Laser-Induced Alignment and Orientation of Quantum-State

173 Pages·2012·11.24 MB·English

by  Jens Hedegaard Nielsen

Laser-Induced Alignment and Orientation
of Quantum-State Selected Molecules and
Molecules in Liquid Helium Droplets
PhD Thesis
Jens Hedegaard Nielsen
Department of Physics and Astronomy
Aarhus University
February 2012

Contents
Contents i
List of publications iii
Acknowledgements v
Summary vii
Dansk Resume ix
1 Introduction 1
2 Theory 9
2.1 Alignment and orientation . . . . . . . . . . . . . . . . . . . . . . . 9
2.2 Droplets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
3 Experimental Setup 23
3.1 Deflector Machine . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
3.2 Droplet Machine . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
3.3 Laser System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
3.4 Electronic synchronization . . . . . . . . . . . . . . . . . . . . . . . 37
3.5 Data acquisition . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
3.6 Helium droplets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
4 Long Pulse Alignment and Orientation of OCS 43
4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43
4.2 Orientation as a Function of Angle and Intensity . . . . . . . . . . 46
4.3 Alignment and Orientation as a Function of Position . . . . . . . . 54
4.4 Effect of Parallel Plates Field on Orientation . . . . . . . . . . . . 57
5 Nonadiabatic alignment of OCS 59
5.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59
5.2 Results. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61
5.3 Alignment Dependent Ionization Yields . . . . . . . . . . . . . . . 73
5.4 Fourier Transform of the Alignment Trace . . . . . . . . . . . . . . 75
5.5 The Effect of Collisional Alignment . . . . . . . . . . . . . . . . . . 77
5.6 Outlook and Conclusion . . . . . . . . . . . . . . . . . . . . . . . . 79
i
ii Contents
6 Deflection, Alignment and Orientation of Asymmetric Tops 81
6.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81
6.2 Benzonitrile . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82
6.3 Iodobenzene . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85
6.4 Outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92
7 Conformer Separation 95
7.1 3-aminophenol . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96
7.2 1,2-diiodoethane . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99
7.3 Outlook and Conclusion . . . . . . . . . . . . . . . . . . . . . . . . 110
8 Alignment in Helium Droplets 113
8.1 Pulsed Helium Droplets . . . . . . . . . . . . . . . . . . . . . . . . 113
8.2 Continuous Droplet Source . . . . . . . . . . . . . . . . . . . . . . 125
8.3 Possible Interpretations of the Observed Dynamics . . . . . . . . . 132
8.4 Outlook and Conclusion . . . . . . . . . . . . . . . . . . . . . . . . 136
9 Conclusion and Outlook 139
Appendix A Analysis of Rotational state composition 143
Bibliography 147
List of publications
The work presented in the thesis has also led to the following
publications.
[A1] L. Holmegaard, J. H. Nielsen, I. Nevo, H. Stapelfeldt, F. Filsinger,
J. Ku¨pper, and G. Meijer. “Laser-Induced Alignment and Orientation of
Quantum-State-Selected Large Molecules.” Phys. Rev. Lett. 102, 023001
(2009).
[A2] F. Filsinger, J. Ku¨pper, G. Meijer, L. Holmegaard, J. H. Nielsen, I. Nevo,
J.L.Hansen,andH.Stapelfeldt. “Quantum-stateselection,alignment,and
orientationoflargemoleculesusingstaticelectricandlaserfields.” J.Chem.
Phys. 131, 064309 (2009).
[A3] F. Filsinger, J. Ku¨pper, G. Meijer, J. L. Hansen, J. Maurer, J. H. Nielsen,
L. Holmegaard, and H. Stapelfeldt. “Pure Samples of Individual Conform-
ers: The Separation of Stereoisomers of Complex Molecules Using Electric
Fields.” Angew. Chem. Int. Ed. 48, 6900 (2009).
[A4] J. H. Nielsen, P. Simesen, C. Z. Bisgaard, H. Stapelfeldt, F. Filsinger,
B. Friedrich, G. Meijer, and J. Ku¨pper. “Stark-selected beam of ground-
state OCS molecules characterized by revivals of impulsive alignment.”
Phys. Chem. Chem. Phys. 13, 18971 (2011).
[A5] J. H. Nielsen, H. Stapelfeldt, J. Ku¨pper, B. Friedrich, J. J. Omiste, and
R. Gonz´alez-F´erez. “Nonadiabatic effects in long-pulse mixed-field orienta-
tion of OCS molecules.” (2012). Submitted to Phys. Rev. Lett.
Other publications that I have contributed to.
[B1] I. Nevo, L. Holmegaard, J. H. Nielsen, J. L. Hansen, H. Stapelfeldt,
F. Filsinger, G. Meijer, and J. Ku¨pper. “Laser-induced 3D alignment and
orientationofquantumstate-selectedmolecules.” Phys.Chem.Chem.Phys.
11, 9912 (2009).
[B2] D. Dimitrovski, M. Abu-samha, L. B. Madsen, F. Filsinger, G. Meijer,
J. Ku¨pper, L. Holmegaard, L. Kalhøj, J. H. Nielsen, and H. Stapelfeldt.
“Ionization of oriented carbonyl sulfide molecules by intense circularly po-
larized laser pulses.” Phys. Rev. A 83, 023405 (2011).
iii
iv List of publications
[B3] J. L. Hansen, L. Holmegaard, J. H. Nielsen, H. Stapelfeldt, D. Dimitrovski,
and L. B. Madsen. “Orientation-dependent ionization yields from strong-
field ionization of fixed-in-space linear and asymmetric top molecules.” J.
Phys. B: At. Mol. Opt. Phys. 45, 015101 (2011).
[B4] J. J. Omiste, M. G¨arttner, P. Schmelcher, R. Gonza´lez-F´erez, L. Holme-
gaard, J. H. Nielsen, H. Stapelfeldt, and J. Ku¨pper. “Theoretical descrip-
tion of adiabatic laser alignment and mixed-field orientation: the need for a
non-adiabatic model.” Phys. Chem. Chem. Phys. 13, 18815 (2011).
[B5] J. L. Hansen, J. H. Nielsen, H. Stapelfeldt, C. B. Madsen, L. B. Madsen,
M. P. Johansson, A. T. Lindhardt, and T. Skrydstrup. “Laser controlled
torsion of molecules.” (2012). In preparation.
Acknowledgements
First of all I am grateful to my supervisor Henrik Stapelfeldt. From the moment
when he first suggested a project based of the alignment of molecules in helium
droplets this has been an interesting journey. The droplet experiments have
suffered a number of setbacks and the content of this thesis is quite different
from what I originally anticipated. Henrik has always worked hard on making it
possible for us to continue the experiments and ultimately succeed.
Early in my phd studies I had the pleasure of spending a month in Regens-
burg in the group of Alkwin Slenczka taking part in a number helium droplet
spectroscopyexperimentsworkingwithDominikPentlehnerwholaterjoinedour
groupandhasplayedakeyroleinthelongprocessofachievinganddemonstrat-
ing alignment in helium droplets. I am grateful to Alkwin Slenczka for allowing
me to join the group and to the rest of the ground welcoming me there.
The people that I worked with on various experiments in “femtolab” are
also greatly acknowledged. In approximate order of appearance they are Lotte
Holmegaard, Iftach Nevo, Jonas L. Hansen, Line Kalhøj, Jochen Maurer, Sofie
L. Kragh, Paw Simesen, Lauge Christensen, Dominik Pentlehner and Sankar De.
They all played a major role in the building, planning and running of the lab
andaregreatlyacknowledged. Inadditionmanyofthemhasplayedakeyrolein
the long days and nights used to acquire the data that I present in this thesis. I
would also like to thank two former members of the group that have contributed
to the continued development of the lab and our experiments beyond their own
projects and time in the group. Simon Viftrup is greatly acknowledged, not only
for writing the original femtolab data acquisition software, but also extending
this to an advanced multithreaded program after having left the group, which I
hadthepleasureofimplementing. ChristerZ.Bisgaardisacknowledgedforboth
performing numerical calculations on our nonadiabatic OCS experiment and for
always being keen to help with my new ides for additional simulations and also
for allowing us to use his simulation program in upcoming numerical simulations
on nonadiabatic experiments in droplets.
The collaboration with Frank Filsinger, Jochen Ku¨pper and Gerard Meijer
of the Fritz-Haber-Institut der Max-Planck-Gesellschaft has played a major role
in many of the experiments. The idea and implementation of the electrostatic
deflectionbasedontheirexpertisehasmovedthealignmentexperimentsforwards
by enormous steps and in addition the good colaboration with Jochen Ku¨pper
and Frank Filsinger has been very significant for the execution of the deflection
experiments. InadditionIwouldliketothankJochenKu¨pperforhostingmeone
week in Berlin at the Fritz-Haber-Institut to allow us to work on the theoretical
v
vi Acknowledgements
simulations of mixed field orientation while these unfortunately only led to the
conclusionthatabettermodelwasneeded. InadditionIamalsogratefulforthe
insightful theoretical discussions with Bretislav Friedrich and Marko H¨artelt of
the Fritz-Haber-Institut concerning mixed field orientation.
When the chances of getting experimental and theoretical agreement for the
long pulse orientation of OCS using an adiabatic model were exhausted Juan
J. Omiste and Rosario Gonz´alez-Ferez took up the challenge of modeling the
problem with a time dependent model, which ultimately brought much more
insight into the problem which I am very grateful for.
I would like to thank Jan Thøgersen for his excellent work on keeping our
laser systems in operation under prime conditions and especially for fighting the
almostconstantproblemswithour“seeded”YAGlaser. Iwouldalsoliketothank
all the people at the mechanics, vacuum and electronics-workshop at physics as
well as the mechanics workshop at chemistry for assistance with big and small
tasks related to our experiments.
In addition I would like to thank all of the people in “Femtolab” for making
these years enjoyable through lunch and coffee breaks, sports activities including
canoetripsandfootballtournamentsaswellasothersocialarrangements. Finally
I would like to thank all friends from my eight years at the university including
my former team mates and friends from FFB for making this an enjoyable time.
Finally I would like to thank my family for always supporting me in my studies
and research even though they claim to understand very little of it.
Summary
The main subjects of the thesis is split are two. The central theme of both
experiments is the manipulation of molecules with external fields. The first half
is molecular beam experiments in a supersonic jet while the second half deals
with experiments conducted in liquid helium droplets.
By the non resonant interaction of a laser pulse with a molecule we may
alignthemostpolarizableaxisofthemoleculeaccordingtothelaserelectricfield
polarization. Two distinct regimes exist. For laser pulses much longer than the
rotationalperiodofthemolecule,themoleculeisadiabaticallytransferredfroma
rotationalstatetoanalignedpendularstate; inthefollowingwewillrefertothis
as adiabatic alignment. On the other hand if the pulse length is much shorter
than the rotational period of the molecule, the interaction creates a rotational
wave-packet by a series of Raman transitions. Shortly after the interaction with
thelaserpulsetherotationalwave-packetcomesintophaseandstrongalignment
iscreated. Astherotationalwave-packetevolvesitdephasesandthealignmentis
lost,however,atwelldefinedtimesthewave-packetcomesintophaseagainanda
revival of the alignment is observed. By the combination of adiabatic alignment
and the interaction with a weak static field molecular orientation, where the
inversion symmetry of alignment is broken, is created.
In chapter 4 we use electrostatic deflection to select the linear molecule Car-
bonyl Sulfide (OCS) in a near rotational ground state. The OCS molecules are
the adiabatically aligned, and we investigate the effect of the rotational selection
on alignment. Furthermore we investigate the mixed field orientation. By di-
rect comparison of experiments with both time dependent and time independent
simulation we demonstrate that the usual adiabatic criterion with the rotational
period is no longer valid for mixed field orientation. In chapter 5 we continue
the experiments on OCS and investigate the nonadiabatic dynamics in the ro-
tationally state selected beam as well as the direct beam. We demonstrate how
the deflection facilitates the selection of a nearly pure rotational state with a
well defined parity. In the parity selected beam the quarter revivals, normally
lost by the averaging over rotational states of different parity, are visible. By
the comparison with simulations we find that 92+2% of the molecules are in the
−5
rotational ground state. In addition we demonstrate that the parity selection
also enhances the visibility of higher order wave-packet dynamics compared to
the direct beam. In chapter 6 we extend the class of molecules that benefit from
rotational state selection by demonstrating significantly enhanced alignment and
orientationoftheasymmetrictopmoleculeiodobenzene(IB)followingrotational
state selection. While the size of this molecule does not allow the same isolation
vii
viii Summary
of a single rotational state, the benefit in terms of alignment and orientation
enhancement is very significant.
Following the demonstration of electrostatic deflection as a tool for the se-
lection of rotational states we extend the use to molecules with multiple stable
conformers in chapter 7. We show how the electrostatic selection can separate
the two conformers of both 3-amino-phenol and 1,2-diiodoethane.
In chapter 8 we investigate the alignment of molecules in helium droplets.
Following a series of failed attempts to probe alignment of molecules in helium
droplets produced in a pulsed droplet valve, we demonstrate alignment of IB in
droplets produced from a continuous nozzle. Both adiabatic and non adiabatic
alignment has been demonstrated. The nonadiabatic alignment shows interest-
ing dynamics very different from the corresponding gas phase dynamics and is
promising for both applications in investigating the droplet molecule interaction
as well as applications in field free alignment due to the long persistence of the
alignment observed in droplets.