Spelling suggestions: "subject:"metastable states."" "subject:"etastable states.""
1 |
Weak Atomic InteractionsSchef, Peter January 2006 (has links)
<p>An atom or ion can change quantum state, usually through emission or absorption of a photon. The photon has the same energy as the energy difference between the states of the transition. The states, or energy levels, of an atom are quantized and light emitted, or absorbed, from the atom is therefore of specific wavelengths, giving spectral lines. The spectrum of an atomic ion is unique and contains information of the structure and energy levels of the ion. The spectrum of an atom can be used as a fingerprint in determinations of the abundance of the element in different objects.</p><p>This thesis is focused on some weak effects observed by spectroscopy. Although the effects are weak, they turn out to be of great importance. According to quantum mechanics transitions between certain states are not allowed. Here the weak effects open the possibility for transitions. Spectral lines from forbidden transitions are very weak and difficult to observe under ordinary laboratorial conditions, but they are commonly observed from astrophysical objects and can be very useful for diagnostics of astrophysical plasmas. The first reported observation of forbidden lines was from an astrophysical object and at that time supposed to be from new, previous unknown, elements. If all possible decay channels from an energy level are forbidden, the energy level is metastable and has usually a lifetime 10$^8$ times longer than an ordinary excited state. Measurements of such long lifetimes are difficult since the ion need to be confined during the observation time. Confinement of ions can be achieved with a storage device, such as a storage ring or a trap, where the ions are stored without interacting with each other or the surroundings.</p><p>A laser probing technique has been developed at the storage ring CRYRING, for measurements of lifetimes of metastable states. The technique has now been improved for measurement of longer lifetimes. The technique has also been modified to fit when measuring on negative ions. Results of lifetime measurements are reported and the techniques and methods used are described.</p><p>Another weak effect is hyperfine interaction, which splits the energy levels of an atom or ion. Hyperfine splitting is very small and usually special spectral techniques are needed to resolve such structure. A laser can, in combination with an electromagnetic radio-frequency field, be used for accurate determination of hyperfine structures in atomic ions. Such measurements are also important for evaluation of astrophysical properties, since hyperfine structure can broaden the spectral lines. An experimental setup for such double resonance measurements has been developed and constructed. Results of experimental measurements are reported and the technique is described.</p>
|
2 |
Study of semiconductor and metal surfaces using a novel scanning Kelvin probePetermann, Uwe January 2000 (has links)
No description available.
|
3 |
Weak Atomic InteractionsSchef, Peter January 2006 (has links)
An atom or ion can change quantum state, usually through emission or absorption of a photon. The photon has the same energy as the energy difference between the states of the transition. The states, or energy levels, of an atom are quantized and light emitted, or absorbed, from the atom is therefore of specific wavelengths, giving spectral lines. The spectrum of an atomic ion is unique and contains information of the structure and energy levels of the ion. The spectrum of an atom can be used as a fingerprint in determinations of the abundance of the element in different objects. This thesis is focused on some weak effects observed by spectroscopy. Although the effects are weak, they turn out to be of great importance. According to quantum mechanics transitions between certain states are not allowed. Here the weak effects open the possibility for transitions. Spectral lines from forbidden transitions are very weak and difficult to observe under ordinary laboratorial conditions, but they are commonly observed from astrophysical objects and can be very useful for diagnostics of astrophysical plasmas. The first reported observation of forbidden lines was from an astrophysical object and at that time supposed to be from new, previous unknown, elements. If all possible decay channels from an energy level are forbidden, the energy level is metastable and has usually a lifetime 10$^8$ times longer than an ordinary excited state. Measurements of such long lifetimes are difficult since the ion need to be confined during the observation time. Confinement of ions can be achieved with a storage device, such as a storage ring or a trap, where the ions are stored without interacting with each other or the surroundings. A laser probing technique has been developed at the storage ring CRYRING, for measurements of lifetimes of metastable states. The technique has now been improved for measurement of longer lifetimes. The technique has also been modified to fit when measuring on negative ions. Results of lifetime measurements are reported and the techniques and methods used are described. Another weak effect is hyperfine interaction, which splits the energy levels of an atom or ion. Hyperfine splitting is very small and usually special spectral techniques are needed to resolve such structure. A laser can, in combination with an electromagnetic radio-frequency field, be used for accurate determination of hyperfine structures in atomic ions. Such measurements are also important for evaluation of astrophysical properties, since hyperfine structure can broaden the spectral lines. An experimental setup for such double resonance measurements has been developed and constructed. Results of experimental measurements are reported and the technique is described.
|
4 |
Estudo do resfriamento em um sistema com múltiplos estados fundamentais / A study of cooling in a system with several ground states.Henrique Santos Guidi 29 October 2007 (has links)
Estudamos um sistema de dois níveis acoplados como um modelo que imita o comportamento de líquidos super-resfriados. Em equilíbrio o modelo apresenta uma fase líquida e uma fase cristalina com diversos estados fundamentais. O modelo é definido numa rede quadrada e a cada sítio é associada uma variável estocástica de Ising. A característica que torna este modelo particularmente interessante é que ele apresenta estados metaestáveis duráveis que podem desaparecer dentro do tempo acessível para as simulações numéricas. Para imitar o processo de formação dos vidros, realizamos simulações de Monte Carlo a taxas de resfriamento constante. Apresentamos também simulações para resfriamentos súbitos a temperatura abaixo da temperatura de fusão. / We study a coupled two level systems as a model that imitate the behavior of supercooled liquids that become structural glasses under cooling. In the equilibrium the model shows a liquid phase and a crystalline phase with many grouond states. The model is defined on a square lattice and to each site a stochastic Ising variable is associated. The feature that makes this model particularly interesting is that it display durable metastables states which can vanish within the time available for numerical simulations. In order to imitate the glass former process, we perform Monte Carlo simulations at constant cooling rate. We present also simulations for quenchs to temperatures below the melting temperature.
|
5 |
Implantação de um espectrômetro de estados metaestáveis de alvos gasososAlessio, Rita de Cássia Polito Vita 09 June 2009 (has links)
Submitted by Renata Lopes (renatasil82@gmail.com) on 2017-06-27T18:11:39Z
No. of bitstreams: 1
ritadecassiapolitovitaalessio.pdf: 1515814 bytes, checksum: ec2256bcf074df2e72a53fe6c0a62c6f (MD5) / Approved for entry into archive by Adriana Oliveira (adriana.oliveira@ufjf.edu.br) on 2017-08-07T21:06:14Z (GMT) No. of bitstreams: 1
ritadecassiapolitovitaalessio.pdf: 1515814 bytes, checksum: ec2256bcf074df2e72a53fe6c0a62c6f (MD5) / Made available in DSpace on 2017-08-07T21:06:14Z (GMT). No. of bitstreams: 1
ritadecassiapolitovitaalessio.pdf: 1515814 bytes, checksum: ec2256bcf074df2e72a53fe6c0a62c6f (MD5)
Previous issue date: 2009-06-09 / CAPES - Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / O estudo de ressonâncias, através de estados metaestáveis pelo impacto de
elétrons, permite observar estruturas bem definidas nos espectros, gerando informações
diretas sobre as energias de formação das ressonâncias, bem como suas formas e
larguras. A investigação destas ressonâncias é muito importante em vários ramos da
ciência, possuindo inúmeras aplicações biológicas e tecnológicas. Um espectrômetro
por impacto de elétrons foi construído e implantado no Laboratório de Física Atômica e
Molecular, permitindo a obtenção das funções de excitação do Argônio, do Hélio e da
molécula de Nitrogênio, sendo esta técnica inédita no Brasil. Neste espectrômetro
aplica-se a técnica dos feixes cruzados, que consiste basicamente de um canhão de
elétrons monocromatizado, um feixe de gás efusivo, uma gaiola de Faraday para a
blindagem da região de colisão, um detector de íons metaestáveis, devidamente
posicionado, e um coletor de Faraday. O canhão de elétrons monocromatizado
apresenta uma alta eficiência, cobrindo continuamente uma faixa de energia de 7 a 200
eV, com uma resolução em torno de 50 meV. Para a aquisição dos espectros, a energia
do canhão foi varrida a partir do limiar de excitação metaestável até uma energia menor
que o primeiro potencial de ionização do alvo estudado. / The study of resonances through metastable states of atoms and molecules by
electron impact shows spectra with well-defined structures, providing direct information
on the energies of resonances formation, as well as its shapes and widths. The
investigation of these resonances is important in several branches of science, such as
many technological and biological applications. In this work we have assembled at
Laboratory of Atomic and Molecular Physics an electron impact spectrometer which
allowed us to collect metastables excitations functions of Argon, Helium and the
Nitrogen molecules. As far we know, that was the first time that this technique was
performed in Brazil. That spectrometer applies the cross beam technique, and consists
of a monochromatized electron gun, an effusive gas beam, a cage to shield the collision
region, a metastable detector properly positioned and a Faraday cup. The
monocromatized electron gun built has a high efficiency, covering continuously the
energy range from 7 to 200 eV, with a resolution of about 50 meV. The three spectras
recorded in this work cover the energy range from the excitation metastable threshold
up to energy lower than the first ionization potential of the targets studied.
|
6 |
Estudo do resfriamento em um sistema com múltiplos estados fundamentais / A study of cooling in a system with several ground states.Guidi, Henrique Santos 29 October 2007 (has links)
Estudamos um sistema de dois níveis acoplados como um modelo que imita o comportamento de líquidos super-resfriados. Em equilíbrio o modelo apresenta uma fase líquida e uma fase cristalina com diversos estados fundamentais. O modelo é definido numa rede quadrada e a cada sítio é associada uma variável estocástica de Ising. A característica que torna este modelo particularmente interessante é que ele apresenta estados metaestáveis duráveis que podem desaparecer dentro do tempo acessível para as simulações numéricas. Para imitar o processo de formação dos vidros, realizamos simulações de Monte Carlo a taxas de resfriamento constante. Apresentamos também simulações para resfriamentos súbitos a temperatura abaixo da temperatura de fusão. / We study a coupled two level systems as a model that imitate the behavior of supercooled liquids that become structural glasses under cooling. In the equilibrium the model shows a liquid phase and a crystalline phase with many grouond states. The model is defined on a square lattice and to each site a stochastic Ising variable is associated. The feature that makes this model particularly interesting is that it display durable metastables states which can vanish within the time available for numerical simulations. In order to imitate the glass former process, we perform Monte Carlo simulations at constant cooling rate. We present also simulations for quenchs to temperatures below the melting temperature.
|
7 |
Distribution of metastable states of spin glassesSchnabel, Stefan, Janke, Wolfhard 09 June 2023 (has links)
The complex behavior of systems like spin glasses, proteins or neural networks
is typically explained in terms of a rugged energy or fitness landscape. Within the high dimensional conformation space of these systems one finds features like barriers, saddle points,
and metastable states whose number – at least in the case of spin glasses – grows exponentially
with the size of the system. We propose a novel Monte Carlo sampling algorithm that employs an
ensemble of short Markovian chains in order to visit all metastable states with equal probability.
We apply this algorithm in order to measure the number of metastable states for the two dimensional and the three-dimensional Edwards-Anderson model and compare with theoretical
predictions.
|
8 |
Development and Benchmarking of Hermitian and non-Hermitian Methods for Negative Ion ResonancesKolathingal Thodika, Mushir ul Hasan, 0000-0002-6837-9710 January 2022 (has links)
Low energy electron (LEE) driven chemistry underpins a wide range of interdisciplinary fields, including radiation biology, redox chemistry, astrochemistry and biomaterial design. A growing interest in the chemistry of LEEs concerns the radiative damage to DNA. Studies have found that LEEs can induce single and double-strand breaks in DNA by forming a negative ion resonance (NIR). These processes are remarkably site-specific and have been utilized to synthesize radiosensitizers, which aid in identifying target cells in hypoxic tumors in radiation therapy. Despite the prevalence of LEE-induced reactions, computational studies of such processes are limited compared to thermal and photochemical reactions. The relative scarcity in computational studies of LEE-induced reactions stems from the difficulties in the theoretical treatment of NIRs. In our work, we report new developments on the application of quantum chemical methods to NIRs. We demonstrate that the combination of approaches developed for resonances with multi reference electronic structure methods enables the computation of various types of NIRs in a single calculation. Additionally, we show that multi-reference methods can also quantify the mixing between NIRs. It is observed that the mixing between resonances can have significant consequences on their lifetimes. We also report the development of a new technique, the continuum remover Feshbach projection operator approach, which uses the conventional methods developed for bound states to characterize resonances. We show that this new approach is straightforward to implement with standard electronic structure packages, it is efficient, and provides promising results. / Chemistry
|
9 |
SINGLE CHAIN BEHAVIOR IN METASTABLE STATES IN SEMICRYSTALLINE POLYMERS AS INVESTIGATED BY SOLID STATE NMRYuan, Shichen 23 May 2018 (has links)
No description available.
|
10 |
An investigation of metastable electronic states in ab-initio simulations of mixed actinide ceramic oxide fuelsLord, Adam 13 November 2012 (has links)
First-principles calculations such as density functional theory (DFT) employ numerical approaches to solve the Schrodinger equation of a system. Standard functionals employed to determine the cohesive system energy, specifically the local density and generalized gradient approximations (LDA and GGA), underestimate the correlation of 5f electrons to their ions in AO₂ systems (A=U/Pu/Np). The standard correction, the "Hubbard +U," causes the multidimensional energy surface to develop a large number of local minima which do not correspond to the ground state (global minimum). Because all useful energy values derived from DFT calculations depend on small differences between relatively large cohesive energies, comparing systems wherein one or more of the samples are not in the ground state has the potential to introduce large errors. This work presents an analysis of the fundamental issues of metastable states in both pure and binary AO₂ systems, investigates novel methods of handling them, and describes why current literature approaches which appear to work well for the pure compounds are not well-suited for systems containing multiple actinide species.
|
Page generated in 0.0564 seconds