Production and sympathetic cooling of complex molecular ions [Elektronische Ressource] / vorgelegt von Chaobo Zhang

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• Publié par: heinrich-heine-universitat_dusseldorf
• Publié le: 01 janvier 2008
• Nombre de lectures: 17
• Langue: English
• Poids de l'ouvrage: 18 Mo

Extrait

Production and Sympathetic Cooling
of Complex Molecular Ions






Inaugural-Dissertation


zur
Erlangung des Doktorgrades der
Mathematisch-Naturwissenschaftlichen Fakultät
der Heinrich-Heine-Universität Düsseldorf


vorgelegt von
Chaobo Zhang
aus Zhejiang, China


Mai 2008
Aus 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. T. Heinzel
Tag der mündlichen Prüfung: 24. Juni 2008 Abstract
This thesis reports on experimental and theoretical studies of the sympathetic cooling of
complex molecular ions demonstrating that this general method for cooling atomic and
molecular ions is reliable and efficient.
For this purpose, complex molecular ions and barium ions have been confined si-
multaneously in a linear Paul trap. The complex molecular ions are generated in an
electrosprayionizationsystemandtransferredtothetrapviaa2mlongoctopoleion
guide. These molecular ions are pre-cooled by room temperature helium buffer gas so
that they can be captured by the trap. The atomic barium ions are loaded from a barium
evaporator oven and are laser-cooled by a 493 nm cooling laser and a 650 nm repumping
laser. Due to the mutual Coulomb interaction among these charged particles, the kinetic
energy of the complex molecular ions can be reduced significantly.
In our experiments we have demonstrated the sympathetic cooling of various molecules
(CO , Alexa Fluor 350, glycyrrhetinic acid, cytochrome c) covering a wide mass range
2
from a few tens to 13000 amu. In every case the molecular ions could be cooled down to
millikelvin temperatures.
138 + 2Photo-chemical reactions of the Ba ions in the ( P ) excited state with gases such
1/2
138 +as O ,CO,orNO, could be observed. If the initial Ba ion ensemble is cold, the
2 2 2
138 +produced BaO ions are cold as well, with a similar temperature as the laser-cooled
138 +barium ions (a few tens of millikelvin). The back-reaction of BaO ions with neutral
138 +CO to Ba is possible and was observed in our experiments as well.
A powerful molecular dynamics (MD) simulation program has been developed. With
this program dynamic properties of ion ensembles, such as sympathetic interactions or
heating effects, have been investigated and experimental results have been analyzed to
obtain, for example, ion numbers and temperatures. Additionally, the feasibility of
nondestructive spectroscopy via an optical dipole excitation and the lineshape of ro-
vibrational transitions of 1.4 and 5.1 μm have been studied.
iDedication
to my wife, Fanzhen Meng.
iiContents
Abstract i
1 Introduction 1
1.1 Motivation................................... 1
1.2 Outline..................................... 2
2 The linear radiofrequency (rf) trap 3
2.1 Traptypes 3
2.2 TraptheoryofthelinearPaultrap..................... 6
2.2.1 Quadrupolemasfilter........................ 6
2.2.2 Longitudinal confinement ...................... 7
2.2.3 Mathieuequations.......................... 8
2.2.3.1 SolutiontotheMathieuequations ............ 10
2.2.3.2 Stability diagrams 10
2.2.4 Ion trajectories ............................ 12
2.2.4.1 Secularmotion....................... 13
2.2.4.2 Micromotion 14
2.2.4.3 Rfheating 15
3 Cooling of trapped ions 17
3.1 Lasercooling ................................. 17
3.1.1 Dopplerlasercooling......................... 18
3.1.1.1 Notationfortheatom-laserinteraction.......... 19
3.1.1.2 BasictheoryofDopplerlasercooling........... 21
3.1.1.3 Dopplercoolinglimit.................... 2
138 +3.1.2 Doppler cooling of Ba ...................... 24
3.1.2.1 Bariumelement....................... 24
138 +3.1.2.2 Transitions of Ba 25
3.1.2.3 Lasercoolingofathre-levelsystem........... 26
3.1.2.4 Zeemansplitting 27
3.2 Buffergascooling............................... 29
3.3 Sympatheticcooling ............................. 30
3.3.1 Overviewofsympatheticcooling................... 30
3.3.2 Masdependencyofsympatheticcooling.............. 32
4 Experimental setup 39
4.1 Vacuumsystem................................ 39
4.1.1 Mainvacuumchamber........................ 39
4.1.2 Differentialvacuumsystem...................... 43
4.2 Iontrap.................................... 4
4.2.1 Mechanicaldesign........................... 4
iiiContents
4.2.2 Electricalpowersupplies....................... 44
4.2.2.1 Rf power supply ...................... 45
4.2.2.2 DC power 47
4.2.3 Magneticfield............................. 48
4.3 Ionsources................................... 48
4.3.1 Bariumionsource .......................... 48
4.3.2 Ionsfrombackgroundgas 49
4.3.3 Complexmolecularionsource-ESI................. 49
4.3.3.1 PrinciplesofESI...................... 50
4.3.3.2 AdvantagesofESI..................... 50
4.3.3.3 StructureofESI 51
4.3.3.4 Masspectra........................ 52
4.3.3.5 Ionflux........................... 52
4.4 Transportofcomplexmolecularions .................... 53
4.4.1 Theoreticalbackgroundofoctopoles................. 53
4.4.2 Structureoftheoctopole....................... 54
4.4.3 Octopoleparameters......................... 5
4.5 Iondetection ................................. 57
4.5.1 CEM.................................. 57
4.5.2 CCD 57
4.5.3 PMT 58
4.6 Lasersystem 59
4.6.1 The 650 nm laser - repumping laser ................. 60
4.6.2 The493nmlaser-coolinglaser................... 61
4.6.3 Laser stabilization .......................... 63
5 Preparation and properties of laser-cooled barium Coulomb crystals 65
5.1 PreparationofbariumCoulombcrystals.................. 65
5.2 Laserdetuning ................................ 67
5.3 Secularfrequency............................... 67
5.3.1 Radial secular excitation ....................... 68
5.3.2 Axial e........................ 69
5.4 Trapparameters 70
5.4.1 Ion confinement ............................ 71
5.4.2 Compensationvoltages........................ 72
5.5 Spatialdistribution.............................. 73
5.5.1 Axialspatialdistribution 73
5.5.2 Radialspatialdistribution...................... 74
5.6 Lifetimeofionensembles........................... 75
138 +6 Chemical reactions of Ba 77
6.1 Introduction.................................. 77
6.2 Investigation of chemical reactions with He, N ,andHgases....... 80
2 2
6.3 Chemical reaction with O .......................... 83
2
6.4 C r with CO ......................... 87
2
6.5 Chemical reaction with NO 91
2
6.6 Back-reactionwithCO............................ 91
ivContents
7 Sympathetic cooling of medium sized organic molecular ions 95
7.1 Loadingofmolecularions .......................... 95
7.2 PhotofragmentationofRhodamineions................... 9
7.3 SympatheticcoolingofAlexaFluor350ions................101
7.4 Sympathetic cooling of glycyrrhetinic acid ions...............108
8 Sympathetic cooling of proteins 115
8.1 Proteinmolecule-cytochromec.......................115
8.2 Loadingofcytcmolecularions17
8.3 Sympatheticcoolingofcyt17proteinions..................18
8.3.1 Simultaneoustrappingofcyt17andbariumions..........19
8.3.2 Evidencesofthesympatheticcoolingofcyt17ions.........19
8.3.2.1 Ion extraction........................121
8.3.2.2 Visibleevidence121
8.3.2.3 Secularexcitation......................12
8.4 TemperaturedeterminationviaMDsimulations..............12
8.5 Sympatheticcoolingofcyt12ions126
9 Molecular dynamics simulations 129
9.1 Introduction..................................129
9.2 Basicapproach................................130
9.2.1 Equationsofmotionandforces...................130
9.2.2 Algorithms ..............................131
9.2.3 Theleapfrogalgorithm........................131
9.2.4 ThevelocityVerletalgorithm....................132
9.3 Timescales13
9.4 Initialconditions...............................134
9.5 Simulationmodels136
9.5.1 Heatingeffects ............................137
9.5.2 Modelling of heating effects .....................140
9.5.3 Coolingmodel.............................145
9.6 SimulationofCCDimages..........................147
9.7 Ionnumberandtemperaturedetermination ................148
9.7.1 Determinationofionnumbers....................148
9.7.2 Sympatheticheatingandcooling ..................149
9.7.3 Temperaturedeterminationoflaser-cooledions ..........151
9.7.4 Temperaturedeterminationofsympatheticallycooledions....153
9.8 Advancedcoolingofcomplexmolecularions154
9.9 Micromotioneffect..............................156
9.10Iondiffusion..................................157
9.1Conclusion...................................161
10 Simulation of secular excitation spectra 163
10.1Introduction163
10.2 Model for the secular excitation .......................16
10.3Spacechargeeffects16
10.3.1Weakcoupling ............................168
10.3.2Strongcoupling168
vContents
10.4 Alternative model for the secul