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.
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.
Abstract A superconducting Fabry–Perot microwave cavity for a molecular trap was designed, constructed and characterized. The cavity was designed to create intense microwave fields that are sufficient to trap cold polar molecules by the AC Stark force. By coating the mirror surfaces with a superconducting material, an unloaded quality factor of up to 1.1 × 106 at 24.087 GHz was achieved at a temperature of 2.24 K. The field strength of 1.06 MV m−1 obtained for a TEM02 transverse mode with an input power of 10 W is sufficiently intense to trap ammonia molecules at a temperature of 85 mK, which is achievable by a conventional Stark molecular decelerator. We have also implemented an ion optics assembly for sensitive detection of molecules inside the microwave cavity by resonance enhanced multi-photon ionization.
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.
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.
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.