansatz for At2 was wellverified by the clustering of TOF data around fitted quadratic curves,as illustrated in Figure 2 for Ar+B produced by Cl+S at four energies.Recoil energies determined in this manner are summarized in Table II .The nearly symmetric ion-atom collisions reported here correspondto impact parameters ~ .5 au. [8]. Recoil energies for similar impactparameters estimated by elastic Coulomb scattering theory, in which EP« (qr)z, where q and r are the projectile and recoil charge states,respectively, are an order of magnitude smaller than those we observe.Classical trajectory Monte Carlo (CTMC) calculations [10] of inelasticmultiple ionization cross sections for 1 MeV/u collisions of bare ions(q - 2-20,44) with the rare gases lead to an expression for recoilenergy which also scales as (qr)2. When present measurements of logrecoil energy are plotted vs log qr (q = 5, r - 6,7,8,9), the lineardependence observed, Figure 3, suggests a power law of the form E r a(qr)n where least-squares fit results give n - 10. Thisextraordinarily steep dependence is due, in part, to the low q (q - 5)used in the fits. Since projectile loss cross sections are largecompared to capture cross sections for Cl+S at these energies, mostargon recoils are accompanied by scattered projectiles of charge statehigher than five. Similar plots for the data obtained in coincidencewith scattered projectile charge state suggest an exponent n - 5. Thisstrong dependence, far steeper than the n » 2 obtained for elasticCoulomb scattering and in the CTMC [10] calculations is due primarilyto the fact that effective charges seen by the target and projectilenuclei at the moment of closest approach are of much greater importancein determining recoil energy than the asymptotic values (q,r). Infact, good agreement with present measurements is obtained byevaluating the CTMC expression for recoil energy [10] using effectivetarget and projectile charges obtained by Hartree-Fock calculations[8].This work was supported in part by the National Science Foundationand by the U.S. Department of Energy under contractNo. DE-AC05-840R21400 with Martin Marietta Energy Systems, Inc. It wasperformed at SSRL which is supported by the U.S. Dept. of Energy,Office of Basic Energy Sciences, and the National Institutes of Health,Biotechnology Resource Program, Division of Research Resources, and atthe ORNL EN Tandem Facility.References[1] R. T. Short, C.-S 0, J. C. Levin, I. A. Sellin, L. Liljeby,S. Huldt, S.-E. Johansson, E. Nilsson and D. A. Church,Phys. Rev. Lett. 56 (1986) 2614.[2] For recent reviews see L. Liljeby et al., Phys. Scr. 33 (1986) 310;S. Kelbch et al., J. Phys. B 18 (1985) 323; J. Ullrich et al.,J. Phys. B 17 (1981) L785; Proceedings of 3n International Symposium onProduction and Physics of Highly Charged Ions, L. Liljeby,ed. (Phys. Scr. T3 (1983)).[3] J. B. Hastings and V. 0. Kostroun, Nucl. Instr. and Meth. 208(1983) 815.W T. A. Carlson, W. E. Hunt, and M. 0. Krause, Phys. Rev. 151 (1966)11.[5 ] T. Tonuma, A. Yagishita, H. Shibata, T. Koizumi, T. Matsuo,K. Shima, T. Mukoyama and H. Tawara, J. Phys. B 20 (1987) L31-[6] M. H. Chen, B. Crasemann, and H. Mark, Phys. Rev. A2H (1981) .177.[7] W. H. McMaster, N. Kerr Del Grande, J. H. Mallett , andJ. H. Hubbell, Compilation of X-Ray Cross Sections, UCRL-50174 Sec. IIRev. 1, Lawrence Radiation Laboratory, University of California,Livermore, 1969.[8] J. C. Levin, R. T. Short, C.-S. 0, H. Cederquist, S. B. Elston,J. P. Gibbons, I. A. S e l l i n , and H. Schmidt-BOcking, submitted forpublication.[9] W. C. Wiley and L. H. McLaren, Rev. Sci . Instrum 26 (1955) 1150.Table I I . Comparison of argon r e c o i l ion energies ( in eV) produced usingunmonochromatized x rays from SSRL with those produced using 18, 23, 2 7 . 5 ,and 33 MeV Cl+ 5 beams from the ORNL EN tandem acce le ra to r .Argon SSRL Cl+S beam (MeV)charge x-rays 18 23 27.6a 27.6b 3323k567890.016(20) eV0.050(23)0.066(21)0.069(35)0.086(36)0.092(71) 1.0.081(56) 4.13.<.2 <.2 <.2 C.2.26(.17) .09(.29) .3K.21) .15(.15).81 (.67) .62(.61) .75(.49) -38(.3D2.69(1.1) 1.84(1.0) 2.28(.8J») 1.50(.92).91(1.6) 8.21(2.6) 7.17(2.2) 6.82(1.9) 5.03(1.1)a September, 1986. b October, 1986.Figure CaptionsFig. 1. (a) Kr charge-state distribution following L-shell vacancyproduction. Plotted is number of ions detected as a function of flighttime [1]. (b) Earlier work by Hastings and Kostroun [3] displaying similarresults, (c) Expanded view of a Kr peak from (a) demonstrating isotopicresolution [1], (d) Expanded view of a Xe peak from a typical Xe spectruraFig. 2. Quadratic behavior of the square of kr** TOF peak widths plotted asa function of (r<)"' (see text) for Cl+5 at four beam energies.Fig. 3. Recoil energy vs product (qr) of incoming projectile charge andrecoil charge for beams of 18, 23, 27.6, and 33 MeV Cl+S. Data have beenfitted to straight lines (see text), both shifted for clarity. 8oth scalesare logarithmic. The 27.6 MeV data plotted are the average of the data inTable I I .10TIME OF FLIGHTBeam (MeV)1/frK)Beam (MeV)45

Production of Very Cold, Highly Charged Ions by Synchrotron Radiation: Comparisons of the ''Scalpel'' and ''Hammer'' Methods

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Production of Very Cold, Highly Charged Ions by Synchrotron Radiation: Comparisons of the ''Scalpel'' and ''Hammer'' Methods

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