598 research outputs found
Spherical Harmonics Models and their Application to non-Spherical Shape Particles
The dissertation investigates spherical harmonics method for describing a particle shape. The main object of research is the non-spherical shape particles. The purpose of this dissertation is to create spherical harmonics model for a non-pherical particle. The dissertation also focuses on determining the suitability of the lowresolution spherical harmonics for describing various non-spherical particles. The work approaches a few tasks such as testing the suitability of a spherical harmonics model for simple symmetric particles and applying it to complex shape particles. The first task is formulated aiming to test the modelling concept and strategy using simple shapes. The second task is related to the practical applications, when complex shape particles are considered. The dissertation consists of introduction, 4 chapters, general conclusions, references, a list of publications by the author on the topic of the dissertation, a summary in Lithuanian and 5 annexes. The introduction reveals the investigated problem, importance of the thesis and the object of research, describes the purpose and tasks of the thesis, research methodology, scientific novelty, the practical significance of results and defended statements. The introduction ends in presenting the author’s publications on the topic of the dissertation, offering the material of made presentations in conferences and defining the structure of the dissertation. Chapter 1 revises the literature: the particulate systems and their processes, shapes of the particles and methods for describing the shape, shape indicators. At the end of the chapter, conclusions are drawn and the tasks for the dissertation are reconsidered. Chapter 2 presents the modelling approach and strategies for the points of the particle surface, spherical harmonics, the calculation of the expansion coefficients, integral parameters and curvature and also the conclusions. Chapters 3 and 4 analize the modelling results of the simple and complex particles. At the end of the both chapters conclusions are drawn. 5 articles focusing on the topic of the dissertation have been published: two articles – in the Thomson ISI register, one article – in conference material and scientific papers in Thomson ISI Proceedings data base, one article – in the journal quoted by other international data base, one article – in material reviewed during international conference. 8 presentations on the subject of the dissertation have been given in conferences at national and international levels
Reconstruction methods for single-shot diffractive imaging of free nanostructures with ultrashort x-ray and XUV laser pulses
With x-ray and XUV single-shot diffractive imaging on free nanoparticles it is possible to investigate structure and shape of the particles. The scattering image of the nanoparticle only contains the intensity distribution but not the phase of the scattered light. Thus, numerical methods are required to infer information from experimental data. In the thesis, different reconstruction methods are implemented, advanced and applied to different scattering scenarios to characterize diffraction patterns for different laser parameters, reconstruct optical properties and 3d shapes of nanotargets.Mit Einzelschussmessungen an freien Nanoteilchen mit Hilfe von Röntgenstrahlung ist es möglich, die Teilchenstruktur zu untersuchen. Damit die Information aus Experimenten gewonnen werden kann, sind numerische Methoden notwendig, da das Streubild der Nanoteilchen nur die Inensitätsverteilung des gestreuten Lichtes, aber nicht die Phase bereitstellt. Es werden verschiedene Rekonstruktionsmethoden implementiert, weiterentwickelt und auf verschiedene Streuszenarien angewandt, um Streubilder für verschiedene Laserparameter zu charakterisieren, optische Eigenschaften und 3D-Formen zu rekonstruieren
Spatio-temporal isolation of attosecond soft X-ray pulses in the water window
We demonstrate experimentally the isolation of single attosecond pulses at
the carbon K- shell edge in the soft-X-ray water window. Attosecond pulses at
photon energies that cover the principal absorption edges of the building
blocks of materials are a prerequisite for time resolved probing of the
triggering events leading to electronic dynamics such as exciton formation and
annihilation. Herewith, we demonstrate successful isolation of individual
attosecond pulses at the carbon K edge (284 eV) with a pulse duration below 400
as and with a bandwidth supporting a 30 as pulse duration. Our approach is
based on spatio-temporal isolation of ponderomotively shifted harmonics and
validates a straightforward and scalable approach for robust and reproducible
attosecond pulse isolation
The power of wavelets in analysis of transit and phase curves in presence of stellar variability and instrumental noise I. Method and validation
Stellar photometric variability and instrumental effects, like cosmic ray
hits, data discontinuities, data leaks, instrument aging etc. cause
difficulties in the characterization of exoplanets and have an impact on the
accuracy and precision of the modelling and detectability of transits,
occultations and phase curves. This paper aims to make an attempt to improve
the transit, occultation and phase-curve modelling in the presence of strong
stellar variability and instrumental noise. We invoke the wavelet-formulation
to reach this goal. We explore the capabilities of the software package Transit
and Light Curve Modeller (TLCM). It is able to perform a joint radial velocity
and light curve fit or light curve fit only. It models the transit,
occultation, beaming, ellipsoidal and reflection effects in the light curves
(including the gravity darkening effect, too). The red-noise, the stellar
variability and instrumental effects are modelled via wavelets. The wavelet-fit
is constrained by prescribing that the final white noise level must be equal to
the average of the uncertainties of the photometric data points. This helps to
avoid the overfit and regularizes the noise model. The approach was tested by
injecting synthetic light curves into Kepler's short cadence data and then
modelling them. The method performs well over a certain signal-to-noise (S/N)
ratio. In general a S/N ratio of 10 is needed to get good results but some
parameters requires larger S/N, some others can be retrieved at lower S/Ns. We
give limits in terms of signal-to-noise ratio for every studied system
parameter which is needed to accurate parameter retrieval. The wavelet-approach
is able to manage and to remove the impacts of data discontinuities, cosmic ray
events, long-term stellar variability and instrument ageing, short term stellar
variability and pulsation and flares among others. (...)Comment: Submitted to A&A. 11 pages, 14 figure
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