Takamasa Momose

Collisions between cold polyatomic molecules and atoms are essential in various processes, including interstellar chemical reactions, quantum tunneling,and precision measurements.The methyl radical was the rst polyatomic radicaltrapped in a magnetic trap. This was achieved by our group in 2017, where the trapped radicals were trapped at a temperature of approximately 130 mK. The methyl radical has been proposed as a potential candidate for sympathetic cooling, which can achieve temperatures below 1 mK. In order to elucidate the collisional properties of the methyl radical, collisions between magnetically trapped, cold methyl radicals, and room temperature helium and argon atoms were investigated experimentally. This work presents the results of these collisional studies and their comparison with theoretically derived trap loss rates. Quantum diffractive collisions dominate the collisions between room temperature helium atoms and cold methyl radicals in a magnetic trap with a trap depth of about 134 mK. More than 10 % of trap loss is attributed to inelastic collisions, in which the methyl radicals are excited to a rotationally excited state. On the other hand, collisions between cold methyl radicals and room temperature argon at this trap depth is near the border between quantum diffractive collisions and classical collisions.

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It is well known that amino acids are the building blocks for proteins, and as such, likely played a critical role in the origin of biological life. When in the solid phase, or in solution, amino acids take on a zwitterionic form, enabling a plethora of stabilizing intermolecular interactions. However, prior to understanding amino acids in the bulk phase, one must first characterize them in their neutral form. Due to the conformational flexibly and thermal instability of amino acids, this is best accomplished through the use of high-resolution matrix isolation Fourier transform spectroscopy in conjunction with high level ab initio calculations. The vibrational spectrum of alpha-serine is reported for the first time within a solid parahydrogen matrix, in which ten conformers were identified, thereby supporting the claim that solid parahydrogen matrices are superior to other noble gas matrices for the analysis of highly flexible molecules. Assignment of the vibrational transitions was accomplished through the use of quantum chemical calculations at both the DFT and MP2 levels of theory. Additionally, preliminary TD-DFT calculations indicate that the first singlet excited state of alpha-alanine is a dissociative state, in which, upon excitation alpha-alanine forms the hydrocarboxyl (HOCO) and ethylamine radicals through a Norrish type I pathway. Interestingly, it is predicted that both the first and second singlet excited states are dissociative states for alpha-alanine conformers displaying strong hydrogen bonds between the carboxyl hydrogen and amine lone pair, indicating that noncovalent interactions play a substantial role in both the ground and excited state potential energy surfaces of amino acids.

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The existence of amino acids in interstellar space has been a hot topic among researchers in astronomical science because of the biological molecules relevance to the origin of life. One of the most popular tools employed for the study of amino acids in relation to interstellar chemistry is matrix-isolation spectroscopy, as the cold and isolated environment provided by the matrix crystal mimics various astrophysical media such as interstellar ice. In the presented work, we reported the conformational and UV photochemistry studies of β-alanine and α-alanine via matrix-isolation Fourier transform infrared (MI-FTIR) spectroscopy in solid parahydrogen. This is the first time β-alanine and α-alanine have been registered in parahydrogen matrices, and the crystal has proven to be a more beneficial host over argon matrices for conformational analysis and in-situ UV irradiation experiment. Our claim on the superiority of solid parahydrogen is supported by the detection of high energy amino acids conformers in solid parahydrogen, which are unobserved in noble gas matrices. These conformers are conformer III for β-alanine, and conformer VI and V for α-alanine. As for UV irradiation experiment in solid parahydrogen, we obtained predominantly conformational change for β-alanine. However, α-alanine underwent complete photodestruction to give CO₂ and other unknown photoproducts we are still attempting to identify. Finally, we observed the possibility of β-alanine zwitterion formation in parahydrogen matrices, and are currently in the process of performing high accuracy quantum calculations on several β-alanine zwitterion conformers to assist us in spectral assignment.

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The population dynamics of a Magneto-Optical Trap (MOT) make it a potential candidate for flux measurements of an atomic beam. This is achieved by determining the collisional cross section of the trapped atom and the beam particle which would result in ejection of a trapped atom. Due to the properties of a MOT it is possible to make spatial and time-of-flight profiles of the beam using this technique. The work of this thesis explores the collisional cross sections and flux profiles of several gaseous beams with a MOT of ⁸⁵Rb or ⁸⁷Rb. Each of the beams, generated through supersonic expansion, produced a loss cross section on the order of the combined van-der-Waals radii of the two particles. The flux and time-of-flight information of the beam was verified with a Residual Gas Analyser (RGA) and high beam rep rate pressure measurements. The MOT was characterized through a combination of fluorescence detection for population and a catalysis process for the trap's depth. A custom built translation mechanism for the MOT's optics and Helmholtz coils was constructed to perform the profiling measurements.

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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.

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The ultimate goal of this research project is to develop an experiment to probe for superfluityin large clusters of molecular hydrogen in ultra-cold helium-4 nanodroplets. Superfluidityhas now been observed in a wide variety of systems and hydrogen is a good candidate toexhibit this macroscopic quantum phenomenon in a molecular system. In this thesis, twomajor advances were made enroute to the eventual search for superfluidity in bulk clustersof molecular hydrogen.1. In the first advance, the fluidity of supercooled molecular H₂ was investigated inhelium-4 nanodroples (~ 10⁵ atoms) at 0.38 K. To clearly demonstrate that the H₂clusters are fluxional, or fluid-like, separate studies of argon and neon clusters werealso made for comparison. To probe the behavior of the clusters, a single tetraceneprobe molecule was also inserted into the droplet and the laser induced fluorescence(LIF) from the tetracene was studied as a function of the cluster size and the pickupmethod. In the prior pickup method, the cluster species is added to the ⁴He dropletbefore to the probe molecule and in the post pickup method, the tetracene is addedand then the cluster species is added. Due to the difference in the pickup order, theconfiguration of the probe molecule and the cluster species can differ. The observedspectral shift of tetracene LIF in the presence of the cluster species was studied forboth pickup methods. For Ar and Ne clusters, the spectral shifts from the prior andpost pickup methods show clear differences. This observation suggests that for priorpickup, the tetracene molecule attaches to the surface of the cluster and does notpenetrate into the centre of the cluster and we conclude that the Ar and Ne clustersare not fluid-like in the helium droplets. For para-hydrogen and normal-hydrogen theLIF spectra of tetracene are independent of pickup order and we conclude that thesupercooled H₂ clusters remain fluid-like at 0.38 K.2. The second advance made in this thesis was to configure the droplet apparatus tostudy the rotational states of probe molecules in ⁴He droplets doped with H₂ clusters.The rotational states are studied by a combination of infrared and mass spectroscopy.Methane is the probe molecule used and when introduced into the ⁴He droplet it issurrounded by the H₂ cluster. If the surrounding H₂ liquid is superfluid, the methanerotates freely with a low moment of inertia. Conversely, if the H₂ remains a normalfluid, the dopant molecule drags hydrogen molecules along as it rotates and has a muchlarger moment of inertia. Rotationally resolved infrared spectroscopy of the methanegives clear information about the state of the surrounding supercooled liquid H₂. Asa first step, the v3 vibrational mode of bare methane in ⁴He droplets was studied. TheR(0) transition of the v3 stretching mode of methane was partially observed and foundto be consistent with the R(0) peak for CH⁴-doped ⁴He droplet systems previouslymeasured by the Miller group [1].

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