1,257 research outputs found

    Linear and nonlinear spectroscopic techniques applied to study of transient molecular species

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    Carbon chain radicals and ions have long been known to exist in the interstellar medium, and considered as potential carriers for diffuse interstellar bands. Measurements on these molecules are difficult because they are short-lived species generated in small number densities. Discharge and laser vaporization sources coupled to a supersonic jet expansion are, so far, the most effective techniques used to generate sufficiently large densities suitable for spectroscopic studies of these transients. The present work describes the combination of these molecular sources with high resolution cavity ring down (CRDS) and four wave mixing (FWM) spectroscopic techniques applied to detection of radicals and ions of astrophysical significance. CRDS offers high sensitivity because of the large absorption path lengths achieved inside the cavity and its immunity towards shot-to-shot laser fluctuations. Four-wave mixing, on the other hand, offers selectivity of the species studied by the application of very short discharge pulse lengths, in the nanosecond time scale. As a consequence, the molecules are separated out in the plasma discharge based on their masses. The 3Π-3Π electronic transition of C6H+ has been measured using CRDS; this is the first gas phase detection of the cation. Partially resolved P lines and observation of band heads permitted a rotational contour fit. Spectroscopic constants in the ground and excited-state were determined. Broadening of the spectral lines indicates the excited-state lifetime to be around 100 ps. The potential of degenerate and two-color FWM applied to selectivity of transient species has been studied extensively in this work using various molecular systems like C3/C4H, C3/HC2S and C2/HC4H+, only by varying the timings between the experimental components (laser/valve/discharge) while applying extremely short (<1µs) discharge pulses. The two color variant is exceptionally powerful in disentangling overlapping features even within the same spectroscopic system. This is demonstrated in the case of HC4H+ where P lines of the Ω=3/2 spin orbit component are effectively separated out from the overlapping Ω=1/2 component in the A←X electronic transition. The first ever detections of ions (HC4H+, C2-) by FWM are also reported. The results suggest convincingly that nonlinear four-wave mixing spectroscopy is applicable to study numerous neutral, cationic and anionic radicals that are produced in plasma environments in low particle densities by applying a discharged free-jet expansion. Both CRDS and FWM have been employed as tools for spectroscopic investigation of non-adiabatic effects in linear polyatomic molecules. The excitation of the ν3 (C-C stretch) and the 2ν7 (C≡C-C bend) levels in the A2Π electronic state of diacetylene cations results in Renner-Teller (R-T) and Fermi interactions. The ν3 and 2ν7 vibronic bands in the A←X transition of HC4H+ have been measured with rotational resolution using CRDS in a supersonic slit jet discharge. A vibronic analysis has been carried out taking into consideration the R-T, spin-orbit, and Fermi resonance interactions between the ν3 and ν7 modes. The spectroscopic constants for the excited electronic state are compared with the ground state. The double resonance four wave mixing approach was used to unambiguously identify the vibronic R-T manifold in the A2Π state up to 700 cm-1 above v=0 of C4H by pumping on the origin B←X electronic transition. On the basis of the experimental linelist, several of the energy levels are assigned to vibrations in the electronic X2Σ+ ground state. An assignment of the levels was carried out by R-T analysis, leading to a relatively large ε6 in the ground state for the second lowest bending mode as previously found in the upper state. This study results in the detection of levels located below the A2Π state because of high R-T interaction. CRDS has also been employed to detect broad absorption features of the B←X transition of H2CCC (l-C3H2). The observations provide evidence that the broad, diffuse interstellar bands (DIBs) at 4881 and 5450 Å are caused by the B←X transition of H2CCC (l-C3H2). The large widths of the bands are due to the short lifetime of the B 1B1 electronic state. The bands in the gas phase show exact matches to the profiles and wavelengths of the two broad DIBs. This makes l-C3H2 a carrier of the DIBs, which have remained a long standing mystery in astronomy. The present work provides an insight to understanding not only the fundamental spectroscopic properties of these transient species but also their astrophysical significance. Moreover, it also demonstrates the highly sensitive and selective capabilities of the employed experimental techniques, which could be of use in other fields like combustion, trace gas analysis, to name a few

    De productie en moleculaire verschijningsvorm van radiotoxisch Po-210 in kernfusie- en splijtingsreactoren = The Production and Molecular Occurrence of Radiotoxic Po-210 in Nuclear Fusion and Fission Reactors

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    In order to counter the climate change, there is a need for a radical revolution in the global energy supply. One of the most realistic scenarios is a substantial shift towards renewable energy sources, supported by a flexible baseload from carbon-free nuclear energy. The main requirements for this new generation of nuclear reactors are intrinsic safety, high fuel efficiency and a minimum amount of (long-lived) radioactive waste. A promising candidate for the near future is the lead (alloy) cooled fast fission reactor. An ideal long-term solution are nuclear fusion reactors, which produce only a small amount of short-lived radioactive waste. A problem that occurs in both these reactor types is the undesired production of the highly radiotoxic polonium-210 isotope. In this PhD thesis, the total production of this isotope in a nuclear fusion reactor is determined using neutron transport and inventory calculations. Next, quantum chemical calculations are performed to predict the molecular form in which Po-210 will occur in both discussed reactor types. Combining both aspects allows to estimate the risk associated with the presence of Po-210 in a nuclear reactor and can help to design efficient Po-210 extraction systems

    Nuclear Magnetic Resonance Spectroscopy

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    Nuclear Magnetic Resonance (NMR) spectroscopy is a nondestructive technique that can be used to characterize a wide variety of systems. Sustained development of both methodology and instrumentation have allowed NMR to evolve as a powerful technology, with applications in pure sciences, medicine, drug development, and important branches of industry. NMR provides precise structural information down to each atom and bond in a molecule, and is the only method for the determination of structures of molecules in a solution. This book compiles a series of articles describing the application of NMR in a variety of interesting scientific challenges. The articles illustrate the versatility and flexibility of NMR

    Interactions of fluorophores with complex surfaces and spectroscopic examinations of ancient manuscripts

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    In the first part of this thesis, it was found by fibre-optic fluorescence spectroscopy, that the greening of fabrics washed in optical brighteners is due to a reabsorption effect. The quantum yield of fluorescence of the optical brighteners OB15, OB36 and OB49 in water are 0.11 0.11 , 0.08 0.08 and 0.71 0.71 respectively. Their respective fluorescence natural lifetimes are 6700±109 6700\pm109 , 5971±712 5971\pm712 and 1685±22 1685\pm22 ps. In solution, the excited state of OB15 experiences more competing relaxation processes as the solvatochromic shift increases. OB49 displays the opposite trend. A literature cellulose model surface is employed as a cotton mimic for evanescent wave fluorescence studies. Two model greases are similarly developed and used, and a third is presented for future work. These are based on surface-specific reactions with glass substrates, and the doping of a regenerated cellulose film with long chain alcohols. On doped cellulose surfaces, some low quantum yields occur compared to clean cellulose and bulk solution. Photobleaching behaviours are also observed. Both dyes physisorb rigidly to cellulose and grease models. The second part of this thesis identifies the pigment palette of the earliest Northumbrian manuscripts pre- and post-1066, by Raman and diffuse reflectance spectroscopy. It develops a suite of multispectral imaging programs in MATLAB for facile classification of pigments across a page ab initio, using data reduction and colour spaces. Raman and reflectance data are meta-analysed using symmetric permutation to split manuscripts and pigments into groups ab initio. It was also generalised, that the palette of the pre-Hastings selected manuscripts contained vergaut, indigo, orpiment, impure red lead, and copper green pigments, as well as orcein purples. Immediately post-1066 white lead, red ochre, vermilion and lapis lazuli appear in the palette in England, though vergaut and indigo disappear and the red lead used is essentially pure

    Theoretical Concepts of Quantum Mechanics

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    Quantum theory as a scientific revolution profoundly influenced human thought about the universe and governed forces of nature. Perhaps the historical development of quantum mechanics mimics the history of human scientific struggles from their beginning. This book, which brought together an international community of invited authors, represents a rich account of foundation, scientific history of quantum mechanics, relativistic quantum mechanics and field theory, and different methods to solve the Schrodinger equation. We wish for this collected volume to become an important reference for students and researchers

    Visualizing the influence of the Fermi surface on superconductivity

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    Tesis Doctoral inédita leída en la Universidad Autónoma de Madrid, Facultad de Ciencias, Departamento de Física de la Materia Condensada. Fecha de lectura: 15-12-2016IN the book of Kittel, a well-known citation attributed to Mackintosh states that metals are “a solid with a Fermi surface”. When metals become superconducting, BCS theory tells us that a gap opens at the Fermi level, so that all electronic states at the Fermi surface are lost. The shape and the properties of electrons at and close to the Fermi surface remain however fundamental to understand the superconducting behavior. Many Fermi surfaces consist of different sheets with a topology that can be varied. For example, in the well-known system MgB2, two-dimensional and three-dimensional sheets are found together in the same material. Electron-phonon coupling in each sheet is different and leads to different values of the superconducting gap. Furthermore, the anisotropy is large and also influences the superconducting density of states. The electronic properties at and close to the Fermi surface also influence other aspects of superconductors. Here we focus on visualizing these aspects by studying the spatial variation in the density of states that characterizes superconductors. We center the attention on the shape of vortices, the orientation of the vortex lattice, the dynamics of vortices and excitations, Fano and Friedel like, due to electronic correlations. We address superconductors with very different electronic properties close to the Fermi level. First, we analyze a system with a complex Fermi surface, yet simple single gap superconducting behavior, b-Bi2Pd. We determine the influence of the complex Fermi surface in the vortex behavior (Chapter 3) and make a detailed visualization of the tilted vortex lattice (Chapter 4). We then analyze vortex dynamics in a crystalline system, Rh9In4S4, where disorder is so strong that it has produced peculiar pinning landscapes leading to unstable vortex lattices (Chapter 5). Finally, we study a highly ordered system where electronic correlations lead to profound changes in the electronic structure close to the Fermi level, URu2Si2 (Chapter 6). By making detailed atomic size imaging and spectroscopy and viewing Friedel-like oscillations at the surface, we unveil how these electronic correlations influence the superconducting state. The level of understanding that we now have from the Fermi surface and electronic interactions is very high and this thesis has hopefully contributed to advance such understanding. However, applications of superconductors and other condensed matter systems need experiments where the parameters controlling the critical temperature or the Fermi surface can be varied and handled at will. This will really bring us closer to superconductors working at room temperature or carrying much more current, or any other interesting application stemming out from electronic correlations (for example topological superconductivity). Strain, changes in the electronic density by gate voltage or very high magnetic fields are routinely used to shape the macroscopic properties of the systems presented in this thesis. Adding such control parameters to microscopic visualizations of the Fermi surface as obtained by photoemission and STM is probably the next important step.EN el libro de introducción a la física de estado sólido de Kittel, se incluye una conocida cita de Mackintosh en la que se define a los metales como “sólidos con una superficie de Fermi”. Cuando los metales transitan al estado superconductor, la teoría BCS nos dice que un gap de energía se abre en el nivel de Fermi debido a nuevas interacciones que surgen entre los electrones que se encuentran en su proximidad. Por ello, estudiar el comportamiento de los estados electrónicos que se encuentran en la superficie de Fermi y su proximidad es fundamental para entender las propiedades de los superconductores. Las superficies de Fermi a menudo están formadas por diferentes capas con topología diversa. Por ejemplo, en el caso conocido del superconductor MgB2, encontramos en el mismo material capas bidimensionales y tridimensionales. El acoplamiento electrón-fonón es muy diferente en cada capa, y conduce a valores distintos del gap superconductor dentro de la misma superficie de Fermi. Asimismo, la fuerte anisotropía de sus propiedades electrónicas tiene una influencia notable en la densidad de estados superconductora. Otras propiedades de los superconductores se ven también afectadas por la estructura electrónica de la superficie de Fermi y sus proximidades. En esta Tesis nos planteamos como objetivo visualizar dichas propiedades mediante el estudio de la variación espacial de la densidad de estados electrónica que caracteriza a los superconductores. En concreto, hemos centrado nuestra atención en la forma de los vórtices superconductores, la orientación y dinámica de la red de vórtices, y la presencia de excitaciones, de tipo Fano y Friedel, que surgen de las correlaciones electrónicas. En esta Tesis Doctoral hemos estudiado superconductores con propiedades electrónicas cerca del nivel de Fermi muy diferentes. Primero analizamos el material b-Bi2Pd, que presenta una superficie de Fermi compleja y al mismo tiempo un gap superconductor único y sencillo. Hemos determinado la influencia de la superficie de Fermi en el comportamiento de la red de vórtices (Capítulo 3) y realizado un estudio detallado de la red bajo campos magnéticos inclinados (Capítulo 4). A continuación, hemos investigado la dinámica de vórtices en un nuevo superconductor cristalino descubierto en 2015, Rh9In4S4, donde el valor extremo de sus parámetros superconductores y la presencia de fuerte desorden producen redes de vórtices inestables (Capítulo 5). Finalmente, hemos estudiado un sistema muy ordenado, URu2Si2, donde las correlaciones electrónicas modifican profundamente la estructura electrónica cerca del nivel de Fermi (Capítulo 6). Mediante un estudio topográfico y espectroscópico a escala atómica y la visualización de oscilaciones tipo Friedel, hemos determinado la influencia de las correlaciones electrónicas en el estado superconductor. El nivel de comprensión que ahora tenemos de la superficie de Fermi y las interacciones electrónicas es muy alto, habiendo contribuido esta Tesis a avanzar en dicha comprensión. Sin embargo, el desarrollo de aplicaciones basadas en materiales superconductores, y en otros sistemas de la materia condensada, requiere aún de experimentos en los que se puedan modificar o manejar a voluntad los parámetros que controlan la temperatura crítica o las propiedades de la superficie de Fermi. Esto nos llevará mucho más cerca de los superconductores que mantengan esta propiedad hasta temperatura ambiente o que sean capaces de transportar corrientes eléctricas mayores, o cualquier otra aplicación interesante que surja de las correlaciones electrónicas (por ejemplo superconductividad topológica). Otros parámetros como la aplicación de tensiones, los cambios en la densidad electrónica por voltajes de puerta o el uso de campos magnéticos elevados se utilizan de manera rutinaria para modificar las propiedades macroscópicas de los superconductores estudiados en esta Tesis. El próximo paso importante será, por tanto, aadir estos parámetros de control a los métodos de visualización microscópica de la superficie de Fermi, como es la microscopía de efecto túnel

    Doing cold smarter

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    Cold has been much neglected in the energy debate. Governments are developing strategies and policies to green everything from electricity to transport to heat, but the energy and environmental impacts of cooling have so far been largely ignored. This is a serious oversight, since making things cold is energy intensive and can be highly polluting, and demand for cooling in all its forms is booming worldwide – especially in developing countries. According to one projection, by the end of this century global demand for air conditioning alone could consume the equivalent of half our worldwide electricity generation today – and most of the increase will come in developing markets. The ‘greening’ of cold is clearly an urgent global problem – but it may also offer Britain a massive business opportunity. Cold may have been ignored but is vitally important to many aspects of modern life. An effective cold chain, for example, is essential for tackling problems such as food waste, food security, water conservation and public health. Cooling is also critical for many less obvious but essential functions: data centres couldn’t operate without it, nor for example MRI scanners in medicine or superconductors in power electronics. Cooling also provides modern levels of comfort in hot countries – and can make the difference between some regions being habitable or not. At the same time, vast amounts of cold are wasted – for instance during the regasification of LNG – which could in principle be recycled to satisfy some of this demand and start to reduce the environmental damage caused by cooling. Such a system-level approach – which starts by asking what energy services we need, and what is the least damaging way to provide them, rather than accepting existing practices as a fait accompli – has recently been coined the ‘Cold Economy’. It is clear the Cold Economy could unleash a wide range of innovative clean cold technologies and provide energy resilience, economic growth and environmental benefits, but there is an urgent need to develop a system-level analysis of this problem and the potential solutions to inform both industry and policymakers. The Birmingham Policy Commission: Doing Cold Smarter was convened to start this work
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