Dissertation submitted to the Combined Faculties for the Natural Sciences and for Mathematics of the Ruperto-Carola University of Heidelberg, Germany for the degree of Doctor of Natural Sciences presented by Dipl.-Phys. Jochen Steinmann born in Stuttgart Oral examination: 18.07.2007 Multiphoton Ionization of Laser Cooled Lithium Gutachter: Priv.-Doz. Dr. Alexander Dorn Junior-Prof. Dr. Selim Jochim Zusammenfassung: Reaktionsmikroskope erm¨oglichen die kinematisch vollst¨andige Erfassung von atomaren und molekularen Fragmentationsprozessen. Wurden bisher uberwiegend¨ Uberschall-Gas-¨ strahlen zur Erzeugung ultrakalter Target-Atome verwendet, kombiniert die im Rahmen dieser Arbeit entwickelte Apparatur erstmalig das Prinzip des Reaktionsmikroskops mit einer magneto-optischen Falle. Diese erm¨oglicht die Pr¨aparation von Lithium-Atomen mit Temperaturen im sub-mK-Bereich. Lithium ist auf Grund seiner einfachen atomaren Struk- tur mit nur drei Elektronen als Modellsystem fur¨ verschiedenste Arten von Ionisations- prozessen von besonderem Interesse. Um die Impulsbestimmung der geladenen Fragmente nicht durch das magnetische Feld der Atomfalle zu beeintr¨achtigen, wird die Falle selbst in einem gepulsten Modus bei einer Schaltrate bis zu 300 Hz betrieben. Hieraus resultieren feldfreie Bedingungen w¨ahrend der Detektionsphasen, gleichzeitig gew¨ahrleistet dies eine effiziente Nutzung der gespeicherten Atome. Mit der neuartigen Apparatur steht nun ein universelles Target zur Untersuchung der Ionisation von Lithium durch Elektronen- und Ionenstoss sowie der Photoionisation zur Verfugung.¨ Erstmalig wurde die Multiphotonioni- sation von Lithium in intensiven Laserfeldern mit Pulsdauern von 25 fs und Spitzeninten- sit¨aten zwischen 1011 W/cm2 und 1016 W/cm2 impulsaufgel¨ost vermessen. Dabei zeigten sich unerwartete Strukturen in den Photoelektronenspektren, wie z.B. eine bevorzugte Emission senkrecht zur Polarisationsachse des Lichtfeldes, welche derzeit noch nicht voll- st¨andig verstanden sind. Entsprechende Rechnungen sind bei mehreren Theoriegruppen in Arbeit. Abstract: Reaction microscopes enable kinematically complete measurements of atomic and molecu- lar fragmentation. An ultracold atomic target is usually provided by a supersonic gas jet. The apparatus developed in the course of this thesis for the first time combines the princi- ple of the reaction microscope with a magneto-optical trap. This allows for the preparation of lithium atoms in the sub-mK range. Being a three-electron system, its simple atomic structure makes lithium a model system of great topical interest for all kinds of ionization reactions. In order not to deteriorate the determination of the momenta of the charged fragments by the magnetic field of the trap, a pulsed mode of operation is adopted, cre- ating field-free conditions during data acquisition and making efficient use of the stored target. The novel apparatus provides a versatile target for investigations on fragmentation of lithium by electron, ion and photon impact. For the first time, momentum-resolved mea- surements on multiphoton ionization in intense laser fields with pulse durations of 25 fs and peak intensities in the range between 1011 W/cm2 and 1016 W/cm2 were performed. The acquired photoelectron spectra exhibit unexpected structures such as a preferred emission in the direction perpendicular to the laser polarization axis, which are not fully understood yet. Presently, corresponding calculations are being done in several theory groups. Contents Introduction and Motivation 1 1 Basics of Multiphoton Ionization 7 1.1 Mechanisms of Multiphoton Ionization . ...... 7 1.1.1 Free Electrons in a Laser Field . 9 1.1.2 Multiphoton Ionization . 11 1.1.3 Above-Threshold Ionization . 13 1.1.4 Ponderomotive Shift and Dynamical Resonances . ..... 14 1.1.5 TunnelingIonization . 18 1.1.6 Over-the-Barrier Ionization . 21 1.1.7 MultipleIonization. 22 2 Laser Cooling and Trapping of Lithium 27 2.1 Principles of Laser Cooling and Trapping . ...... 27 2.1.1 TheSpontaneousForce . 27 2.1.2 DopplerCooling ............................. 30 2.1.3 The Magneto-Optical Trap . 32 2.1.4 Dynamics of the Magneto-Optical Trap . 35 2.1.5 Temperature of the Magneto-Optical Trap . 38 2.1.6 Density of the Magneto-Optical Trap . 39 2.2 CoolingandTrappingofLithium . 40 2.2.1 General Properties of Lithium . 40 2.2.2 Spectroscopic Properties of Lithium . 41 2.2.3 Resonance Fluorescence and Atom Number . 44 3 Experimental Setup 47 3.1 ReactionMicroscopes ............................. 47 3.1.1 TargetPreparation. .. .. .. .. .. .. .. .. .. .. 49 3.1.2 Reconstruction of Momenta . 51 3.1.3 Particle Detection . 55 3.2 Combination of a Reaction Microscope with a MOT . ..... 56 3.2.1 ModeofOperation............................ 56 i Contents 3.2.2 OverviewoftheSetup .. .. .. .. .. .. .. .. .. 58 3.3 Experimental Chamber and Vacuum System . 60 3.4 Spectrometer and Detectors . 62 3.4.1 TimeFocusingSpectrometer . 62 3.4.2 Detector System and Data Acquisition . 64 3.4.3 Magnetic Electron Extraction Field . 67 3.5 MOT-Coils and Magnetic Field Switching . ..... 68 3.5.1 MOT-Coils ................................ 68 3.5.2 Magnetic Field Switching . 71 3.5.3 CompensationCoils . .. .. .. .. .. .. .. .. .. 75 3.6 LaserSystems................................... 79 3.6.1 BroadAreaDiodes............................ 80 3.6.2 Two-ModeDyeLaser .......................... 89 3.6.3 TaperedAmplifier ............................ 91 3.7 AtomicBeamSource............................... 93 3.7.1 ThePrincipleofZeemanSlowing . 93 3.7.2 ZeemanSlower ..............................101 3.7.3 LithiumOven...............................108 3.8 ExperimentalControl . .. .. .. .. .. .. .. .. .. .. 111 3.8.1 TheADwin-System . .. .. .. .. .. .. .. .. .. .112 3.8.2 UserInterface...............................114 4 Characterization of the Lithium Target 117 4.1 DiagnosticMethods ............................... 117 4.1.1 Photodiode ................................118 4.1.2 FluorescenceImaging . .119 4.2 MOTCharacteristics.............................. 121 4.2.1 Loading Rate and Atom Number . 121 4.2.2 Temperature ...............................124 4.2.3 TargetDensity ..............................126 4.3 RecaptureEfficiency ............................... 127 4.3.1 ReleaseTime ...............................128 4.3.2 OtherParameters ............................130 5 First results 135 5.1 ExperimentalProcedureandConditions . 135 5.1.1 Thefs-LaserSystem . .. .. .. .. .. .. .. .. .. .136 5.1.2 DataAcquisition .............................137 5.1.3 IntensityCalibration . 138 5.1.4 ExperimentalEffects. .140 5.2 LithiuminIntenseLaserFields . 143 5.3 ResultsandDiscussion. 148 ii Contents 5.3.1 Ionization from the Ground State . 148 5.3.2 Ionization from Ground- and Excited State . 150 5.3.3 Two-DimensionalSpectra . 150 5.3.4 PossibleMechanisms. .154 Conclusion and Outlook 159 Appendix A 163 Appendix B 164 Bibliography 167 iii Contents iv Introduction and motivation Atoms, molecules and ions are the basic building blocks of all complex structures in the universe and the understanding of their structure and the dynamics of their interaction is of fundamental relevance not only in physics, but also in chemistry, biology, astronomy, medicine and material science and a scientific treatment of many systems on a quantum physical level is becoming of increasing importance. There has been a tremendous progress in both the theoretical description and experimental determination and verification of the properties of stationary quantum systems, i.e. atoms, ions and molecules in terms of their energy levels and wave functions. On the experimental side, there is a continued devel- opment in spectroscopic tools and techniques. This concerns radiation sources, as well as methods of preparation of atoms, ions and molecules for precision spectroscopy and means of detection. State-of-the-art atomic structure calculations, incorporating quantum-electrodynamic ef- fects, can in many cases match the degree of precision of the most refined measurements, as impressively demonstrated by experimental determination [Fis04] and theoretical pre- diction of the 1s–2s transition energy of the hydrogen atom [Jen05]. For the dynamics of interacting quantum systems at the most basic level, the picture looks quite different. As yet, there is no unified theoretical approach to describe real few-body time-dependent quantum systems, such as single or double ionization of a multi-electron atom by charged particle impact. Even today, in times of almost ubiquitous availability of computing power, a brute force approach to a numerical solution of the full multidimen- sional Schr¨odinger equation (for the simplest non-relativistic case) is still beyond present day’s computing capacities. Most of the calculational techniques rely on an approximative treatment of the system under consideration, making them applicable only under certain conditions, such as a particular range of impact energies or collision geometries. It was not until 1999, when it was claimed, that an exact description of the most fundamental three-body Coulomb problem, the ionization of a hydrogen atom by an electron ’has been reduced to practical computation’ [Res99]. It shall be noted here, that despite all the difficulties to find an adequate theoretical description within the framework of quantum mechanics, many of the processes and fragmentation channels can be cast into appealingly intuitive and simple mechanistic pictures, which greatly facilitate their identification and the