Cooling of Anionic Metal Clusters Stored in an Electrostatic Ion Beam Trap

Cooling of anionic metal clusters stored in an electrostatic ion beam trap Inauguraldissertation zur Erlangung des akademischen Grades eines Doktors der Naturwissenschaften der Mathematisch-Naturwissenschaftlichen Fakultät der Ernst-Moritz-Arndt-UniversitätGreifswald vorgelegt von Christian Breitenfeldt geboren am 22.10.1986 in Greifswald Greifswald, den 7. Dezember 2016 Dekan: Prof. Dr. Werner Weitschies 1. Gutachter: Prof. Dr. Lutz Schweikhard 2. Gutachter: Prof. Dr. Karl-Heinz Meiwes-Broer Tag der Promotion: 24.04.2017 Contents List of Figures iii List of Tables iii Nomenclature v 1 Introduction 1 2 Experimental Setup 3 2.1 The electrostatic ion-beam trap . .3 2.2 The cryogenic trap for fast ion beams . .4 2.2.1 Metal Ion Sputter Source (MISS) . .6 2.2.2 Laser VAPorization ion source (LVAP) . .6 2.3 Operation . .8 2.3.1 Experimental control system . .8 2.3.2 Room temperature operation . .9 2.3.3 Cryogenic operation . .9 2.4 Data evaluation . 10 3 Beam dynamics of an electrostatic ion-beam trap 11 3.1 Beam losses . 11 3.2 Self-synchronization and self-bunching . 11 3.3 Thesis article I . 12 4 Anionic clusters 13 4.1 Decay of highly excited clusters . 13 4.1.1 Electron emission . 13 4.1.2 Fragmentation . 16 4.2 Radiative cooling of excited clusters . 19 4.3 Geometric cluster structure . 20 4.4 Thesis article II . 21 4.5 Thesis article III . 22 5 Summary and Outlook 23 6 Cummulative thesis articles 33 6.1 Author contribution . 33 6.2 Spreading times of ion-bunches in the Cryogenic Trap for Fast ion beams 35 6.3 Decay processes and radiative cooling of small anionic copper clusters . 41 6.4 Long-term monitoring of the internal energy distribution of isolated cluster systems . 53 7 Eigenständigkeitserklärung 67 i List of Figures 2.1 Schematic view of the EIBT at the CTF . .4 2.2 Schematic view of the CTF setup . .5 2.3 Photograph of the pick-up electrodes . .6 2.4 Schematic view of the MISS . .7 2.5 Schematic view of the LVAP . .7 2.6 Schematic experimental cycle . .8 4.1 Calculated electron emission rate constants for Co4− ........... 15 4.2 Decay curves for Co4− ............................ 17 List of Tables 2.1 Applied voltages at the EIBT . .5 iii Nomenclature CSR cryogenic storage ring CTF cryogenic trap for fast ion beams EIBT electrostatic ion-beam trap LVAP laser-vaporization source MCP micro-channel plate detector MISS metal-ion sputter source MR-ToF multi-reflection time of flight OPO optical parametric oscillator 1 A2 Einstein coefficient for spontaneous emission Bd rotational constant of the daughter cluster Bp rotational constant of the parent cluster 2 B1 Einstein coefficient for absorption D0 binding energy of the most weakly bound atom in a cluster E energy EA adiabatic electron affinity Eph photon energy Ei energy of level i E0 kinetic energy of the reference particle Jd rotational moment of the daughter cluster max Jd maximum rotational momentum of the daughter cluster Jp rotational moment of the parent cluster L orbital angular momentum of the fragments Lmax maximum orbital angular momentum of the fragments Lmin minimum orbital angular momentum of the fragments L0 orbital length for the reference particle M total number of states for an electron in a box N internal energy distribution of an ion ensemble R rate of decaying ions R electron emission rate An− An+e− R ! electron attachment rate An+e− An− R ! single atom emission rate A− A− +A n ! n 1 R − single atom attachment rate A− +A A− n 1 ! n U − potential between the fragments T temperature V volume c speed of light e elemental charge f spin degeneracy f0 revolution frequency of the reference particle g reaction path degeneracy h Planck constant ~ reduced Planck constant v Nomenclature kat rate constant for electron attachment kcomplex rate constant for forming a collision complex kem rate constant for electron emission kfrag fragmentation rate constant krad photon emission rate constant m electron mass n number of atoms in a cluster nx;y;z quantum numbers of an electron in a box r distance of the fragments v relative velocity of the electron and the cluster or the daughter cluster and the atom v0 mean velocity of the reference particle Γ total number of orbital angular momentum states ∆E energy difference between the ion of interest and the reference ion ∆f frequency difference between the ion of interest and the reference ion ∆L difference of the orbit length between the ion of interest and the reference ion ∆v difference of the mean velocity between the ion of interest and the reference ion Λ constant describing the height of the rotational barrier α polarizability d relativ translational energy of the fragments max d maximum relative kinetic energy of the fragments min d minimum relative kinetic energy of the fragments d;ges sum of rotational and kinetic energy of the fragments electron kinetic energy ηE slip factor µ reduced mass ν photon frequency ρ density of states ρd density of states in the daughter cluster ρelectron electron density of states ρp density of states in the parent cluster ρproduct density of states after electron emission ρreactant density of states before electron emission ρrot rotational density of states ρtrans density of translational states σ electron-attachment cross section σP h photon-absorption cross section vi 1 Introduction Already early mankind recognized the potential application of thermal radiation. A simple example is warming in the sun or at a fire. But even more complex applica- tions such as monitoring the annealing temperature of a metal workpiece for forging are common since thousands of years. With the formulation of the Maxwell's equa- tions [MAX65] and the related understanding of light being an electro-magnetic wave, increasing interest in the question about the nature of thermal radiation arose. Slightly earlier it was discovered that the spectrum of an atom is composed of discrete lines [KIR60]. Along with the development of the atomic model these lines were later interpreted as the energy difference between electronic levels [BOH13]. Then it took more than 40 years finding a description for the thermal radiation of bulk material. At the beginning of the 20th century Max Planck published his law of thermal radiation [PLA00, PLA01]. Planck's famous formula solved the problem of the so called ultraviolet catastrophe, a result of the Rayleigh-Jeans law [RAY00, JEA05], which reproduces the radiation spectrum at low photon energies. His formula merged the Rayleigh-Jeans law with the Wien approximation [WIE97], the latter one describing the short-wavelength spectrum of thermal radiation. Planck's formula marks the beginning of quantum physics. Clusters bridge the gap between single atoms and bulk material. Mie theory [MIE08] describes successfully the thermal radiation of large cluster systems containing hun- dreds of atoms. For small clusters consisting of only a few atoms Mie theory fails and experimental data are sparsely available. Thermal radiation is one of the basic observables of clusters. Studying its depen- dence on the cluster size allows a deeper understanding of the general evolution from single atoms to the bulk material. The experimental environment is crucial for the reliability of the gathered information. Due to the high reactivity and the large sur- face to volume ratio, storage and accumulation of clusters on surfaces or embedded in matrices often lead to faulty results. Thus, measurements in the gas phase are desired. Nowadays ion storage on timescales comparable to thermalization processes of many species is possible. But low production yields and limits of storage capabilities often prohibit direct measurements of the thermal radiation. Hence, indirect measurements are required. Here electrostatic ion-storage devices are suitable setups. These devices offer the possibility to store ions over long times and have field-free regions in order to perform experiments without perturbation of electric or magnetic fields. Due to the purely electrostatic nature the absence of any mass limitation offers unique experimental conditions covering a wide range of different cluster sizes. In this thesis the cooling of small anionic cobalt and copper clusters is addressed. First the experimental setup, based on an electrostatic ion-beam trap, is presented. Here, studies on basic trap characteristics are discussed. These sections are followed by a discussion on the decay of anionic clusters. Measurements on anionic copper clusters consisting of four to seven atoms are discussed. Here, the decay of hot clusters is observed in order to draw conclusions on the internal temperature and the cooling process itself. In the last part of this thesis measurements on Co4− are discussed. A measurement scheme is presented enabling to monitor the internal energy distribution 1 1 Introduction of the clusters over storage time in a temperature-controlled environment. The cooling of initially hot clusters as well as the heating of initially cold clusters were observed. 2 2 Experimental Setup 2.1 The electrostatic ion-beam trap Almost 50 years ago electrostatic ion mirrors were introduced to the field of time- of-flight mass spectrometry [MAM73]. Ions with higher kinetic energy can penetrate deeper into the ion mirror and thus travel a longer path. The basic idea is tuning the ion mirror in such a way that the longer traveled path compensates for the higher velocity. Thus, the mass resolution can be improved compared to a simple, so called linear, time- of-flight mass spectrometer. To further increase the resolving power a more advanced mirror setup including several stacks of reflection plates has been developed. The refined ion potential creates a closed ion orbit between the mirrors and thus increases the flight time dramatically [WOL90].

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