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
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
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.
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.
This thesis describes contributions to the development of a new method to control the translational motion of atoms and molecules in supersonic beams.This method, Rydberg-Stark deceleration, is in principle applicable to all atoms and molecules in the gas phase.It relies on the Stark effect exhibited by Rydberg states and enables the production of samples of translationally and internally cold atoms and molecules which have possible applications in high-resolution spectroscopic experiments, in the studies of collisions at low temperatures and in quantum information science.This method has been developed and tested in deceleration and trapping experiments on hydrogen and deuterium atoms, and hydrogen molecules initially moving in supersonic beams.Its power originates from the large dipole moments, easily exceeding 1000 Debye, induced in Rydberg states by electric fields.Large forces are exerted on Rydberg atoms and molecules
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.
Cold molecules are unique samples that can be exploited for a broad set of novel experiments, such as cold collision studies and precision spectroscopy. An important remaining challenge in this field is a widely applicable method to manipulate the motion of neutral polar molecules in high-field-seeking states, i.e., the molecular ground state and states of more complex medium-sized and large molecules. For molecules in these states, true three-dimensional field maxima in free space are required, which are an inherent property of electromagnetic waves. The experiments presented in this thesis exploit the interaction of polar molecules with near-resonant microwave fields in open Fabry–Pérot type resonators and in closed cylindrically symmetric resonators. Ammonia (14NH3) was chosen as a prototypical test molecule for these microwave manipulation experiments because well-controlled ammonia packets can be generated using Stark deceleration in single quantum states. These packets are ideal starting points for developing and characterizing novel motion manipulation techniques. In particular, the rovibronic ground state of para-ammonia has a strong interaction with microwave radiation close to 23.7 GHz corresponding to a wavelength of 12.7 mm. In a first set of experiments, transverse focusing of packets of ammonia molecules with mean forward velocities ranging from 20 m/s to 50 m/s was demonstrated in a closed cylindrically symmetric resonator. The resonator mode used for these experiments features an electric field maximum on the resonator axis that coincides with the molecular beam axis. In order to also manipulate the forward motion of the molecular packets, several consecutive electric field maxima on the molecular beam axis are required and the microwave field has to be rapidly switched at the appropriate times. In a modified closed cylindrically symmetric resonator with twelve consecutive electric field maxima on the axis, the first microwave guiding with full phase-space control was demonstrated for packets of ammonia molecules with a mean forward velocity ranging from 12 m/s to 25 m/s. With the same setup, also the first microwave acceleration and deceleration were accomplished with an extraction of as much as 30% of the initial kinetic energy and a gain by up to 60%. The main advantage of the microwave manipulation technique is its applicability to larger molecules that cannot be manipulated with conventional methods, such as Stark-deceleration. Successful deceleration is complicated for a number of reasons. For example, these larger molecules generally carry much higher kinetic energy. Furthermore, a precise theoretical calculation of the interaction with microwave fields becomes much more challenging. In the last part of this thesis, methods are proposed to increase the performance of the microwave decelerator, developed and characterized in this thesis, to eventually provide slow samples of medium-sized molecules, such as aminobenzonitrile, and trap ammonia.
Cold molecules are unique samples that can be exploited for a broad set of novel experiments, such as cold collision studies and precision spectroscopy. An important remaining challenge in this field is a widely applicable method to manipulate the motion of neutral polar molecules in high-field-seeking states, i.e., the molecular ground state and states of more complex medium- sized and large molecules. For molecules in these states, true three- dimensional field maxima in free space are required, which are an inherent property of electromagnetic waves. The experiments presented in this thesis exploit the interaction of polar molecules with near-resonant microwave fields in open Fabry–Pérot type resonators and in closed cylindrically symmetric resonators. Ammonia (14NH3) was chosen as a prototypical test molecule for these microwave manipulation experiments because well-controlled ammonia packets can be generated using Stark deceleration in single quantum states. These packets are ideal starting points for developing and characterizing novel motion manipulation techniques. In particular, the rovibronic ground state of para-ammonia has a strong interaction with microwave radiation close to 23.7 GHz corresponding to a wavelength of 12.7 mm. In a first set of experiments, transverse focusing of packets of ammonia molecules with mean forward velocities ranging from 20 m/s to 50 m/s was demonstrated in a closed cylindrically symmetric resonator. The resonator mode used for these experiments features an electric field maximum on the resonator axis that coincides with the molecular beam axis. In order to also manipulate the forward motion of the molecular packets, several consecutive electric field maxima on the molecular beam axis are required and the microwave field has to be rapidly switched at the appropriate times. In a modified closed cylindrically symmetric resonator with twelve consecutive electric field maxima on the axis, the first microwave guiding with full phase-space control was demonstrated for packets of ammonia molecules with a mean forward velocity ranging from 12 m/s to 25 m/s. With the same setup, also the first microwave acceleration and deceleration were accomplished with an extraction of as much as 30% of the initial kinetic energy and a gain by up to 60%. The main advantage of the microwave manipulation technique is its applicability to larger molecules that cannot be manipulated with conventional methods, such as Stark-deceleration. Successful deceleration is complicated for a number of reasons. For example, these larger molecules generally carry much higher kinetic energy. Furthermore, a precise theoretical calculation of the interaction with microwave fields becomes much more challenging. In the last part of this thesis, methods are proposed to increase the performance of the microwave decelerator, developed and characterized in this thesis, to eventually provide slow samples of medium-sized molecules, such as aminobenzonitrile, and trap ammonia.