Studies of trapped, cooled ion ensembles [Elektronische Ressource] / vorgelegt von David Offenberg

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• Physik

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• Publié par: heinrich-heine-universitat_dusseldorf
• Publié le: 01 janvier 2010
• Nombre de lectures: 20
• Langue: English
• Poids de l'ouvrage: 9 Mo

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Studies of trapped, cooled ion ensembles
Inaugural-Dissertation
zur Erlangung des Doktorgrades
der Mathematisch-Naturwissenschaftlichen Fakultät
der Heinrich-Heine-Universität Düsseldorf
vorgelegt von
David Offenberg
aus Leverkusen
Düsseldorf, Oktober 2009Aus dem Institut für Experimentalphysik
der Heinrich-Heine-Universität Düsseldorf
Gedruckt mit der Genehmigung
der Mathematisch-Naturwissenschaftlichen Fakultät
der Heinrich-Heine-Universität Düsseldorf
Referent: :::::: Prof. S. Schiller, Ph.D.
Koreferent::::: Prof. Dr. R. Weinkauf
Tag der mündlichen Prüfung::::::16.12.2009Abstract
This work presents various novel results in the field of experimental and theoretical trapped
138 +ion studies. All investigations involve laser-cooled Ba ions confined in a linear quadru-
pole ion trap, which serve as coolant for complex molecular ions, as target for collisions
with neutral atoms, or as model system in theoretical analyses.
The special feature of the apparatus used in this work is a molecular ion source based on
electrospray ionization that allows for the production of gas-phase molecular ions of almost
any species with a maximum mass-to-charge ratio of 2,000 Da. Trapped together with laser-
138 +cooled Ba ions, the molecular ions can be cooled down, kept cold and investigated for
many minutes, and in principle for hours, in a nearly collisionless environment. In this
work, the sympathetic cooling of complex molecular ions to translational temperatures
below 1 K is demonstrated for different types of molecules covering a mass range from 182
to 12,400 Da. For example, the protein cytochrome c, with a mass of about 12,400 Da,
is the most massive molecular species sympathetically cooled in an ion trap so far. The
methods for detecting successful trapping and cooling, and for the determination of the
temperature are presented in detail. In one case, multiply protonated molecules of the
protein cytochrome c have been cooled to less than 0.75 K.
Furthermore, two methods for measuring photodestruction rates of cold, trapped com-
plex molecular ions have been developed that can, for example, be applied in destructive
spectroscopy schemes. Both techniques are demonstrated using singly protonated gly-
+cyrrhetinic acid molecules (GAH ) dissociated by an ultraviolet (UV) laser. Measurements
of the photodestruction rate were performed at different intensities of the UV laser, and
-1rates as low as 0.05 s and less have been determined, which is only possible due to the
long ion storage times that can be achieved with this apparatus. For the UV laser wave-
−17 2 +length of 266 nm, a destruction cross section of (1.1± 0.1)· 10 cm for GAH has been
determined.
In this work, feasibility studies towards internal cooling of trapped complex molecular ions
have been performed. Such an additional internal cooling could be realized via collisions
with laser-cooled neutral atoms. To test their suitability, neutral atoms of Li, Na, K, Rb,
+Cs, and Yb have been collided with trapped laser-cooled and non-cooled Ba ions as well as
+with complex molecular ions. For all species charge exchange reactions with Ba ions have
been observed, which excludes these species for use in collisional cooling. While K, Rb,
+and Cs showed charge exchange reactions with ground state and laser-excited Ba ions,
+the charge exchange rates of Yb and Ba were found to be state dependent. Collisions
of neutral Yb atoms were found to be reactive and lead to the formation of adducts and
fragments, in contrast to the alkali metals for which no such reactions have been observed.
In addition, the motion of cooled, trapped ions has been studied theoretically via simula-
tions. When the translational energy of a trapped ion ensemble is sufficiently reduced, a
first-order phase transition from a liquid to a solid state occurs with the ion ensemble chang-
ing from a disordered ion cloud to an ordered ion crystal. As ions in a linear quadrupole
trap arrange in shell structures, a free motion in the radial direction is disabled, and ions
can only move to neighboring shells when their kinetic energy is high enough to overcome
a certain potential barrier. The simulations have shown that intershell diffusion rates in
the considered ion crystals increase exponentially when the ion temperatures exceed values
that well agree with the predictions of other established models. Ion ensembles of differ-
138 +ent ion numbers (300, 500, and 1000 Ba ions) and symmetries (prolate and spherical)
have been investigated showing a clear tendency for the phase transition temperatures: the
higher the ion number, the lower the transition temperature.AContents
1 Introduction 7
1.1 Motivation..................................... 7
1.2 Outline ...................................... 8
2 Trapping and cooling of ions 9
2.1 Linearquadrupoletrap ............................. 9
2.1.1 Radial confinement - The quadrupole mass filter . . . . . . . . . . . 10
2.1.2 Axialconfinement ............................ 12
2.1.3 Ionmotioninalinearquadrupoletrap................. 12
2.2 Coolingoftrappedions ............................. 14
2.2.1 Coolingtechniques............................ 15
2.2.2 Fundamentals of Doppler laser cooling . . . . . . . . . . . . . . . . . 16
138 +2.2.3 Doppler laser cooling of Ba ions.................. 19
2.3 Coulombioncrystals............................... 21
2.3.1 Coulomb crystallization . . . . . . . . . . . . . . . . . . . . . . . . . 21
2.3.2 Ioncrystalstructures........................... 23
3 Experimental setup 27
3.1 Vacuumsystem.................................. 27
3.2 Molecularionsource............................... 29
3.3 Trapsetup..................................... 31
3.4 Lasersystem................................... 34
4 Analysis of trapped ion ensembles 37
4.1 Secularexcitation................................. 37
4.2 Ionextraction................................... 40
4.3 Moleculardynamicssimulations......................... 42
5 Sympathetic cooling of proteins 47
5.1 Preparationofcoldcomplexmolecularions.................. 47
5.1.1 Loadingofmolecularions........................ 47
5.1.2 Preparationprocedure.......................... 50
5.1.3 Cooledmolecularspecies......................... 51
5.2 Coolingofcytochromec............................. 53
5.2.1 Multipleprotonation........................... 53
5.2.2 Detectionoftrappingandcooling.................... 5
5.2.3 Temperaturedetermination....................... 56
6 Measurement of photodestruction rates 63
6.1 Testmolecule:Glycyrhetinicacid....................... 63
6.2 Secularexcitationmethod............................ 6
6.3 Ionextractionmethod.............................. 68
6.4 Results....................................... 71
6.5 Comparison.................................... 73
6.6 Outlook...................................... 74
5Contents
7 Collisions of neutral atoms and trapped ions 77
7.1 Neutralatomsources............................... 77
7.2 Chargeexchangereactions............................ 79
+7.2.1 Charge exchange of K and Rb with Ba ................ 80
+7.2.2 Charge exc of Cs with Ba .................... 82
+7.2.3 Charge exchange of Li and Na with Ba ................ 83
+7.2.4 Charge exc of Yb with Ba .................... 86
7.3 Collisions of neutral atoms and trapped complex molecular ions . . . . . . . 89
8 Theoretical studies on Coulomb crystallization 95
8.1 Simulatedionensembles............................. 95
8.2 Analysisofionmotion.............................. 96
8.3 Intershelionmotion............................... 9
8.3.1 Intershelldiffusionrates......................... 99
8.3.2 Potentialbarriers.............................104
8.4 Intrashelionmotion...............................108
8.5 Phasetransitiontemperatures..........................11
9 Summary and outlook 115
9.1 Summary.....................................15
9.2 Outlook......................................117
9.3 Zusammenfasung(SummaryinGerman)...................18
Bibliography 123
Publications 129
Acknowledgments 131
61 Introduction
1.1 Motivation
In recent years, enormous progress has been made in the production of cold neutral and
charged molecules. By common convention, molecules are designated as cold when ex-
hibiting translational temperatures between 1 and 1000 mK [1]. In this low-temperature
regime effects of light-molecule, atom-molecule, and molecule-molecule interactions become
accessible that do not occur or cannot be observed at higher temperatures.
One wide field of current cold molecule research is the investigation of cold collisions
[2, 3, 4]. Low collision energies allow one to study quantum mechanical details of collisional
processes, with the deBroglie wavelength of the involved particles becoming comparable
to the scale of their interaction. For example, elastic and inelastic collisions at very low
temperatures can be strongly dominated by resonances [5], and also chemical reactions are
expected to feature resonances in the cross section as a function of their collision energies
[6].
Another important field where cold molecules are finding application is in high-resolution
spectroscopy, where molecular transition frequencies can be measured with greater accu-