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.