NL39C0217 INIS-mf--11421 I ELECTRON SPECTROSCOPY OF COLLISIONAL EXCITED ATOMS Coincidence analysis of the Li+-He collision system ELEKTRONSPECTROSCOPIE VAN BOTSINGSGEINDUCEERDE AANGESLAGEN ATOMEN Coïncidentie analyse van het Li+-He botsingssysteem (met een samenvatting in het Nederlands) PROEFSCHRIFT Ter verkrijging van de graad van Doctor aan de Rijksuniversiteit te Utrecht, op gezag van de Rector Magnificus Prof.Dr. J.A. van Ginkel, volgens besluit van het College van Decanen in het openbaar te verdedigen op woensdag 25 november 1987 des namiddags te 2.30 uur door PETER VAN DER STRATEN geboren op 10 augustus 1959 te Hilversum Promotor: Prof.Dr. A. Niehaus, verbonden aan de fakulteit Natuur- en Sterrenkunde van de Rijksuniversiteit Utrecht Promotor: Prof.Or. R. Morgenstern, verbonden aan de fakulteit der Natuurwetenschappen van de Rijksuniversiteit Groningen Aan mijn ouders, Wilma en Lieke Tekstverwerking Corine Lamberts Tekeningen Wim Post Fotografie Onderwijs Media Instituut Drukkerij Elinkwijk B.V. The work described in this thesis was performed as part of the research programme of the 'Stichting voor Fundamenteel Onderzoek der Materie' (FOM) with financial support from the 'Nederlandse Organisatie voor Zuiver Wetenschappelijk Onderzoek' (ZWO). CONTENTS Chapter 1 Introduction and summary 1 References 5 Chapter 2 Theory 7 2.1. Introduction 7 2.2. Angular distribution of electrons 10 2.3. Energy distribution of electrons 12 2.A. Less restrictive experiments 14 a. Coincident angular distribution of electrons 14 b. Non-coincident energy distribution of electrons 15 c. Non-coincident angular distribution of electrons 16 2.5. Galilean transformation 17 a. The transformation 18 b. The heavy particle collision 19 c. The impact parameter 20 2.6. Collision amplitudes 22 a. Scaling the amplitudes 23 b. Rotation of the coordinate frame 23 c. The observables of the collision 26 Appendix A: Atomic units 28 Appendix B: Rotation of the coordinate frame 29 a. Rotation from collision to rotated frame 29 b. Rotation from collision to natural frame 30 References 31 Chapter 3 Experimental set-up 33 3.1. Introduction 33 3.2. Ion beam preparation 34 3.3. Target beam preparation 37 3.4. Electron spectrometer 38 3.5. Projectile spectrometer 40 a. Ion spectrometer 40 b. Neutral spectrometer 43 3.6. The coincidence technique 45 a. The electronics 45 b. The statistics of a coincidence analysis 47 c. Width of the time peak 51 3.7. Limitations of the coincidence method 53 References 58 Chapter 4 Interference of autoionislng transitions in fast ion-atom collisions 59 4.1. Introduction 59 4.2. The transition amplitude for autoionlsation 61 4.3. Interference of autoionising transitions 68 4.4. Application to experimental data 71 a. Excitation cross sections at high collision energies 71 b. Emission angle-dependent post-collision interaction 73 4.5. Angular shift induced by PCI 79 4.6. Conclusions 83 References 84 Chapter 5 Angular dependent post-collision interaction in Auger processes 85 5.1. Introduction 85 5.2. Theory 87 5.3. High excess energies 91 5.4. Angular dependent line shift 93 5.5. Results 96 105 5.6. Conclusions 106 Appendix 108 References Chapter 6 Quasimolecular or kinematical broadening of autolonisation electron spectra from Li /He collisions? A coincidence study 109 6.1. Introduction 109 6.2. Experiment 112 6.3. Results 113 6.4. Analysis of results 115 a. Analysis of coincidence measurements 115 b. Coincident electton spectra 119 c. Coincident angular distributions 121 d. Inelastic differential cross section for He -excitation 121 e. Non-coincident elsc.tr,n spectra 123 6.5. Dls-.usslon and conclusions 126 References 128 Chapter 7 A coincidence analysis of the excitation mechanisms for two electrons in low energy collisions of Li and He 129 7.1. Introduction 129 7.2. Experiment 131 7.3. Theory 132 7.4. Results 134 7.5. Discussion 147 References 152 Chapter 8 Shapes of excited atoma and charge motions induced by ion-atom collisions 153 8.1. Introduction 153 8.2. Angular distributions of autoionisation electrons and atomic shapes 155 8.3. Asymmetric atomic shapes and charge oscillations induced by L-coherences 161 8.4. Comparison with related work 8.5. Do oscillating charge motions correspond 165 to extra correlations? 166 Appendix 168 References 170 Samenvatting 173 Nawoord 175 Curriculum Vitae 179 Parts of chapter A are published in J. Phys. B: At. Mol. Phys. _19_ (1986) 1361-1370 and in Phys. Rev. A_34_(1986) 4482-4484. Chapter 5 is submitted to Z. Phys. D: Atoms, Molecules and Clusters. Chapter 6 is published in Z. Phys. A - Atoms and Nuclei _320_, 81-88 (1985). Chapter 7 is submitted to J. Phys. B: At. Mol. Phys. Chapter 8 is published in Comments At. Mol. Phys. J7_ (1986) 243-260. A simplified version of chapter 8 appeared in Ned. Tijdsch. v. Nat. B 52 (1986) 121, 124. Inspection of the Ion Spectrometer (photograph Werry Crone). CHAPTER 1 INTRODUCTION AND SUMMARY From the onset of laboratory research in atomic physics at the beginning of this century, collision experiments played an important role. In collisions details of structure and interactions of particles determine the outcome and information on these details can be investigated by observing the collision partners after the collision. The advent of quantum mechanics has increased the importance of collision' experiments, because they allow to observe phenomena, unknown in classical mechanics, that reveal the quantum mechanical laws of nature. Given all initial properties of a given collision system, one can deduce in classical mechanics the evolution of the system in the future. Therefore one can obtain all the information obtainable for the given initial conditions by observing one single collision. In quantum mechanics this is no longer the case. When the laws of quantum mechanics apply, one has to observe many identically prepared collisions to obtain the complete information. In the main part of this thesis we will describe measurements, where we observed many collisions under well defined initial conditions for one special collision system: the LI+-He system. There are various ways the collision process can be observed, yielding information in varying degrees of detail. Van Eck and Kistemaker (1962) measured for the Li -He system the total scattering cross section by determining the probabilities for scattering the projectiles out of the beam by the target atoms. In this way information is obtained, which can be interpreted in terms of - 1 - theoretical models that contain only little details on the physical quantities governing well defined collision events. More detailed information can be gained by measuring the cross section, differential in one of the quantities, determining the initial conditions. Such measurements were performed for the Li -He system by Lorents and Conklin (1972) and Francois et al (1972), who measured the cross section differential in the scattering angle of the projectile. This more detailed information puts a more stringent test on theoretical models used. Another piece of information can be gained by observing the "particles", emitted by the system as a result of the collision, such as electrons and photons. In the case of photons these experiments were performed by Tsurubuchi and Iwai (1981) and in the case of electrons by Thielmann (1974), Yagishita et al (1980) and Zwakhals et al (1982). Because "particles" can only be emitted, if sufficient excitation occurs during the collision, these experiments yield only information on inelastic collisions. If one only detects either scattered projectiles or emitted "particles", information on the respective "other part" of the collision process is not obtained. Therefore these measurements are called incomplete measurements. To put it in terms of quantum mechanics, the system can not be described by a wave function, but one has to apply the density matrix formulation for its description. If one performs ab initio calculations for a certain system, one essentially calculates, in some approximation, the evolution of the corresponding wave function during the collision. To compare these calculations with incomplete experiments, one has to average the wave function over all unobserved quantities. It is obvious that this does not lead to a very direct test of the theoretical methods used. The situation can be improved, if one measures both, the scattered projectile and the "particle" emitted in the same scattering event. For certain collision systems, and under certain experimental conditions, such a measurement can be regarded as a "complete experiment". Complete in the sense, that the maximum - 2 - information obtainable according to the quantum mechanics Is In fact obtained. Such experiments became feasible with the introduction of coincidence techniques, which have been applied to the Li -He system only recently. Andersen et al (1985) measured coincidences between scattered projectiles and photons. This yields information on collisions, in which one electron is excited. In this thesis we will describe measurements, in which coincidences are detected between scattered projectiles and emitted electrons. This yields information on two-electron excitation processes. Together with the results of Andersen et al, our results form a complete experimental investigation of excitation mechanisms in the keV-region for the Li+- He collision system. In order to show, what can be learned from coincidence experiments, a detailed theoretical analysis thereof is given in chapter 2. The transition amplitudes are introduced, which contain all the information, which can be obtained from a collision. These amplitudes are related to quantities, which enbody more physical significance for a collision process, such as the angular momentum transferred from the projectile to the target or the alignment of the excited state after the collision. The corresponding relations are derived. Furthermore it is shown, what kind of information is lost, when one confines oneself to detect only the electrons. In chapter 3 the experimental set-up is shown. After an overview of the components, generally used in collision physics (ion source, detectors), the coincidence technique is discussed in detail.