Laser-Induced Alignment and Structure Determination of Molecular Dimers Embedded in Helium Nanodroplets

Laser-induced Alignment and Structure Determination of Molecular Dimers embedded in Helium Nanodroplets PhD Thesis Constant Schouder Department of Physics and Astronomy Aarhus University September 2019 Contents Contents 1 Acknowledgments......................................... i Summary ............................................. iii Dansk Resumé .......................................... iv List of Publications........................................ v 1 Introduction 1 2 Helium droplets5 2.1 Superfluid helium...................................... 5 2.2 Helium droplets....................................... 8 3 Structure determination of molecular complexes 13 3.1 Non-convalent interactions................................. 13 3.2 Studying molecular complexes............................... 14 4 Ion imaging and covariance mapping 19 4.1 Ion imaging......................................... 19 4.2 Covariance ......................................... 23 5 Laser-induced alignment 29 5.1 Rotational Hamiltonian and quantum numbers ..................... 30 5.2 Interaction between a molecule and an electric field................... 32 5.3 Quantum mechanical dynamics.............................. 36 6 Experimental Setup 41 6.1 Vacuum chambers ..................................... 41 6.2 Detection system...................................... 45 6.3 Optical setup........................................ 47 7 Field-free alignment of molecules inside helium droplets 53 7.1 Background......................................... 53 7.2 Spectrally truncated chirped pulses............................ 56 7.3 Results............................................ 57 7.4 Conclusion ......................................... 67 8 Structure determination of the tetracene dimer inside helium droplets 69 8.1 Background......................................... 69 8.2 Experimental setup..................................... 71 8.3 Results............................................ 71 8.4 Simulations......................................... 82 1 CONTENTS 8.5 Conclusion ......................................... 86 9 Structure determination of I2-bromobenzene and bromobenzene dimers inside helium droplets 87 9.1 Background......................................... 88 9.2 Experimental setup..................................... 89 9.3 Results............................................ 93 9.4 Simulations.........................................106 9.5 Conclusion .........................................110 10 Wave function imaging 111 10.1 Background.........................................111 10.2 Argon ............................................112 10.3 Carbon disulfide ......................................119 10.4 Conclusion .........................................124 11 Conclusion and outlook 125 A Experimental islands from Timepix3 data and 3D aligned images 129 B Two-body filtering procedure 133 C Velocity map imaging calibration 137 Bibliography 139 2 CONTENTS Acknowledgments There are key times in a person’s life, that may require some time to realize what just happened or how impactful it had been on that person’s future. One of mine is for sure my first meeting with Henrik three years ago, which led me to discover the land of Denmark and share astonishing moments with the Femtolab team. Therefore, I would first like to thank my supervisor, Henrik Stapelfedt. His dedication, faultless commitment and tireless determination has been a profound inspiration to my own work. His door has always been opened for me, and I will always be grateful for his unconditional support. Proof reading some chapters of this manuscript is one example among many, and I would like to honor him for that. Finally, not only was he supportive of many ideas that emerged from the team, but he proved himself to be a tireless stream of new experiments and ideas that led me to discover a broad range of scientific subjects. My thoughts also go out to the Femtolab group, which has proved itself to be an extremely pleasant environment. I mainly worked with Adam C. Chatterley, who I would describe as a fountain of scientific knowledge. His expertise and laboratory skills were an incommensurable source of learning. Moreover, he gladly shared it, and has always been accessible to discuss and answer (most of) the (numerous) questions that emerged during my time alongside him. I would like to thank him a lot for both his help and the moment we shared together, inside and outside the laboratory. I am also grateful to him for proof reading parts of this manuscript. I would also like to thank Jan Thøgersen who was also a crucial help for many scientific questions. His broad knowledge in optics and lasers has been extremely useful, and my understanding of the laser system has been dramatically improved by watching him repair the laser the few times it failed us. I would like to honor him for the stability of the laser system I used during these past years, and for the necessary repairs of some of its components. During my time in Femtolab, I had the chance to meet many talented people. The office has always fostered an healthy emulation of ideas and debates, on scientific and non-scientific topics, that could start in the workplace and go on during the π-break. We shared memorable moments, as the canoe trip, the Christmas dinner, or the bowling night, that will stick into my memories. I would like to thank particularly Anders Vestergaard, Marlene Madsen, James Pickering, Jacqueline Arlt, Lars Christiansen, Mette Heidemann Rasmussen, Benjamin Shepperson, Bjarke Hübschmann, Mia Baatrup and Albert Vinas Munoz. I am also grateful to Albert for proof reading many chapters of this manuscript. Apart from the Femtolab, I would like to thank all the persons in the ASPIRE network. The project of each ESR was for me a path to broaden my knowledge on the challenges the scientific community will have to face and resolve for the upcoming years. I also had the chance to meet a lot of brilliant minds, professors and researchers, and to be introduced to a large number of technical and scientific subjects. I would like to thank all the persons from the CMI group of Hamburg, with whom I spent a month during my secondment. My thoughts particularly go to Melby Johny, the person I worked the most with. Her enthusiasm, sympathy and experimental knowledge has been extremely helpful and inspiring. I would also like to thank Sebastian Trippel, Jolijn Onvlee, Ahmed Al-Refaie for their sympathy and their efforts to make my trip especially pleasant and instructive. I would finally like to thank Jochen Küpper making this collaboration possible, and for all the discussions we had during my time there. I would like to thank Franck Jensen who has always been kind and helped me solve many questions that emerged from the quantum chemistry calculations done throughout this thesis. His expertise in this field is indisputable and was extremely fruitful for my work. Outside of my research field, I would also like to thank Jørgen Johansen Rørstad, Anders Vester- gaard and Ming-Tsung Chung for the many hours spent together playing board-games. The time spent together was extremely relaxing and pleasant to escape, for a moment, all the scientific dilem- i ACKNOWLEDGMENTS mas encountered during the day. I would also like to address a special thank to Katia Verteletska for the wonderful nights we spent together saving the world from zombies and for the fantastic cheese and wine trays that came together. Finally, I would like to thank my family who has always been extremely supportive. The encouragement and the love were a real fortress during the difficult times I encountered. My thoughts go notably to my mother, my brother and my father who have always been there for me. This thesis is an achievement that would have not been possible without their investment and their support. ii CONTENTS Summary In the first part of my PhD project, I have developed a new experimental technique, based on the use of a strong laser pulse, to align large molecules sharply and keep the molecules aligned for > 10 picoseconds after the laser pulses is turned off. This is sufficiently long that intramolecular and intermolecular reactions and processes can be followed throughout their duration. The method applies to both 1-dimensional and 3-dimensional alignment and is generally applicable to a wide class of molecules and molecular complexes – in fact, the larger the molecule the longer it can be field-free aligned. The key to the long-lasting, field-free period of sharply-aligned large molecules is to conduct the experiments on molecules embedded in helium nanodroplets. First, the laser-induced alignment becomes exceptionally strong thanks to the 0:4 K temperature of the droplets, shared with the molecules. Second, the helium environment impedes the rotational motion. The strong alignment, created by an adiabatically turned-on laser pulse, lingers when the alignment field is rapidly turned off at its peak. This creates a window of 10-20 ps where the degree of alignment is high and the residual field so weak that it does not perturb molecules in fragile states, which I show explicitly through the demonstration of survival of molecular parent cations. Third, the droplets can accom- modate large molecular species and molecular complexes, thereby extending the systems amenable to alignment much beyond what is possible in the gas phase. I illustrate this by studies on five different molecules as well as three different molecular dimers. In the second part of my PhD project, I have determined

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