Photoionization and Excitation of Free Variable Size van der Waals Clusters in the Inner Shell Regime Dissertation zur Erlangung des naturwissenschaftlichen Doktorgrades der Bayerischen Julius-Maximilians-Universit¨at Wurzburg¨ vorgelegt von Ioana Lavinia Bradeanu aus Bucharest Wurzburg¨ 2005 Eingereicht am: bei der Fakult¨at fur¨ Chemie und Pharmazie 1. Gutachter: 2. Gutachter: der Dissertation 1. Prufer:¨ 2. Prufer:¨ 3. Prufer:¨ des Offentlichen¨ Promotionskolloquiums Tag der mundlichen¨ Prufung:¨ Doktorurkunde ausgeh¨andigt am: Abstract Photoionization from core levels provides element-specific information on the electronic structure of atoms, molecules, and condensed matter. Post-collision interaction (PCI) is investigated in free, variable size krypton and argon clusters near the Kr 3d- and Ar 2p-ionization energies. Photoionization of free Van der Waals clusters is reported using zero kinetic energy (ZEKE) photoelectron spectroscopy in the Ar 2p, Ne 1s, Kr 3d and N 1s excitation regime. ZEKE photoelectron spectroscopy is used as an experimental technique for investigating the core ionization process as a function of cluster size. It is found that the asymmetry, which is a consequence of PCI, is characteristically smaller for clusters than for isolated atoms. Moreover, there is less asymmetry for bulk-sites than for surface-sites in variable size rare gas clusters. The emission of ultraviolet fluorescence from variable size argon clusters is investigated in the Ar 2p-excitation regime (240–270 eV). The results are assigned in terms of plausi- ble relaxation processes occurring in the 2p-excitation regime. This implies that charge separation mechanisms are active, which occur after electronic relaxation of the core hole. Besides electron impact ionization within clusters caused by Auger electrons, the inter- atomic Coulombic decay (ICD) mechanism appears to be a plausible way to rationalize the experimental findings. Cluster size effects in core excitons below the N 1s ionization energy of nitrogen clusters are investigated in the energy regime 405-410 eV. These results are compared to the molecular Rydberg states as well as the corresponding bulk excitons of condensed nitrogen. The experimental results are compared with ab initio calculations, providing an improved spectral assignment of core exciton states in weakly bound molecular clusters and the corresponding condensed phase. To my Parents “ Run rabbit run Dig that hole, forget the sun And when at last the work is done Don’t sit down It’s time to start another one” v List of Publications I) I.L. Bradeanu, H. Setoyama, T. Hatsui, N. Kosugi and E. R¨uhl, “Post Collision Interaction of Free Van der Waals Clusters” In manuscript II) I.L. Bradeanu, H. Setoyama, T. Hatsui, N. Kosugi and E. R¨uhl, “Photoelectron Profiles of Free Variable Size van der Waals Clusters” In manuscript III) I.L. Bradeanu, R. Flesch and E. R¨uhl, “ZEKE Photoelectron spectroscopy of free Van der Waals clusters” In manuscript IV) T. Hatsui, H. Setoyama, N. Kosugi, B. Wassermann, I.L. Bradeanu and E. R¨uhl, “Photoionization of Small Krypton Clusters: Evidence for Site-Specific Photoemis- sion” J. Chem. Phys. 123, 154304 (2005) V) I. L. Bradeanu, R. Flesch, M. Meyer, H.-W. Jochims and E. R¨uhl, “Radiative Relaxation of 2p-Excited Argon Clusters: Evidence for the Interatomic Coulombic Decay Mechanism” Eur. J. Phys. D, 173–178 (2005). VI) R. Flesch, N. Kosugi, I. Bradeanu, J.J. Neville and E. R¨uhl, “Cluster Size Effects in Core Excitons of 1s-Excited Nitrogen” J. Chem. Phys. 121, 8343-8350 (2004). The following are papers to which I have contributed but they are not discussed in the thesis. I.L. Bradeanu, R. Flesch, N. Kosugi and E. R¨uhl, “Core (C 1s and N 1s) → π∗ transition in pyridine clusters In manuscript I.L. Bradeanu, R. Flesch, N. Kosugi, A.A. Pavlychev and E. R¨uhl, “C 1s → π∗ transitions in benzene clusters” Submitted to Phys. Chem. Chem. Phys. vi Participation at International Conferences I) 7–9 September 2005 International Workshop on Free Clusters and Nanoparticles using Short Wavelength Radiation, Bad Honnef, Germany contributed talk: I.L. Bradeanu “Photoelectron profiles of free variable size van der Waals clusters” II) 27 July—1 August 2005 International Workshop on Photoionization, IWP2005, Campinas, Brazil poster contribution: I.L. Bradeanu, H. Setoyama, T. Hatsui, N. Kosugi and E. R¨uhl “Post Collision Interaction of Free Van der Waals Clusters” poster contribution: I.L. Bradeanu, R. Flesch, M. Meyer, H.-W. Jochims and E. R¨uhl “Radiative Relaxation of 2p-Excited Argon Clusters: Evidence for the Inter- atomic Coulombic Decay Mechanism” III) 8–9 October 2004 Studiecentrum Soeterbeeck, Ravenstein, The Nethelands Invited talk: I.L. Bradeanu “Spectral shifts in core-to-valence transitions of molec- ular van der Waals clusters” IV) 3–5 September 2003 Workshop on free, neutral clusters studied by synchrotron radiation, Sigtuna, Swe- den Invited talk: I.L. Bradeanu “Zero kinetic energy photoelectron spectroscopy of free Van der Waals clusters” Comments on my own participation All papers in this thesis are the result of teamwork as is necessary in experimental physics. My contribution to these papers always lies in the theoretical, experimental and data analysis side. My contribution to papers I and II covers the performance of the theoretical model of post collision interaction in clusters, of the experimental data analysis and the preparation of the manuscript. In paper III my contribution lies in the experiment and data analysis, theoretical calculations and preparation of the manuscript. In paper IV my contribution is the data analysis and the preparation of Fig. 2 and Table 1, and the text around them in the manuscript. In paper V, I have participated in the experimental work, data analysis and the discussion of the manuscript. For paper VI, my contribution consists of preparing figure 4. Contents 1 Introduction 1 2 Core Level Photoelectron Spectroscopy 3 2.1 Direct photoionization . 6 2.2 Auger electron spectroscopy . 6 2.3 In vicinity of the core-ionization threshold . 7 2.4 At the threshold . 9 3 From Atoms to Clusters 11 3.1 Clusters as finite objects . 11 3.2 Bonding rare gas dimers . 12 3.3 Rare gas clusters . 13 3.4 UV/Vis Fluorescence decay . 16 4 Synchrotron Radiation 19 4.1 The synchrotron as a light source . 19 4.2 Beamline . 23 5 Experimental setup 25 5.1 Clusters production in supersonic jets . 25 5.2 Detection methods . 32 6 Post Collision Interaction 45 6.1 Inner-shell photoionization . 46 6.2 ZEKE line shape . 49 6.3 Semi-classical approach . 51 6.4 Dynamics of PCI in clusters . 57 6.5 Dynamics of PCI in solid . 62 7 Model Calculations: Argon Clusters 71 7.1 Binding energies in Ar clusters . 72 8 Photoionization of Small Krypton Clusters 77 8.1 Experimental Results . 77 8.2 Model Calculations . 81 9 Photoelectron Line Shape of Free van der Waals Clusters 85 9.1 PCI in variable size Ar clusters . 85 viii CONTENTS 10 PCI in small Kr clusters 99 10.1 Experimental results and discussion . 99 10.2 Comparison with experimental results . 105 11 ZEKE Photoelectron Spectroscopy of Free Clusters 109 11.1 Ar 2p excited argon clusters . 110 11.2 Kr 3d excited krypton clusters . 118 11.3 Ne 1s excited neon clusters . 122 11.4 N2 1s excited nitrogen clusters . 127 12 Radiative Decay of Core-Hole State: Ar 2p 133 12.1 Experimental results: Fluorescence excitation in Ar clusters . 133 13 Size-dependent Core-to-Rydberg Excitations: N2 143 13.1 Theoretical calculations . 144 13.2 Experimental results . 148 Conclusions 153 Zusammenfassung 155 Outlook 157 Acknowledgments 159 Chapter 1 Introduction Core levels have properties which make them interesting for the study of variable size matter: they are atomic-like and localized even in molecules and solids, and are very short-lived. This means that core level spectroscopy can give information not only about which atoms are present in the sample, but also the number and the type of different sites which these occupy, and how the local electronic and geometric structure depends on the type of site. The short lifetime of core holes enables studies of the dynamics development occurring during their lifetime, i.e. a few femtoseconds (10−15 s). This thesis is dedicated to the study of core level excitation of clusters – which is a unique approach to probe element and site specific size-dependent properties of free clusters in the gas phase. Clusters are aggregates of a finite number of atoms or molecules that bridge the gap between an isolated atom/molecule and an infinite solid [1]. It is therefore of primary interest to be able to follow size dependent changes of properties of matter by specific experimental and theoretical approaches in order to understand whether changes of cluster size occur gradually, in distinct steps, or if there are size regimes with unique properties. Free clusters produced in specially prepared gas beams give a possibility of following the gradual development of the size-dependent properties with cluster size. Information about the atom-to-solid evolution obtained from free cluster investigations have the ad- ditional advantage that the density of states is low in comparison to studies of outer valence shells. As clusters are small objects, a large and size dependent fraction of their constituent atoms are located at their surface. The properties of clusters are thus to a high degree determined by the surface electronic and geometric structure. It is thus desirable to be able to separately probe the atoms located at the surface and those in the bulk interior. Spectroscopic methods focusing on the core electronic levels allow such surface and bulk specific studies to be made. These experimental methods have proven to be powerful tools for the investigations of the electronic and geometric structure of the conventional forms of matter: gas-phase atoms and molecules, solids, and surfaces [2, 3].
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