The subject of this thesis’s work is research of cold molecules on a microchip. On the one hand, cold and ultracold molecules might allow high- precision measurements of molecular properties and the determination of their behavior at the quantum level. On the other hand, one could try to study highly controlled chemical reactions or to build a quantum computer with their help. In the experiments presented here, electrically neutral, polar molecules in low-field-seeking states are trapped and decelerated on a microchip with the help of electric fields. This exploits the Stark effect: the energy level shift of atoms and molecules in an electric field. After manipulation on the chip the molecules can be detected quantum-state specifically via the REMPI method. At first, CO molecules were prepared in the upper Λ-doublet component of the a3Π1, v = 0, J = 1 state, trapped on the chip and get vibrationally excited by an infrared laser in the middle of the chip to the J = 1 or J = 2 rotational state of the vibrational state v = 1. This could be shown experimentally and was analyzed theoretically for molecules which were trapped at all times. Secondly, CO molecules were detected on the chip and the structure of previously trapped molecule clouds of several of the only 120 μm spaced traps was spatially resolved. For this purpose the chip was extended with an ion lens setup, which enabled the determination of the temperature of the clouds by following the free ballistic expansion of the molecules in time; either by comparing the data to trajectory simulations (T = 11 ± 1 mK) or by applying a ballistic expansion model (T = 13 ± 3.5 mK), comparable to the method used by the ultracold atom community. The spatial imaging enabled the observation of a phasespace manipulation scheme, which adiabatically expanded the molecules in the trap and significantly reduces their temperature to approximately one third of their initial value. The third goal was to trap ground state ammonia molecules, which should extend the trapping time and would show that also other molecules could be trapped on the chip. The ammonia isotopologues 14NH3, 14ND3, and 15ND3 in the |vInv = 1, J = 1,K = 1> state of the electronic ground state X were used. No signal could be measured from trapped molecules, which is attributed to large nonadiabatic transitions to untrappable quantum states, leading to severe trap loss. At last, a new chip- based Stark decelerator is presented, which is a miniaturized version of the so-called “traveling-wave” ring decelerator. This chip-based decelerator should be able to trap more molecules compared to the chip that was used so far, and due to improved electronics it should be able to decelerate molecules from molecular beam velocities of approximately 360 m/s down to a standstill, in contrast to large-scale traveling-wave decelerators so far. The chip is still in development to achieve operation with the full specification (voltage). Performance simulations show the usefulness of this design, which needs to be tested in experiments.
The subject of this thesis’s work is research of cold molecules on a microchip. On the one hand, cold and ultracold molecules might allow high- precision measurements of molecular properties and the determination of their behavior at the quantum level. On the other hand, one could try to study highly controlled chemical reactions or to build a quantum computer with their help. In the experiments presented here, electrically neutral, polar molecules in low-field-seeking states are trapped and decelerated on a microchip with the help of electric fields. This exploits the Stark effect: the energy level shift of atoms and molecules in an electric field. After manipulation on the chip the molecules can be detected quantum-state specifically via the REMPI method. At first, CO molecules were prepared in the upper Λ-doublet component of the a3Π1, v = 0, J = 1 state, trapped on the chip and get vibrationally excited by an infrared laser in the middle of the chip to the J = 1 or J = 2 rotational state of the vibrational state v = 1. This could be shown experimentally and was analyzed theoretically for molecules which were trapped at all times. Secondly, CO molecules were detected on the chip and the structure of previously trapped molecule clouds of several of the only 120 μm spaced traps was spatially resolved. For this purpose the chip was extended with an ion lens setup, which enabled the determination of the temperature of the clouds by following the free ballistic expansion of the molecules in time; either by comparing the data to trajectory simulations (T = 11 ± 1 mK) or by applying a ballistic expansion model (T = 13 ± 3.5 mK), comparable to the method used by the ultracold atom community. The spatial imaging enabled the observation of a phasespace manipulation scheme, which adiabatically expanded the molecules in the trap and significantly reduces their temperature to approximately one third of their initial value. The third goal was to trap ground state ammonia molecules, which should extend the trapping time and would show that also other molecules could be trapped on the chip. The ammonia isotopologues 14NH3, 14ND3, and 15ND3 in the |vInv = 1, J = 1,K = 1> state of the electronic ground state X were used. No signal could be measured from trapped molecules, which is attributed to large nonadiabatic transitions to untrappable quantum states, leading to severe trap loss. At last, a new chip- based Stark decelerator is presented, which is a miniaturized version of the so-called “traveling-wave” ring decelerator. This chip-based decelerator should be able to trap more molecules compared to the chip that was used so far, and due to improved electronics it should be able to decelerate molecules from molecular beam velocities of approximately 360 m/s down to a standstill, in contrast to large-scale traveling-wave decelerators so far. The chip is still in development to achieve operation with the full specification (voltage). Performance simulations show the usefulness of this design, which needs to be tested in experiments.
Cold molecules are potentially beneficial for several areas of research. They have applications in spectroscopy, collision studies and, for instance, the study of the effects of dipole-dipole interactions in BECs. One method to decelerate and thereby cool down neutral dipolar molecules is to use a Stark decelerator. In my thesis, I show the advantages of cold, Stark-decelerated molecules in a high-resolution spectroscopy experiment, using the pure inversion motion of ammonia molecules. The transition associated with this motion is measured both on a warm and on a cold beam, resulting in an increased resolution in the latter case. For many of the applications of cold molecules mentioned above, molecules in high-field seeking states are necessary. To induce molecules to form a BEC, ground state molecules, which are always high-field seeking, are necessary for the use of preliminary cooling methods like evaporative cooling, as trap loss due to inelastic collisions is absent in this state. Many of the more interesting spectroscopy experiments, like experiments with biomolecules or experiments involved with the testing of fundamental physics theories, need to be performed on heavy molecules. Most states in these molecules are high-field seeking as well. To be able to better study high-field seeking molecules, it is important that we are able to confine a cold sample of such molecules in a trap. For this, a so-called AC trap is needed. The first electrical AC trap for molecules is demonstrated in this thesis. It utilizes two fields that are shaped like a saddle point. One of the fields is rotated over 90 degrees with respect to the other. By switching between these two fields, a molecule will alternately experience a focusing (maximum in the electric field strength) and defocusing force (minimum in the electric field strength) in each direction. At the appropriate switching frequency, the overall force is focusing and the molecules remain trapped
Ultracold matter offers a unique capability to reach the best high-precision spectroscopy measurements and leads to exciting perspectives in many different areas of science and technology. Molecules, thanks to their complex spectra, couple with a broader range of photons compared to atoms, so they are extremely attractive for fundamental science or to design new quantum technologies. However, to date, only a few molecular species have been brought to temperatures of the order of the microkelvin. This was done by laser cooling, a process that has been demonstrated only for species featuring an unpaired electron that does not participate to the chemical bond. \n \nA different approach, which is indifferent to the molecule electronic structure and thus potentially universal, is sympathetic cooling, where neutral molecules are cooled in a bath of ultracold atoms. However, inelastic collisions between molecules and the coolant is a major obstacle that has hindered the advances of this method thus far. Trapping the molecules in their absolute ground state would circumvent this problem or at least greatly simplify it. In this thesis we simulate the feasibility of an experiment in which metastable CO molecules are first slowed down to a few m/s with a microstructured Stark decelerator, then they are stopped by a strong DC electrical barrier and finally they are transferred irreversibly to their absolute ground state and captured in an optical trap. \nUnfortunately, the results of the simulations indicate that the total number of molecules that can be accumulated in the optical trap is rather low, far behind the observation threshold. Therefore, we concluded that other approaches to produce ultracold molecules must to be searched.
In the last years, it was demonstrated that neutral molecules can be loaded on a microchip directly from a supersonic beam. The molecules are confined in microscopic traps that can be moved smoothly over the surface of the chip. Once the molecules are trapped, they can be decelerated to a standstill, for instance, or pumped into selected quantum states by laser light or microwaves. Molecules are detected on the chip by time-resolved spatial imaging, which allows for the study of the distribution in the phase space of the molecular ensemble.
A lithographically-etched chip containing 1254 microscopic electrodes at a\ncenter-to-center spacing of 40 micron is used to produce an array of electric\nfield strength minima 25 micron above the surface with a spacing between\nneighboring minima of 120 micron. These minima are produced by applying a\nseries of six potentials to the electrodes. For polar molecules in low field\nseeking states, the electric field minima are potential energy minima and thus\nact as traps. By varying the potentials in a controlled manner, the field\nminima can be translated with respect to the electrodes at a constant distance\nfrom the surface. Experiments were performed with CO in the metastable a\n3-Pi-1, v = 0, J = 1 state, which has a strong, linear Stark shift with low\nfield seeking components, a 6 eV internal energy that enables easy detection\nthrough an Auger process, and a radiative lifetime of 2.6 ms, which is long\nenough for the experiments here. CO molecules in a pulsed molecular beam are\ndirectly excited to the metastable state with a 5 ns laser pulse. The\nmetastable CO molecules pass through a skimmer and arrive to the chip\ntraveling parallel to the surface and perpendicular to the electrodes. While\nabove the chip, the molecules are manipulated by the electric fields and\nsubsequently ejected from the other side toward the detector. Metastable 12-CO\nmolecules have been guided over the chip at constant velocities and\ndecelerated. As the final velocity decreases (i.e. the deceleration\nincreases), the decelerated molecules arrive at the detector successively\nlater, and the number of decelerated molecules reaching the detector also\ndecreases. While this effect is expected, the fraction of molecules lost is\nmuch greater than trajectory simulations predict. Most of this excess loss at\nhigher decelerations results from non-adiabatic transitions to untrapped\nstates. At zero electric field, the low field seeking level of the a 3-Pi-1, v\n= 0, J = 1 state of 12-C-16-O is exactly degenerate with a level that is only\nweakly affected by the fields. As the molecules pass near the zero field\nregion at the center of a microtrap, they can transition to this level, after\nwhich they are lost from the trap. One way to avoid this loss channel is to\nuse 13-C-16-O instead of 12-C-16-O, which results in a breaking of this\ndegeneracy by 50 MHz due to hyperfine structure. Because of this splitting,\nensembles of metastable 13-C-16-O molecules can and have been decelerated to\nzero velocity above the chip and held for variable durations ranging from 0 ms\nto 2.5 ms. It is observed that fewer molecules are detected after longer\ntrapping times on the chip, but the rate of this loss is consistent with the\n2.6 ms radiative lifetime of the metastable CO molecules. Further experiments\ndemonstrate that the Zeeman splitting due to an external magnetic field\nperpendicular to the electric field is a valid and more general way to prevent\nnon-adiabatic losses in 12-CO.
Research in the field of cold polar molecules is progressing rapidly. An array of interesting\ntopics is being developed including precision measurement and fundamental tests, quantum phase transitions, and ultracold chemistry. In particular, dipolar molecules in well-defined quantum states enable exquisite control of their interactions via applied electric fields. The long-range, anisotropic interaction between dipolar molecules leads to new collision dynamics that could be used for novel collective effects, quantum state engineering, and information processing. The focus of this Thesis is the production of cold samples of neutral OH molecules via Stark deceleration and magnetic trapping for novel collision experiments. This novelty results from our combination of cold external beams and trapped target molecules which facilitates observation of dipolar effects as well as low temperature collision resonances. The large permanent electric dipole moment of OH allows us to precisely tune the lab-frame velocity of the molecular packets from ∼ 500 m/s to rest. We have magnetically trapped OH packets at the terminus of our Stark decelerator at a temperature of 70 mK and density of 106 cm−3. With improved understanding of Stark deceleration, we optimize the decelerator efficiency and its coupling to the magnetic trap. Our latest trap design uses permanent ring magnets to create a three-dimensional magnetic quadrupole field. Use of magnetically trapped OH molecules for collision experiments with external beams allow us the unique opportunity of observing both elastic and inelastic collisions. In addition, the trap confinement yields sensitivity to differential elastic cross sections at low collision energies. This open magnetic trap has allowed measurement of collision cross sections between trapped OH and external supersonic beams of He and D2, the latter of which is of astrophysical interest due to the role of H2-OH collisions in pumping interstellar OH megamasers. The combination of trapped OH molecules and temperature tuned beams of He and D2 has facilitated measurement of the lowest-energy D2-OH collision cross sections yet reported. More recently, we report the first observation of electric-field dependent cross sections between two different species of cold polar molecules - OH and ND3 - thereby demonstrating control over molecular scattering in the cold regime. By combining for the first time the production techniques of Stark deceleration and buffer gas cooling, we increase the molecular interaction time by∼ 105 over traditional crossed-beam experiments to gain enhanced sensitivity at the characteristic densities of cold molecule production.
Research of cold and ultracold molecules is currently a burgeoning field in experimental and theoretical physics. New experimental techniques, involving an increasingly large set of molecule types under high levels of control, are currently opening up new avenues of research with a vast array of potential applications. From understanding the role of quantum mechanics in molecular scattering and cold chemistry, to testing the fundamental symmetries of nature and realizing quantum computing with dipolar molecular qubits, experiments are accessing regimes not dreamed of even a few years ago. Theoretical interest and computing capabilities are also at an all time high, spurred on by the possibility of creating ultracold dipolar gases as tunable realizations of strongly interacting quantum Hamiltonians, creating exotic phases of matter, and the investigation of controlling molecular interactions with applied electromagnetic fields.\nLess than a decade ago, cold molecule experiments had seemingly reached a technological plateau, being capable of creating moderate densities of 106-107 molecules/cm3 at temperatures of 10-100 mK. With many applications requiring colder temperatures and higher densities, the field was ripe for new advances. Today, via a plethora of methods such as direct molecular laser cooling, electro-optical cooling, magneto- and photo-association, and new molecular beam deceleration techniques, the field is just beginning to have the tools capable of producing truly interesting systems for study.\nThis work will discuss a couple of major steps taken in the direction of achieving scientific goals using cold molecules. The first experimental advancement discussed will be the development of a co-trap environment for studying interactions and collisions between ultracold atoms and Stark decelerated cold polar molecules. In this experiment, rubidium atoms are trapped using magnetic fields, and ammonia atoms are decelerated and trapped using electric fields. The two traps are spatially overlapped in order to investigate inter-species interactions. The co-trap environment provides exceedingly long interaction times, many orders of magnitude longer than typical beam-based interaction studies. As a result, it provides extremely high sensitivity to weak interaction mechanisms. The second experimental advancement discussed will be the development and construction of a new style of Stark decelerator, capable of producing much larger densities of cold molecules. This apparatus has the potential to expand the realm of possible experiments with chemically interesting species, and provide an unprecedented amount of control over molecular beams and traps. The gains haven't come easily though, as a new class of custom high-voltage amplifiers have needed to be developed. This part of the experiment alone took approximately two years of consistent effort to bring to fruition. After many years of development, this experiment is poised to come online, finally fulfilling its potential.