Dissertations / Theses on the topic 'Hard Electron Energy Spectra'

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Chemistry, 2002.
Vita.
Includes bibliographical references.
In this thesis, several problems regarding dynamics and spectra in condensed phases are theoretically analyzed via analytical models. The thesis consists of four main topics. First, a theoretical description of single molecule spectroscopy is presented in order to study time-dependent fluctuations of single molecule spectra in a dynamic environment. In particular, the photon counting statistics is investigated for a single molecule undergoing a generic type of spectral diffusion process. An exact analytical solution is found for this case, and various physical limits are analyzed. Second, motivated by recent experimental observations of anomalous spectral fluctuations in quantum dot systems, both the lineshape phenomenon and the photon counting statistics are explored when spectral fluctuations are characterized by power-law statistics, for which there is no finite timescale. Unique features of the power-law statistics are demonstrated in spectral properties of those systems. Third, a spectral analysis method is developed for the non-adiabatic electron transfer reactions, which allows a unified treatment of diverse kinetic regimes in the electron transfer process. The method is applied to electron transfer reactions in mixed-valence systems in order to explore the possibility of electronic coherence. Finally, effects of the nonequilibrium bath relaxation on the excitation energy transfer process are investigated by generalizing the Forster-Dexter theory of excitation energy transfer to the case of the nonstationary bath relaxation.
by YounJoon Jung.
Ph.D.

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Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)
Tese (Doutoramento)
IPEN/T
Instituto de Pesquisas Energeticas e Nucleares - IPEN/CNEN-SP
FAPESP:01/09049-5

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Dissertação (Mestrado)
IPEN/D
Instituto de Pesquisas Energeticas e Nucleares - IPEN-CNEN/SP

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Os catalisadores eletroquímicos binários de PtSn/C, PdSn/C e PtPd/C foram sintetizados em diferentes proporções pelo método da redução via borohidreto, posteriormente estes foram caracterizados por microscopia eletrônica de transmissão, difração de raios X, espectroscopia no infravermelho por transformada de Fourier (PtSn/C e PdSn/C) e energia dispersiva de raios X. As atividades eletroquímicas dos diferentes materiais preparados foram avaliadas por intermédio de voltametria cíclica, cronoamperometria e curvas de polarização em célula a combustível alimentada diretamente por etileno glicol em eletrólito alcalino. As curvas de densidade de potência indicaram que os catalisadores eletroquímicos contendo Sn e Pd são mais ativos para a reação de oxidação do etileno glicol, especialmente a composição 70%:30% - relação molar entre os metais suportados em carbono - dos catalisadores PtSn/C, PdSn/C e PtPd/C todos superando as medidas de potência do Pt/C. Este resultado indica que a adição de Sn e Pd favorece a oxidação do etileno glicol em meio alcalino. O melhor desempenho observado para os catalisadores eletroquímicos PtSn/C, PdSn/C e PtPd/C (70%:30%) poderia estar associado à sua maior seletividade quanto a formação de oxalato, ou seja , a formação deste produto resulta em um maior número de elétrons, por consequência em maiores valores de corrente.
Tese (Doutorado em Tecnologia Nuclear)
IPEN/T
Instituto de Pesquisas Energéticas e Nucleares - IPEN-CNEN/SP

Le centre d#-, ou ion donneur negatif, est l'analogue en physique du solide de l'ion hydrogene negatif h#-. Il s'agit donc d'un systeme correle simple, avec seulement deux electrons. Les transitions magneto-optiques permettant l'etude des donneurs peu profonds (neutres ou charges negativement) dans les multi-puits gaas/algaas se situent dans la gamme de l'infra-rouge lointain. Notre appareillage experimental est essentiellement constitue d'un spectrometre a transformee de fourier, couple a un aimant supraconducteur ou resistif. Nous avons etudie le mecanisme de formation des centres d#- confines dans des multi-puits quantiques. Dans cette configuration, contrairement au materiau massif, les centres d#- sont formes par construction, ce qui permet leur etude a l'equilibre thermodynamique. L'importance de la geometrie de dopage est soulignee, et la possibilite de realiser un dopage d#- de puits quantiques est mise en evidence. Le spectre d'energie des centres d#- confines a ete etudie pour une large gamme de champs magnetiques (jusqu'a 20 t) et pour differentes largeurs de puits. Ces mesures permettent d'observer, outre la non-parabolicite de bande de gaas, les effets de correlation electronique augmentes par la baisse de dimensionalite. L'etude des centres d#- en presence de desordre d'alliage dans des puits de al-gaas met en evidence la possibilite de coexistence, dans ce type de systeme, de centres profonds et de centres peu profonds, tous deux charges negativement

The widely popular and successful standard fireball model for Gamma Ray Burst (GRB) afterglows is based on ultra-relativistic external shocks sweeping up matter around the explosion site to accelerate electrons up to GOV energies and boost the magnetic field to values close a few Gauss in its downstream. According to the model, the afterglow radiation is the synchrotron emission from these electrons gyrating around the enhanced magnetic field. A contribution from inverse Compton scattering may also appear in the total flux at higher frequencies. The synchrotron spectrum is characterized by 'breaks' which arise due to various physical processes. The spectral slope changes due to the synchrotron self-absorption below a frequency Va. The synchrotron peak frequency (I'm) corresponds to the emission by electrons at the lower limit of the power law distribution of energies and the cooling break 14 corresponds to the electron energy above which synchrotron radiation loss becomes very significant. Apart from these, the light curves exhibit achromatic slope changes due to dynamical processes within the fireball. The ejected matter is collimated and initially undergoes a radial expansion. Later, the lateral expansion of the jet takes over and this is reflected as an achromatic break (jet break) in the light curve. The next achromatic change of slope marks the transition of the fireball into the non-relativistic regime. The spectrum of afterglow radiation itself evolves with time, reflecting the expansion of the fireball, hence a data set well sampled in both spectral and in temporal domain is essential for useful study. Multiband modelling of GRB afterglow (AG) light curves is at present the best available tool to understand the true nature of the explosion and its surroundings. Apart from that, detailed modelling also holds the key to the secrets of particle acceleration processes in collision less shocks. By modelling the well-sampled data set of an afterglow, the energy content (Etot) of the jet, its angle of collimation (00), the density profile of the ambient medium (n(r) where r is the distance from the site of the explosion) and some relevant parameters of shock microphysics (p, the power law index of the distribution of electrons which are radiating via synchrotron mechanism, G, the fraction of energy in those electrons and that in downstream magnetic field) can be obtained. Afterglow data of the nearby (z 0.16, one of the nearest GRBs) GRB 030329 was unprecedentedly rich in both optical and radio bands (but unfortunately poor in x-rays) which enabled detailed and well constrained modelling attempts. The rigorous monitoring campaign revealed an unexpected behavior of the radio flux, for which one explanation was that that the early optical emission and the late radio emission arose in two different jets. However, our detailed modelling using the rich data set allowed us to propose a new mechanism in which the initial outflux of energy is 'refreshed' by a later episode of injection. The standard fireball model uses certain simplistic assumptions owing to our lack of knowledge of the shock acceleration process. One common assumption is that of a universal spectrum of the accelerated electrons, a steep non thermal energy distribution with power law of index 2.2. It owes its origin to theoretical simulations of shock acceleration which often produce a steep (p > 2) spectrum. This also fits many observed cases of such energy distributions. Further, this assumption leads to a simplification in theoretical models, since the upper cut off energy of the distribution plays virtually no role. The presence of harder, p < 2 spectrum, in a minority of cases, has hence not received a fair share of attention. Calculations to derive the physical parameters of the burst in such cases are often not done consistently. Early attempts to model GRB afterglows with hard electron energy spectrum had several loopholes. In this thesis, we have done these calculations consistently and applied them to a few afterglows with good temporal and spectral coverage. Apart from multiband modelling, this thesis also presents late time observations of the GRB030329 afterglow in low frequency radio bands. Radio observations have always been special since they allow the estimation of the self-absorption frequency, thus giving a direct clue to the size of the fireball. Afterglows are long lived in low radio frequencies (< 1 GHz) while they quickly decay below visibility in all other bands, even at high radio frequencies (say 15 GHz). Hence monitoring at low radio frequencies is the only way to study the late time evolution including the transition from relativistic to non-relativistic dynamics. GRB030329 had one such rare bright radio afterglow and we followed it up in low frequencies (1280 MHz and 610 MHz) using the Giant Meter wave Radio Telescope (GMRT). The follow up campaign is continuing thanks to the slow evolution in low radio frequencies. This afterglow has hence become the longest (N 1000 days) observed, beating the earlier record of 500-day long observations of Radio afterglow of GRB970508. It also is the only one which is seen in frequencies below 1 GHz. This thesis is organized in the following manner: Chapter 1 gives a general introduction to GR13s and their afterglows. After describing the properties of the burst and the afterglow, we proceed to explain the standard fireball model in detail. The dynamics of the external shock and the profile of the bulk Lorentz factor (I") vs. r is described. We explain the jet break (tj) and non-relativistic transition (tnr), two major developments in the life of the fireball. We then give a detailed description of the synchrotron radiation mechanism, which is the source of afterglow radiation. The spectral breaks (va, I'm and 14) and their time evolution is explained. We conclude this chapter by listing a few unanswered questions relevant to this thesis. In Chapter 2, we present the theoretical modifications required for the standard model to accommodate electron energy spectra with power-law indices less than 2. The energy spectrum requires a new parameter 7i, which is the Lorentz factor corresponding to the upper cut-off of the hard energy distribution. Above 'i, the distribution either terminates or steepens (double slope electron distribution) to a value of p larger than 2. The functional form of this cut-off is decided by the particle acceleration processes, which are at present poorly understood. We therefore parameterized the temporal evolution of in terms of the bulk Lorentz factor of the shock. We discuss two possible origins for the cut off. As a result of this cut-off in the energy spectrum, a new break Vi is introduced in the radiation spectrum, which is the synchrotron frequency corresponding to 7i. Apart from that, the expressions for l.'m and va differ from the standard scenario. We have calculated the shock dynamics using the method adopted by Huang et. al. 2000, which allows a smooth transition from ultra-relativistic to nonrelativistic regime of the fireball. Using this profile of I' vs. observed time, we calculated the synchrotron spectral evolution from a double slope electron energy distribution semi-analytically. The self-Compton emission also is calculated. For ultra-relativistic and non-relativistic regimes, analytical solutions are presented for both ISM n(r) r0] and stellar wind driven [n(r) ambient medium density profiles. The way one identifies potential candidates which could have an underlying hard electron energy spectrum, is by looking at the light curve decay index past the jet break. The choice is confirmed by the optical and x-ray spectral indices. According to the standard model, the flux in higher frequencies, past jet break, decay as a power-law of index p; the spectrum below 14 should have a slope of (p — 1)/2 and above it should fall as pp. The value of p one thus obtains from all these methods is expected to be consistent. In chapter 3, we chose three such afterglows (GRB010222, GRB020813 and GRB041006), which show shallow decay of fluxes in the optical as well as in x-ray bands and relatively flat spectra. Out of a dozen such afterglows, these three have well sampled multi-band light curves. We fitted the data set with the model and estimated the physical parameters. For we have estimated the contribution of the associated supernova by subtracting the afterglow model from the total emission. We found the contribution from Compton emission to be negligible in all these cases. Interestingly, all these afterglows had relatively low cooling frequency, which could perhaps be due to some unknown relation to the acceleration mechanism itself. Chapter 4 and 5 are devoted to GRB030329, one of the best monitored afterglows till date. The 4th Chapter focuses on the radio observations of the afterglow done with the GMRT at low frequencies. To begin with, we give a brief introduction to the interferometric techniques and the instrument. GMRT, an interferometric array with 30 elements, each of diameter 45 meters has an excellent sensitivity at low frequencies which allowed it to detect and monitor the afterglow for a long time. We then present observations in 1280 MHz and 610 MHz bands during the second year of the afterglow Thanks to this long coverage, we were able to pin-point the location of va and the transition of the fireball to the Newtonian regime. Chapter 5 describes the multiband modelling of this afterglow. The evolution of the afterglow was complex. While the afterglow flux in optical as well as in x-ray exhibited a jet break around half a day, the radio flux past 0.5 days did not follow the expectations from a jet which has already entered the lateral expansion regime. Instead, it showed an achromatic steepening around 10 days. Hence, a novel suggestion of two co-aligned jets, one narrow and one wide, together giving rise to the observed flux has emerged (Berger et. al. 2003). We test the predictions of this conjecture and get a refined set of parameters, prompted primarily by the additional data from GMRT. We then proceed to suggest a different scenario in which the initial jet which gave rise to the x ray and optical flux is reenergized by the central engine during its lateral expansion that makes it once again collimated, now to a wider opening angle. This new jet enters a lateral expansion phase around 10 days, resulting in the jet break seen in radio bands. One peculiarity of this GRB was its association with a supernova (SN2003dh) which dominated the optical flux beyond a week. The refined afterglow flux calculation allowed us to subtract the afterglow contribution from the total optical flux and compare the resulting supernova contribution with the stereotype SN1998bw. While being similar in light curve, SN2003dh is fainter compared to a redshifted SN1998bw. The contribution of this thesis lies in presenting a consistent modelling platform for 'hard' electron energy spectra as well as in the low frequency campaign of GRB030329 afterglow and the interpretation of its evolution. Chapter 6 concludes the thesis along with a few suggestions for future directions.

No
The effect of a self-induced electric field is investigated analytically and numerically on differential and mean electron spectra produced by beam electrons during their precipitation into a flaring atmosphere as well as on the emitted hard X-ray (HXR) photon spectra. The induced electric field is found to be a constant in upper atmospheric layers and to fall sharply in the deeper atmosphere from some "turning point" occurring either in the corona (for intense and softer beams) or in the chromosphere (for weaker and harder beams). The stronger and softer the beam, the higher the electric field before the turning point and the steeper its decrease after it. Analytical solutions are presented for the electric fields, which are constant or decreasing with depth, and the characteristic "electric" stopping depths are compared with the "collisional" ones. A constant electric field is found to decelerate precipitating electrons and to significantly reduce their number in the upper atmospheric depth, resulting in their differential spectra flattening at lower energies (<100 keV). While a decreasing electric field slows down the electron deceleration, allowing them to precipitate into deeper atmospheric layers than for a constant electric field, the joint effect of electric and collisional energy losses increases the energy losses by lower energy electrons compared to pure collisions and results in maxima at energies of 40-80 keV in the differential electron spectra. This, in turn, leads to the maxima in the mean source electron spectra and to the "double power law" HXR photon spectra (with flattening at lower energies) similar to those reported from the RHESSI observations. The more intense and soft the beams are, the stronger is the lower energy flattening and the higher is the "break" energy where the flattening occurs.

博士
國立交通大學
電子研究所
82
The energy spectra of a symmetric quantum well and single barrier within an in-plane magnetic field are exactly derived and the numerical results are calculated by iteration. A critical dis- tance between the edge of electron spiral radius and the barrier boundary is introduced to describe the relation of well or width and the critical magnetic field beyond which the Landau levels will dominate. We can predict by a simple equation when bulk Landau levels will dominate in the middle of the well or barrier region. The energy spectra of an electron in an asym- metric quantum well and an asymmetric quantum double barrier within in-plane magnetic field and electric field perpendicular to the barrier interfaces are exactly derived and the numerical results are calculated by iteration. The ground and excited are calculated for various asymmetric potential barriers, well widths, barrier widths and electric fields. Major different cha- racteristics of energy spectra between wide and narrow wells are discussed. The behaviors of bulk Landau level in well region of a larger well and the elevation of the ground state in a smaller well width are explained. It is found that the density distribu- tion of electrons in a large well region is strongly affected by the asymmetric potential configuration and electric field in the growth direction. The characteristics of the energy spectrum are discussed by the superposition of well-and barrier- potential sub- systems. Major different characteristics of energy spectra with variation of wire radius and potential configuration are The behaviors of bulk Landau levels under large perpendicular netic field are explained. The characteristics of crossing points within neighboring modes is explained by the combined effect of well-and barrier-subsystems.

Thin metallic films with magnetic properties like magnetite are an interesting material in current technological applications. In the presented work the iron oxide films are grown by molecular beam epitaxy on MgO(001) substrates at temperatures between room temperature and 600K. The film and surface structure are investigated by x-ray reflectometry (XRR), x-ray diffraction (XRD) and low energy electron diffraction (LEED). The chemical properties are investigated by x-ray photoelectron spectroscopy (XPS). Furthermore, charge transfer multiplet (CTM) calculations are performed as a means to gain additional information from photoemission spectra. It is shown that only for temperatures higher than 500K the oxide film forms a spinel structure. A previously unobserved (2x1) surface reconstruction in two orthogonal domains is found for various preparation conditions. The application of CTMs results in good quantitative and qualitative agreement to other methods for the determination of the film stoichiometry. In addition CTMs can well describe the segregation of Mg atoms into the oxide film either during film growth or during film annealing. It is found that initially Mg substitutes Fe on all possible lattice sites, only for prolonged treatment at high temperature do Mg atoms favour the octahedral lattice sites of divalent Fe.