Microwave radiation coupled into a cylindrically symmetric resonator is used to focus, guide, decelerate, and accelerate ammonia molecules in high-field-seeking states. In a first series of measurements, we demonstrate that the focal length of a microwave lens is directly proportional to the molecules’ velocity and to the inverse of the fourth root of the microwave power. In a second series of measurements employing a modified set-up, we use a multi-stage decelerator exploiting the TE1,1,12 mode of a cylindrical resonator and various switching schemes. We can deliberately choose between spatial focusing and velocity focusing while guiding, and we demonstrate kinetic energy reduction by as much as 30% of a 12 m/s slow packet of molecules utilising the 12 deceleration stages.
Microwave radiation coupled into a cylindrically symmetric resonator is used to focus, guide, decelerate, and accelerate ammonia molecules in high-field-seeking states. In a first series of measurements, we demonstrate that the focal length of a microwave lens is directly proportional to the molecules’ velocity and to the inverse of the fourth root of the microwave power. In a second series of measurements employing a modified set-up, we use a multi-stage decelerator exploiting the TE1,1,12 mode of a cylindrical resonator and various switching schemes. We can deliberately choose between spatial focusing and velocity focusing while guiding, and we demonstrate kinetic energy reduction by as much as 30% of a 12 m/s slow packet of molecules utilising the 12 deceleration stages.
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.
We report on the experimental realization of a microwave decelerator for neutral polar molecules, suitable for decelerating and focusing molecules in high-field-seeking states. The multistage decelerator consists of a cylindrical microwave cavity oscillating in the TE11n mode, with n=12 electric field maxima along the symmetry axis. By switching the microwave field on and off at the appropriate times, a beam of state-selected ammonia molecules with an incident mean velocity of 25 m/s is guided while being spatially focused in the transverse direction and bunched in the forward direction. Deceleration from 20.0 to 16.9 m/s and acceleration from 20.0 to 22.7 m/s are demonstrated.
We here report on the implementation of a microwave lens for neutral polar molecules suitable to focus molecules in both low-field-seeking and high-field-seeking states. By using the TE{11m} modes of a 12 cm long cylindrically symmetric microwave resonator, Stark-decelerated ammonia molecules are transversally confined. We investigate the focusing properties of this microwave lens as a function of the molecules' velocity, the microwave power, and the frequency detuning Δ from the molecular resonance. Such a microwave lens can be seen as a first important step towards further microwave devices, such as decelerators and traps.
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.
Abstract A numerical approach that employs a multi‐level dressed state method to determine the AC‐Stark shifts of molecular rotational energy levels is described. This approach goes beyond the two‐level approximation often employed for simpler molecules, such as ammonia and acetonitrile, and is applicable to a variety of molecules. The calculations are used to develop experiments aimed at focusing, guiding, decelerating and trapping neutral, polyatomic, asymmetric‐top molecules by using microwave fields. Herein, numerical calculations are performed for acetonitrile and 4‐aminobenzonitrile. Based on these results, trajectory simulations are performed to predict the outcome of microwave focusing experiments in the TE1,1,p mode of a cylindrically symmetric microwave resonator. Simulations show that, for such an experimental setup, microwave focusing and guiding of 4‐aminobenzonitrile requires starting longitudinal velocities close to, or below, 100 m s−1, that is, much lower than values obtained with standard molecular beam techniques, such as supersonic expansion. Therefore, alternative beam‐generation techniques, for example, buffer‐gas‐cooled molecular beams, are required to extend microwave manipulation methods to larger and more complex molecules.
We demonstrate the manipulation of the translational motion of a polar molecule in its J = 0 rotational ground state by the microwave (MW) dipole force combined with a counter-rotating nozzle. A cold molecular beam of CH3CN seeded in Kr with a longitudinal velocity of about 100 m s−1 was created by a pulsed counter-rotating nozzle. The cold beam was then introduced into a cylindrical MW cavity, in which a standing wave, TM01p mode MW field, nearly resonant to the |J, K〉 = |1, 0〉 ← |0, 0〉 rotational transition of CH3CN was created. By choosing an appropriate MW frequency, we successfully observed focusing and deflection of the cold beam of CH3CN due to the lens effect of the MW standing wave. The present result indicates that the combination of a counter-rotating nozzle and an MW cavity will be a versatile method for making cold and ultracold ensembles of various polar molecules in their rotational ground state.