Dissertations / Theses on the topic 'Velocity spectrometry'

Photofragment velocity-map imaging (VMI) has generally been employed to investigate the photodissociation dynamics of relatively small molecular systems (< 5 atoms). The work reported in this thesis focuses on the application of this technique for the investigation of the unimolecular photodissociation of larger chemical systems, which are of interest to a broad cross section of the chemical community. Typically, VMI studies involve state-selective detection of one particular fragmentation product, and so are often limited to the investigation of a single dissociation channel. By employing vacuum ultra-violet (VUV) photoionization, we are able to detect most, if not all of the fragments resulting from the dissociation of a neutral species, with ‘universal’ ionization being achieved in the ideal case when the fragment ionization energies are all lower than the VUV photon energy. This capability becomes particularly important when investigating larger systems, since these often display complex dynamics with multiple competing fragmentation pathways. Our approach allows us to investigate the different photofragmentation processes occurring for a particular system, to evaluate the relative importance of the active dissociation channels, and to gain insight into the energy partitioning amongst the fragments. A study of the UV photodissociation of two neutral alkyl iodide molecules demonstrates the first use in our laboratory of ‘universal’ ionization in combination with VMI. Studies into the photofragmentation processes resulting from 193 nm photoexcitation of neutral N,N-dimethylformamide, a small-molecule model for a peptide bond, and a number of neutral cyclic alkenes, which undergo the retro-Diels-Alder reaction, are also presented. The remaining studies presented in this thesis have investigated the photofragmentation processes of ionic species, generated by means of VUV photoionization. In the case of ion dissociation each fragmentation channel necessarily produces one charged species, which may be detected using the VMI technique. Therefore, such studies provide an insight into all of the active channels. An in-depth VMI study of the UV photodissociation of two ethyl halide cations is presented, which demonstrates the successful investigation of the multiple photofragmentation pathways of these ionic species. The remainder of the cation photodissociation studies are of relevance to a number of common processes known to occur in mass spectrometry, including the McLafferty rearrangement, the retro-Diels-Alder reaction, and ‘peptide’ bond fragmentation. By velocity-map imaging the products of these reactions, further information is obtained concerning these dissociation processes, which are no doubt of interest to the wider chemical community. This work forms part of the velocity-map imaging mass spectrometry (VMImMS) project. VMImMS involves imaging each of the fragmentation products that result from dissociation of a parent molecule of interest, with the aim of increasing the amount of information that can be obtained from a mass-spectrometry-type experiment. The work presented in this thesis demonstrates that VMImMS allows us to unravel details of the dissociation dynamics of both neutral and ionic species, and is potentially a powerful technique for investigating the fragmentation processes of increasingly complex systems.

Image-charge detection is an analytical technique in which a highly-charged particle is detected by the magnitude of the image current that it generates in a detecting electrode. This current is represented as a voltage between the charged particle and the sensing electrode. It is a single particle detection method, ideal for the analysis of large, variable mass particles such as biological cells. Some of the physical properties of Bacillus subtilis spores were explored using different applications of image-charge detection. B. subtilis is a gram-negative spore-forming bacteria that has been shown to exhibit extremophile behavior. The particular extremophile behavior that was investigated in this study is the resistance to extreme mechanical stress. The effects of high-velocity impacts upon these spores were studied using image-charge detection. The elastic properties of these spores as well as spore survivability to high-velocity impacts were investigated. Spores were shown to survive impacts at velocities up to 299 ± 28 m/s. The average kinetic energy loss experienced by impacting spores, regardless of velocity at impact, was between 71 and 72%. Both conventional and novel image-charge detection techniques were used for these studies. The novel version of a charge detector that was demonstrated was fabricated using patterned metal electrodes on printed circuit boards. The simplicity and versatility of this method was demonstrated with a multi-stage charge detector, a unique bouncing detector, and charge-detection mass spectrometry detector which is capable of measuring the absolute mass of a single highly-charged particle.

The growing interest in the study of gas phase photodissociation dynamics has led to the development of experimental techniques to aid in the understanding of these processes in small (diatomic or triatomic molecules) and in larger systems. In addition, deposition of molecules on large clusters provides information of the clustering dynamics, which can shed light on the processes taking place in the condensed phase. The experiments described in this thesis use nanosecond and femtosecond velocity map imaging to explore the photo dissociation dynamics of a variety of systems on different timescales, and mass spectrometry to investigate the pick-up dynamics of several molecules on clusters. The experiments described in Chapter 3 concern iodine atom loss following the UV dissociation of iodocyclohexane. The bimodal kinetic energy distributions observed are attributed to the dissociation of axial and equatorial conformers of this molecule. The experimental results were complemented by ab initio calculations and two impulsive models. The results were also compared to the dissociation of CH3I, which is generally considered a benchmark system in the field of photodissociation dynamics. In Chapter 4 a time-resolved investigation of the photophysics of electronically excited chlorophenols is presented. The results emphasise the effect of intramolecular hydrogen bonding. To validate these results, the excited state decay of phenol has also been studied and used as a benchmark, since its dynamics have been studied in more detail. Furthermore, there is an introduction to on going high accuracy ab initio calculations, which aim to investigate the various conical intersections by which the electronically excited molecules can transfer to lower electronic states. Chapter 5 presents the experimentally determined pick-up cross-sections of several molecules (NO, HC1, etc) on ice nanoparticles. Particular emphasis is devoted to the pick-up of water molecules by large water clusters. For this process MD simulations have also been carried out. The experimental and theoretical results have been compared to the geometrical cross-sections, which are widely used in atmospheric models. The differences are discussed and the conclusions suggest that the (larger) experimentally determined cross-sections should be used in atmospheric models. Finally, Chapter 6 is dedicated to the recent upgrades of the Bristol VMI spectrometer. The spectrometer was fitted with new ion optics, for which the design details, the simulations and the results from experimental testing are presented. In general the new ion optics design performs better than the one used till now, giving better velocity resolution. However, the final resolution achieved thus far is limited 'by factors other than the configuration of the assembly. These factors are investigated and an effort is made to quantify their effect on the achievable resolution.

We are developing a stable and precise spectrograph for the Large Binocular Telescope (LBT) named "iLocater." The instrument comprises three principal components: a cross-dispersed echelle spectrograph that operates in the YJ-bands (0.97-1.30 mu m), a fiber-injection acquisition camera system, and a wavelength calibration unit. iLocater will deliver high spectral resolution (R similar to 150,000-240,000) measurements that permit novel studies of stellar and substellar objects in the solar neighborhood including extrasolar planets. Unlike previous planet-finding instruments, which are seeing-limited, iLocater operates at the diffraction limit and uses single mode fibers to eliminate the effects of modal noise entirely. By receiving starlight from two 8.4m diameter telescopes that each use "extreme" adaptive optics (AO), iLocater shows promise to overcome the limitations that prevent existing instruments from generating sub-meter-per-second radial velocity (RV) precision. Although optimized for the characterization of low-mass planets using the Doppler technique, iLocater will also advance areas of research that involve crowded fields, line-blanketing, and weak absorption lines.

Few experimental techniques have found such a diverse range of applications as has ion imaging. The field of chemical dynamics is constantly advancing, and new applications of ion imaging are being realised with increasing frequency. This thesis is concerned with the application of a fast pixelated imaging sensor, the Pixel Imaging Mass Spectrometry (PImMS) camera, to ion imaging applications. The experimental possibilities of such a marriage are exceptionally broad in scope, and this thesis is concerned with the development of a selection of velocity-map imaging applications within the field of photoinduced molecular dynamics. The capabilities of the PImMS camera in three-dimensional and slice imaging applications are investigated, in which the product fragment Newton-sphere is temporally stretched along the time-of-flight axis, and time-resolved slices through the product fragment distribution are acquired. Through experimental results following the photodissociation of ethyl iodide (CH₃CH₂I) at around 230 nm, the PImMS camera is demonstrated to be capable of recording well-resolved time slices through the product fragment Newton-sphere in a single experiment, without the requirement to time-gate the acquisition. The various multi-hit capabilities of the device represent a unique and significant advantage over alternative technologies. The details of a new experiment that allows the simultaneous imaging of both photoelectrons and photoions on a single detector for each experimental acquisition cycle using pulsed ion extraction are presented. It is demonstrated that it is possible to maintain a high velocity resolution using this approach through the simultaneous imaging of the photoelectrons and photoions that result from the (3 + 2) resonantly enhanced multi-photon ionisation of Br atoms produced following the photodissociation of Br₂ at 446.41 nm. Pulsed ion extraction represents a substantial simplification in experimental design over conventional photoelectron-photoion coincidence (PEPICO) imaging spectrometers and is an important step towards performing coincidence experiments using a conventional ion imaging apparatus coupled with a fast imaging detector. The performance of the PImMS camera in this application is investigated, and a new method for the determination of the photofragment detection efficiencies based on a statistical fitting of the coincident photoelectron and photoion data is presented. The PImMS camera is applied to laser-induced Coulomb explosion imaging (CEI) of an axially chiral substituted biphenyl molecule. The multi-hit capabilities of the device allow the concurrent detection of individual 2D momentum images of all ionic fragments resulting from the Coulomb explosion of multiple molecules in each acquisition cycle. Correlations between the recoil directions of the fragment ions are determined through a covariance analysis. In combination with the ability to align the molecules in space prior to the Coulomb explosion event, the experimental results demonstrate that it is possible to extract extensive information pertaining to the parent molecular structure and fragmentation dynamics following strong field ionisation. Preliminary simulations of the Coulomb explosion dynamics suggest that such an approach may hold promise for determining elements of molecular structure on a femtosecond timescale, bringing the concept of the `molecular movie' closer to realisation. Finally, the PImMS camera is applied to the imaging of laser-induced torsional motion of axially chiral biphenyl molecules through femtosecond Coulomb explosion imaging. The target molecules are initially aligned in space using a nanosecond laser pulse, and torsional motion induced using a femtosecond 'kick' pulse. Instantaneous measurements of the dihedral angle of the molecules are inferred from the correlated F+ and Br+ ion trajectories following photoinitiated Coulomb explosion at various time delays after the initial kick pulse. The technique is extended to include a second kick pulse, in order to achieve either an increase in the amplitude of the oscillations or to damp the motion, representing a substantial degree of control of the system. Measurements out to long kick-probe delays (200 ps) reveal that the initially prepared torsional wave packet periodically dephases and rephases, in accordance with the predictions of recent theoretical work.

Ablation controlled electrothermal (ET) plasma discharge devices consist of a small diameter capillary through which a large amount of energy is discharged. The high energy in the discharge ablates an inner sleeve material, ionizes the material, and a high energy-density plasma jet accelerates out the open end. ET devices can find applications in internal combustion engines, Tokamak fusion fueling and stabilization, hypervelocity launchers, and propulsion. The ballistic properties of an ET device are highly dependent on the propellant and ablated material. A useful noninvasive technique to characterize a propellant in these types of devices is spectroscopy. The purpose of this study is to design and conduct experiments on the ET facility called PIPE to verify results and assumptions in the ETFLOW simulation code as well as resolve data collection issues such as equipment triggering as spectrometer saturation. Experiments are carried out using an Ocean Optics LIBS2500plus high resolution spectrometer and a Photron FASTCAM SA4 high speed camera. Electron plasma temperatures are estimated using copper peaks in the UV region with the relative line intensity method, and electron plasma density is estimated by measuring the full width at half maximum (FWHM) of the stark broadened H--β line at 486 nm. Electron temperatures between 0.19 eV and 0.49 eV, and electron densities between 4.68*1022 m-3 and 5.75*10²² m⁻³ were measured in the expanding plasma jet about an inch outside the source with values as expected for this region. Velocity measurements of PIPE match well with simulations at around 5333 m/s. This study concluded that the assumption that the propellant Lexan is completely dissociated is a valid assumption, and that the ETFLOW results for electron temperature, density, and bulk plasma velocity match experimental values.
Master of Science

Grâce à ses propriétés (cohérence, brillance, durée), le rayonnement XUV femtoseconde produit par génération d'harmoniques d'ordre élevé est utilisé comme un processus de sonde pour l'étude de dynamiques atomiques et moléculaires, avec une bonne résolution spatiale et temporelle (femtoseconde voire attoseconde). Ainsi, des dynamiques rotationnelles ont été résolues sur des petits systèmes moléculaires (N2, CO2). Les travaux de cette thèse ont consisté à étendre les méthodes de spectroscopie harmoniques et les appliquer à des systèmes moléculaires complexes d'intérêt femtochimique. Parmi elles, nous présenterons la génération d'harmoniques à deux sources, le réseau transitoire d'excitation ou encore la génération d'harmoniques à deux couleurs. Ces techniques nous ont permis de résoudre des dynamiques femtosecondes dans N2O4 et SF6. La HHG est aussi utilisée comme source de rayonnement XUV, en jouant le rôle d'impulsion pompe lors d'expériences de type pompe-sonde. Cette approche a été utilisée pour l'étude du dichroïsme circulaire de photoélectrons de molécules chirales ionisées par un champ XUV harmonique de polarisation quasi circulaire. Nous nous attarderons à détailler la découverte de cette nouvelle source XUV femtoseconde de polarisation quasi circulaire
High harmonic generation (HHG) spectroscopy has proven to be a promisingtool (like probe in pump-probe experiments) in revealing the atomic and molecular dynamicswith the potential for subangstrom spatial resolution and subfemtosecond temporalresolution. Then, rotational dynamics have been resolved on small molecular systems (N2,CO2). This thesis looks to extending HHG spectroscopy methods to probe the structureand the dynamic of complex molecular systems. We will describe the two sources highharmonic generation, the transient grating of excitation and the two-color high harmonicgeneration. We enable to resolve the femtosecond nuclear dynamics in N2O4 and SF6. HHGis also used like a XUV radiation source, playing the role of pump pulse. This approach hasbeen used for the study of photoelectron circular dichroism. An XUV harmonic field witha quasi-circular polarization ionizes chiral molecules. In this manuscript, we will developthis new femtosecond XUV and quasi circular polarization radiation

Cette thèse fait l'étude expérimentale de dynamiques de relaxations ultrarapides au sein d'atomes et de molécules (Ar, NO2, C2H2). Les méthodes expérimentales qui sont utilisées sont basées sur l'interaction d'un rayonnement laser avec le système atomique ou moléculaire étudié et font intervenir le processus de génération d'harmoniques d'ordres élevés, ainsi que la spectrométrie d'imagerie de vecteur vitesse. Au cours de cette thèse, deux approchesexpérimentales de type pompe-sonde ont été mises en œuvre. Une première approche exploitela sensibilité du processus de génération d'harmoniques à la structure électronique dumilieu pour la sonder. Cette méthode a été utilisée sur la molécule de dioxyde d'azote pourobserver sa relaxation électronique à travers l'intersection conique des états X2A1-A2B2suite à une excitation autour de 400 nm. Une seconde approche utilise le rayonnementharmonique comme source de photons dans le domaine de l'extrême ultraviolet (EUV)pour exciter ou sonder les espèces d'intérêt. Cette approche a été couplée avec l'utilisationd'un spectromètre d'imagerie de vecteur vitesse (VMIS), qui a été développé durant lathèse. Des expériences menées sur un système modèle comme l'argon ont permis de validerle dispositif expérimental, qui a ensuite été mis en application pour étudier la photodissociationde la molécule d'acétylène, après excitation autour de 9,3 eV du complexe deRydberg 3d-4s. Les deux méthodes mises en œuvre permettent toutes-deux de réaliserdes études dynamiques résolues en temps à l'échelle femtoseconde.

碩士
國立清華大學
物理系
102
H_3^+, consists of three protons and two electrons, is the simplest polyatomic molecule. Due to its simple structure, it is the benchmark of highly accurate calculation. In the early time of cosmos, H_3^+ plays a crucial role in cooling down the environment and also in forming the first star. H_3^+ interacts with carbon and water and forms carbohydrates which are the essential elements of life. Through the technique of high resolution spectroscopy, it provide precise values for theoretical calculation. It also has important impacts on quantum chemical calculations, planetary science, and astronomical observation. 　　So far, most of H_3^+ transitions are observed by velocity modulation spectroscopy which eliminates the strong absorption line of neutral gas. The transition frequency accuracy is about 150~300 MHz. Recently, our lab and McCall’s group have measured the saturated absorption spectrum of H_3^+. Schlemmer’s group in Germany has observed H_3^+ transition using cryogenic ion trap. Those works achieve frequency accuracy less 1 MHz with the help of optic frequency comb (OFC). Although above methods provide high accuracy, but the systems are more complex and the signal are smaller. 　　In this dissertation, we use the method of velocity modulation spectroscopy to detect the signal of vibration-rotation absorption transition of H_3^+ molecular ion, and measure the frequency by an OFC system. We expect that the frequency accuracy of the weak absorption transitions can be improved to < 10 MHz. 　　The repetition rate and offset frequency of our Ti:sapphire-based OFC are phase-locked to a global positioning system (GPS) disciplined Rb clock. The accuracy of our OFC is better than 〖10〗^(-12) at 1000 sec. It can be used to measure the absolute frequency of wavelength from 500 to 1100 nm. After phase locking, the standard deviation of repetition rate and offset frequency are 2 mHz and 10 mHz respectively at 1 s gate time of the frequency counter. We measure R(56)32-0 a_10 the frequency of hyperfine line the molecular iodine to check our OFC. Our result show the difference is less 17 kHz from CIPM. 　　Our light source is a PPLN difference frequency generation (DFG) laser generated using a Ti:sapphire laser and a Nd:YAG laser. In order to improve the signal-to-noise ratio of absorption spectrum, we split the DFG light into two beams which go through the AC discharge tube with opposite direction. Then we collected the signals by two different detectors. The signals from the two detectors are substracted to increase the signal and decrease the noise. The zero point of absorption signal is also nearer to the line center than the one beam configuration. For the R(1,0) line of H_3^+, the frequency measured is 1.5 MHz less than McCall’s result. 　　In the feature, we will improve the stability of the frequency locking of our Ti:sapphire frequency comb and PPLN DFG and test the measurement accuracy of a weak absorption line. Finally, we will measure extensively the weak absorption line of H_3^+.

碩士
國立交通大學
應用化學系碩博士班
106
This thesis is consist of two parts. Part I is designing the image capture system. The image technology for physical dynamics has been developed since 1970s, and also be applied in many studies. We can get the data of the product speed and information about the angle vector distribution to analyze the relationship between the different vectors. Therefore, we construct a set of chamber, which include the detection and auxiliary parts for experiment using. Part II is according to (1+1) REMPI spectrum for phenol, using time-resolved pump-probe technique measure mode dependent lifetime of phenol, and figure out some particular vibrational state will accelerate the lifetime of phenol.

碩士
國立交通大學
應用化學系碩博士班
106
We design a thick-lens velocity map imaging (VMI) spectrometer with high resolution for wide energy range of electrons and ions. By adding an adjustable electrode in combination with repeller and extractor electrodes, which consists of 12 electrodes in total, the energy resolution reaches ~2% in the kinetic energy range between 0.02 ~ 7.5 eV from SIMION simulation. Appling the idea of multimass ion imaging, a microsecond long pump-probe delayed VMI experiment, which can distinguish neutral photofragment signals from dissociative ionized background, is able to be conducted with high velocity resolution by simple modification of electrodes. Extra functions of this long delayed VMI experiment are proposed in order to minimize the dissociative ionization and detect the photoelectron of energy selected fragment. The observation of striking dependence of product branching ratio on different vibrational modes of thioanisole indicate the existence of dynamical resonance. We examined the resonance on the conical intersection between initial excited S1 state (ππ*) and the dissociative state S2 state (nσ*) with picosecond time-resolved pump-probe photoionization method. From the time and frequency resolved measurements, we found that the lifetime changes with different frequency component of the initial excited vibronic band, or the absorption (ionization) spectra of excited state varies with time alternatively. It could be explained with the influence of its lifetime by the relative phase change in the supposition with the dark continuum, which was known as a Fano resonance.