Home >Documents >Electron-transfer processes in fast ion-atom 198048/FULLTEXT01.pdf · PDF...

Electron-transfer processes in fast ion-atom 198048/FULLTEXT01.pdf · PDF...

Date post:02-Sep-2018
Category:
View:212 times
Download:0 times
Share this document with a friend
Transcript:
  • Electron-transfer processesin fast ion-atom collisions

    Kristian Stchkel

    AKADEMISK AVHANDLING

    som med tillstnd av Stockholms Universitetframlgges till

    offentlig granskning fr avlggandet avfilosofie doktorsexamen

    mandag den 5 december 2005, kl 10:00 i rum FA31AlbaNova Universitetscentrum, Roslagstullsbacken 21, Stockholm.

    Department of PhysicsStockholm University

    2005

  • ii

    Electron-transfer processes in fast ion-atom collisionsKristian StchkelISBN 91-7155-165-4 pp i-x, 1-59 Kristian Stchkel, 2005Stockholm UniversityAlbaNova University CenterDepartment of PhysicsSE - 106 91 StockholmUniversitetsservice US ABStockholm 2005

  • iii

    Abstract

    The subject of this thesis is experimental studies of electron-transfer processesin ion-atom collisions at velocities significantly higher than typical orbital velocitiesof electrons in bound states of atoms or molecules. The experimental techniqueapplied combines the high beam intensity of heavy-ion storage rings with a super-sonic gas-jet target equipped with a recoil-ion-momentum spectrometer. In single-electron capture to fast protons from helium atoms, we have for the first timeachieved a complete separation of the kinematic and Thomas transfer mechanismsand are able to perform a quantitative comparison with the many theoretical resultson a much more detailed level than what was previously possible. For the processof transfer ionization in proton-helium collisions we have determined the velocitydependence of the Thomas transfer ionization cross section to be the expected v11pwhen the projectile velocity, vp, is sufficiently high. Further, we have determinedthe velocity-dependent probability for shake-off of the second electron from heliumprovided that the first one is transferred in a kinematic capture process. Finally, wehave considered collisions between protons and hydrogen molecules. Here we havefound a strong variation in the cross section for transfer and excitation processeswhen the angle between the direction of the incoming projectile and the internuclearaxis of the target molecule is varied. The variation can be explained as a resultof quantum mechanical interference related to the two indistinguishable atomiccenters of the molecule.

  • iv

  • Publications included in thisthesis

    I Experimental separation of the (p-e-N) Thomas charge trans-fer process in high velocity p-He collisions,D. Fischer, K. Stchkel, H. Cederquist, H. Zettergren, P. Reinhed,R. Schuch, A. Kllberg, A. Simonsson, and H. T. Schmidt,Submitted to Physical Review Letters

    II Transfer ionization in p+He collisions,H. T. Schmidt, A. Fardi, J. Jensen, P. Reinhed, R. Schuch, K. Stchkel,H. Zettergren, H. Cederquist and C.L. Cocke,Nuclear Instruments and Methods B 233, 43 (2005)

    III Recoil-ion momentum distribution for Transfer Ionization infast proton-He collisions,H. T. Schmidt, J. Jensen, P. Reinhed, R. Schuch, K. Stchkel, H. Zetter-gren, H. Cederquist, L. Bagge, H. Danared, A. Kllberg, H. Schmidt-Bcking, and C. L. Cocke,Phys. Rev. A. 72, 012713, (2005)

    IV Observation of two-center interference effects in proton-H2collisions,K. Stchkel, H. T. Schmidt, P. Reinhed, R. Schuch, H. Zettergren,H. Cederquist, S. B. Levin, V. N. Ostrovsky and C. L. Cocke,Accepted for publication in Physical Review A (Rapid communication)

    v

  • Other publications

    1 Non-Gaussian velocity distributions in optical lattices,J. Jersblad, H. Ellmann, K. Stchkel, A. Kastberg, L. Sanchez-Palencia, and R. Kaiser,Phys. Rev. A. 69, 013410 (2004)

    2 Fragmentation and ionization of C70 and C60 by slow ions ofintermediate charge,H. Zettergren, P. Reinhed, K. Stchkel, H. T. Schmidt, H. Ced-erquist, J. Jensen, S. Tomita, S. B. Nielsen, P. Hvelplund, B. Manil,J. Rangama, and B. A. Huber,Accepted for publication in European Physical Journal D (2005)

    3 Charge exchange of fast protons on oriented hydrogen mole-cules,V. N. Ostrovsky, S. B. Levin, O. Eidem, H. Cederquist, K. Stchkel,and H. T. Schmidt,in manuscript

    vii

  • Contents

    Contents ix

    1 Introduction 1

    2 Experimental technique 52.1 Recoil-ion-momentum spectroscopy . . . . . . . . . . . . . . . 52.2 Experimental setup . . . . . . . . . . . . . . . . . . . . . . . . 72.3 Experiment specific spectrometer details . . . . . . . . . . . . 11

    3 Single electron capture 193.1 Experiment . . . . . . . . . . . . . . . . . . . . . . . . . . . . 203.2 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

    4 Transfer ionization 274.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . 274.2 Experiment and results . . . . . . . . . . . . . . . . . . . . . 29

    5 Proton-H2 355.1 Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 365.2 Experiment . . . . . . . . . . . . . . . . . . . . . . . . . . . . 405.3 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42

    6 Conclusions 49

    7 The authors contribution 51

    Acknowledgements 53

    ix

  • x CONTENTS

    Bibliography 55

  • Chapter 1

    Introduction

    In this work, electron transfer in fast ion-atom collisions is studied. Elec-tron transfer is defined as the transfer of electrons between the collidingparticles (called the projectile and target). When the projectile velocity islow compared to the speed of the electrons in the target, the collision iscalled slow. In such collisions, the electrons have sufficient time to arrangethemselves according to the changing positions of the nuclei. Thus, if atany time during the collision the internuclear distance is sufficiently smallfor electron transfer to take place, this will happen with unit probability.Electron transfer cross sections for slow collisions are thus determined bythe critical distance for transfer and are of the order of the geometrical sizeof the involved particles or larger. We study fast collisions where the speedof the projectile is much larger than the typical speed of the electrons in thetarget. This is the opposite extreme to the case discussed above. Here verylittle time is available during the collision for the electrons to respond to themotion of the nuclei. Also in this velocity range, electron transfer is possi-ble for all impact parameters smaller than the critical distance. However,the lack of an efficient mechanism to accelerate the electron from the targetsystem to the projectile system results in very small electron transfer prob-abilities. For this reason the cross sections for electron transfer processes infast collisions are much smaller than for slow collisions and decrease rapidlywith increasing velocity. The electron in hydrogen has an average speed ofc = v0 (where c is the speed of light and 1/137 is the fine structureconstant) and this is the velocity unit (v0) when atomic units are used. Thismeans that a collision between a proton projectile and a hydrogen target is

    1

  • 2 CHAPTER 1. INTRODUCTION

    considered fast if the projectile energy is much above 25 keV.For an electron to be transfered in a fast collision, a mechanism must

    exist that accelerates the electron to a matching velocity. Different mech-anisms exist and those relevant in this velocity regime are kinematic anddouble scattering processes (Thomas processes). The kinematic process canbest be described as a single projectile-electron interaction in the field ofthe target nucleus. In the quantum mechanical description of the targetand projectile states there are high velocity tails of the electron wave func-tion. Transfer is possible due to the overlap between these velocity tails[SS79]. Hence, we still see capture, even though the projectile speed is muchlarger than the typical target electron speed. It is clear that this overlap,and consequently the cross section for this process, decreases very rapidlywith the speed of the projectile. The predicted asymptotic velocity depen-dence is v12p where vp is the projectile velocity. It is therefore difficult toinvestigate this process. For the double scattering processes the velocitydependence is v11p , and so this transfer mechanism gradually becomes rela-tively more important. The cross sections for the double scattering processesare, however, also small. Due to the very small cross sections for the pos-sible transfer mechanisms, an experimental setup with high luminosity isrequired to study fast electron transfer processes. With cryring at theManne Siegbahn Laboratory we have access to unique experimental condi-tions where these processes can be studied. With the internal gas-jet targetwe can study electron transfer collisions at high projectile velocities andwith good resolution. Hereby we have the opportunity to study fast elec-tron transfer collisions in detail. In this thesis several new and interestingresults on fast atomic collisions are presented. The results focus on differentaspects of charge transfer from a multi-electron target to a fast proton, andare all obtained with the same experimental setup, where the main partsare: a fast proton beam, a neutral gas-jet target, and a recoil-ion momentumspectrometer.

    The experimental setup is described in chapter 2 along with a brief intro-duction to recoil-ion momentum spectroscopy (rims). This technique allowsfor measurements of capture cross sections much smaller than those acces-sible in single-pass experiments as the storage ring provides a high protoncurrent. The recoil-ion momentum spectrometer is discussed in detail inchapter 2. The following chapters describe the experiments and results pre-sented in the appended articles. In chapter 3 we investigate single-electroncapture (SC) in high-velocity collisions between protons and helium, and

  • 3

    determine the relative contributions of the Thomas and kinematic mecha-nisms.

Click here to load reader

Reader Image
Embed Size (px)
Recommended