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.
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.
The main contribution of this project to the field of cold and ultracold molecules is we firstly demonstrated successful manipulation of the motion of polyatomic molecular beam in the rotational ground state, which has the lowest temperature of all possible states. Chapter 1 gives a summary of this field, including the application of cold and ultracold molecules, the methods to obtain them, and each method’s advantages and disadvantages. Once we decelerate molecular ensembles, we would like to trap the cold and ultracold molecules in electric trap, megnetic trap and megneto-optical trap. Chapter 2 starts with an introduction of the concept of supersonic beam and the basic knowledge of it, such as the specific features. The experimental setup will also be presented and explained in this chapter. The main highlight of this project is that we are manipulating molecules in the real ground state. Our molecular source is a Counter Rotating Nozzle(CRN), which can precool (or slow in alternate terminology) molecules in all states including the rotational ground state. The principle and performance of CRN will be presented and explained in Chapter 3. After obtaining a well precooled molecular beam, the microwave lens effect on various species is presented in Chapter 4. Meanwhile, the principle of AC Stark shift, AC dipole force and the microwave standing wave modes, such as TE modes and TM modes, will be explained in this chapter as well. Finally, I’ll summarize, draw conclusion of this work and describe the future expectation of this project in Chapter 5.
Focusing of a molecular beam in a high-field seeking state by an intense microwave standing wave is demonstrated. An 18 GHz microwave field that is near resonant to a rotational transition deflects a molecular beam due to the first-order ac Stark shift. The intense standing wave is obtained with a 47 cm long superconducting microwave resonator with a high unloaded quality factor of about 10 6 . A cold molecular beam of PbO in the rotational ground state with a center velocity of 145 ms −1 is efficiently focused to have about a factor 10 enhancement of the flux density by the resonator with a microwave input power of about 1 W, despite the heaviness of the PbO molecule.
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 polar molecules provide unique opportunities to test fundamental physics and chemistry. Their permanent electric dipole moments and rich internal structure arising from their vibrational and rotational motion, makes them sensitive probes for new physics. These features also make them ideal for studying ultracold chemistry, for simulating the behaviour of strongly interacting many-body quantum systems, and for quantum information science. This thesis describes a number of advances in cold molecule physics. The optimum method for producing an intense, pulsed, supersonic beam of cold CH molecules is investigated, resulting in a beam with 3.5 × 109 ground state CH molecules per steradian per shot. The beam has a translational temperature of 400 mK and a velocity that is tuneable between 400 and 1800 m s−1 . The lowest-lying Λ-doublet transitions of ground state CH, at 3.3 GHz and 0.7 GHz, are exceptionally sensitive to variations in the fine-structure constant α and the electron-to-proton mass ratio µ. Many modern theories predict that these constants may depend on time, position, or local matter density. Using a novel spectroscopic method, the frequencies of these microwave transitions are measured with accuracy down to 3 Hz. By comparing to radio astronomical observations, the hypothesis that fundamental constants may differ between the high and low-density environments of the Earth and the interstellar medium of the Milky Way is tested. These measurements find no variation and set upper limits of |∆α/α| < 2.1 × 10−7 and |∆µ/µ| < 4.3 × 10−7 . The frequency of the lowest millimetre-wave transition of CH, near 533 GHz, is also measured with an accuracy of 0.6 kHz. The development of a novel type of trap for ground-state polar molecules is presented. Trapping polar molecules is a necessary condition to cool them to ultracold temperatures of 1 µK and below. The trap uses a high intensity microwave field in a Fabry-P´erot resonator. Experimental and theoretical investigations are presented that explore the modes of the cavity, how to obtain the highest possible quality factor, and how to optimally couple the microwave power into the cavity. Finally, a cold supersonic beam of BH molecules is developed. This molecule appears to be particularly well-suited to direct laser cooling due to its favourable rotational structure and Franck-Condon factors. The laser cooling concept is described, and a first spectroscopic investigation of the relevant molecular structure is presented.
In this experimental study, the trapping of the translational motion of a supersonic beam of polar ammonia (NH₃) molecules in the rotational ground state (J = 0) of the para species using a microwave field is demonstrated. A cold beam of ammonia molecules seeded in argon gas passes through a superconducting Fabry–Perot resonator, where a standing-wave microwave field in the TEM₀₂ transverse mode is generated near the molecular transition frequency of 23.7 GHz. The mixed gas trapped at the center of the vacuum chamber is cooled to a temperature of 2.3 mK and ionized using the Resonance-Enhanced Multiphoton Ionization (REMPI) technique with the 16101 cm⁻¹ line of a Rhodamine dye laser. The emission spectrum is then recorded with the microwave field switched on and off. The change in the angular momentum of the polar ammonia molecule without the required excitation energy being supplied indicates a quantum tunneling effect induced by the Stark shift. This article presents the first report of the experimental results of this research conducted at the University of British Columbia.
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.