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Portable X-ray fluorescence and nuclear microscopy techniques applied to the characterisation of southern African rock art paintingsSteyn, Ruan 04 1900 (has links)
Thesis (MSc)--Stellenbosch University, 2014. / ENGLISH ABSTRACT: Non-destructive portable X-ray Fluorescence (pXRF) and Particle Induced X-ray
Emission (PIXE) were used to measure the elemental concentration of rock art
fragment paintings. For pXRF the Amptek Silicon Drift Detector (SDD) and Niton
XL3t spectrometers were used to perform the measurements. These two
spectrometers use different spectrum analysis methods. The Peak Deconvolution
(PD) analysis method is used for the Amptek SDD and an Inverse Overlap Matrix
(IOM) method is used for the Niton XL3t spectrometer.
The pXRF methods were validated by using alloys, coins and rock standards. The
validation is important to establish if the pXRF technique is properly understood and
used and to advance the investigation to more complex rock art paintings, with
heterogeneous and layered properties. The elemental concentrations obtained for
the Standard Reference Materials (SRMs), which were used for the validation, were
in good agreement with that of the known concentration of the SRMs.
The two rock art fragments which were analysed from the Mount Ayliff and Ha
Khotso caves were part of larger rock art painting prior to it being naturally exfoliated
from the rock. For the Mount Ayliff rock art, seven paint points, two unpainted rock
(varnish) point adjacent to the paint and the back of the rock were analysed. The
colour of the paint ranged from black, shades of brown and shades of red. The black
paint is due to manganese or charcoal. The red colour is due to iron oxide and the
red-brown colour is due to Hematite (a type of ferrous oxide) [1]. For the Ha Khotso
fragment the paint on the front of the rock and the rock substrate (back of the rock)
were analysed.
For the Mount Ayliff rock art fragment the results for both pXRF spectrometers
indicated that the elemental concentration was uniform across the fragment. This is
due to the formation of a uniform layer of minerals such as silica and calcium
introduced by the seepage of water through the cracks of the cave. Therefore no
correlation could be established between the colour of the rock art paint and the
elements detected, as was found with the work done by Peisach, Pineda and
Jacobson [1]. For the Ha Khosto rock fragment a relation between the Ca
composition and the cream colour of the rock art paint was established. Both the
PIXE and pXRF techniques were used to identify the compound concentrations of
the Ha Khotso rock art fragment. The comparison between the two techniques
highlights the complexity of rock art paint analysis. The results from the PIXE
elemental mapping indicated the non-uniform distribution of the elements in the
analysed region. From the rock art fragment measuring the analysed points 5 times and obtaining the
same results, indicated that the particle size and inhomogeneities did not have much
effect on the compound compositions.
In order to obtain high accuracy results with pXRF, sound scientific methodology with
specific knowledge and expertise, not only about the XRF technique, but also about
the sample under investigation is required. For alloy analysis pXRF is well suited, the
analysis of geological material however more complex, since they are composed
predominately of low atomic elements e.g. silicon, aluminium, magnesium, sodium,
oxygen and carbon – all of which are excited with very low efficiencies. / AFRIKAANSE OPSOMMING: Nie-beskadigended X-straal Fluoresensie (pXRF) en Deeltjie Geinduseerde X-straal
emmissie (PIXE) was gebruik om die elementêre konsentrasie van die rotstekeninge
in hierdie studie te bepaal. Vir die pXRF-tegniek is die “Amptek Silicon Drift Detector
(SDD)” en die “Thermo Scientific Niton XL3t” spektrometers gebruik gemaak om die
metings uit te voer. Die twee spektrometers maak gebruik van verskillende spektrum
analiseringsmetodes.Die “Peak Deconvolution (PD)” analiseringsmetode is gebruik
vir die “Amptek SDD” en die “Inverse Overlap Matrix (IOM)” analiseringsmetode is
gebruik vir die “Thermo Scientific Niton XL3t” spektrometer.
Vir die validasie van die pXRF-metode is van allooie, muntstukke en rots
standaarded gebruik gemaak. Die validasie is belangrik om vas te stel of die pXRF
tegniek behoorlik verstaan en gebruik word en om die ondersoek te bevorder na
meer komplekse rotstekeninge, met heterogene en lae eienskappe. Die element
konsentrasies wat vir die “Standard Reference Material (SRM)” wat gebruik is vir die
validasie, was in 'n goeie ooreenkoms met die van die konsentrasie van die SRM,
wat bekend is.
Die twee rotstekeninge wat ontleed is van die Mount Ayliff en Ha Khotso grotte en
was deel van 'n groter rots kuns skildery voordat hul natuurlik afgebreek het. Vir die
Mount Ayliff rotskuns, is sewe verf punte, twee ongeverfde rots (vernis) punte
aangrensend aan die verf en die agterkant van die rots ontleed. Die kleur van die
verf het gewissel van swart, skakerings van bruin en skakerings van rooi. Die swart
verf kan toegeskryf word aan mangaan of houtskool. Die rooi kleur is as gevolg van
ysteroksied en die rooi-bruin kleur is as gevolg van Hematiet ('n tipe van yster
oksied) [1]. Vir die Ha Khotso rotskuns is die verf aan die voorkant van die rots en
die rots substraat (agterkant van die rots) ontleed.
Vir die Mount Ayliff rotstekening het die resultate vir beide pXRF spektrometers
aangedui dat die elementele konsentrasie uniform oor die rotstekening is. Dit is as
gevolg van die vorming van 'n uniforme lagie van silica en kalsium, wat deur die
sypeling van water deur die krake van die grot na die oppervlak van die rotstekening
beweeg het. Daarom kon geen korrelasie tussen die kleur van die rotstekening en
die elemente wat gemeet is bepaal word nie, soos gevind deur die werk van
Peisach, Pineda en Jacobson [1]. Vir die Ha Khotso rotstekening is ‘n verband
tussen die room kleur van die rotstekening verf en Ca konsentrasie gevind. Beide die
PIXE en pXRF tegnieke is gebruik om die konsentrasies van die Ha Khotso
rotstekening te identifiseer. Die vergelyking tussen die twee tegnieke beklemtoon die
kompleksiteit van rotstekening verf analise. Die resultate van die PIXE elementele
karakterisering het aangedui die nie-eenvormige verspreiding van die elemente in
die ontlede area. Deur die meting van die ontlede punte 5 keer te herhaal, en dieselfde resultate
verkry, is ‘n aanduiding dat die deeltjie grootte en inhomogeniteite nie veel invloed
op die elementele konsentrasies het nie.
Ten einde 'n hoë akkuraatheid resultate te kry met pXRF, moet goeie wetenskaplike
metode toegepas word met spesifieke kennis en kundigheid, nie net oor die XRF
tegniek, maar ook oor die rotstekening wat ondersoek word vereis. pXRF is wel
geskik vir die ontleding van allooie, die ontleding van geologiese materiaal is egter
meer kompleks, aangesien die materiaal hoofsaaklik bestaan uit lae atoomgetal
elemente bv silikon, aluminium, magnesium, natrium, suurstof en koolstof - wat almal
met lae doeltreffentheid opgewek en baie afgerem word in die materiaal.
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Translocation of a polymer chain under geometric confinementGumede, Sthembiso R. 04 1900 (has links)
Thesis (MSc)--Stellenbosch University, 2014. / ENGLISH ABSTRACT: The advent of the synthesis or manufacturing of controlled structures on submicron
scales as well as experimental developments enabling the investigation
of physics in speci c biological systems at extremely small length scales underlines
the need for dealing with the statistical physics of small systems which
are geometrically con ned. A typical example of a system for which physical
questions can be answered by means of theoretical modelling is the virus,
where polymer genetic material is encapsulated in a protein shell.
In this project the role of con nement on polymer chains will be investigated.
We investigate how the translocation of polymer from one region to
another through a small opening depends on various electrolytic, polymer concentration
and wall interaction conditions. This is an extension of the simple,
purely entropic, picture in that the interaction terms enter the picture. We
employ a variational scheme in deriving our results. / AFRIKAANSE OPSOMMING: Sowel die moontlikheid van beheerbare sintese of vervaardiging van strukture
op sub-mikrometer lengteskale asook die koms van eksperimentele metodes
vir die ondersoek van biologiese stelsels op baie klein lengteskale onderstreep
hoe nodig dit is om die statiestiese sika van klein stelsels met geometriese
beperkings te verstaan. 'n Tipiese voorbeeld waar teoretiese metodes vir siese
vrae aangewend word is 'n virus, waar die polimeriese genetiese materiaal in
'n proteïen skil beweeg.
In die huidge projek word die rol van 'n spesi eke geometriese beperking op
polimeerkettings ondersoek. Ons ondersoek hoe die oorplasing van 'n polimeer
deur 'n klein opening van een gebied na die ander deur verskillende elektrolietiese,
polimeer-konsentrasie en wandinteraksie eienskappe afhang. Dit is 'n
uitbreiding van die eenvoudige, volledig entropiese beeld vir oorplasing deurdat
wisselwerkings ingesluit word. 'n Variasiebeginsel word aangewend om die
resultate af te lei.
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Development of VUV tunable laser spectroscopy techniques for characterizing calcium fluorideMatindi, Tresor 12 1900 (has links)
Thesis (MSc)--Stellenbosch University, 2014. / ENGLISH ABSTRACT: The large band gap (approximately 11.5-12.1 eV) and high transmission of calcium fluoride
(CaF2) crystal in the ultraviolet (UV) and vacuum ultraviolet (VUV) region makes it
an important material for optics for laser applications in UV. However, CaF2 degrades
during long exposure to UV irradiation due to defect generation. The formation of selftrapped
excitons (STE) is considered the first step in defect generation. In this project
the possibility of observing STE states in CaF2 using a narrow bandwidth tunable VUV
laser source is investigated. This is the first spectroscopy study of an alkaline earth fluoride
using VUV tunable laser radiation instead of a fixed wavelength laser. The use of a VUV
tunable laser source has potential for determining the energies of the STE states, which are
unknown. Our main objective is addressed by developing techniques to measure absorption
spectra of pure and doped CaF2 samples, using a VUV scanning monochromator and a
tunable VUV laser, and by doing a literature study. The results obtained with the scanning
monochromator show absorption features in 126-180 nm range of all our samples. These
vary for different samples and correlate with information from the supplier on the samples’
fluorescence spectra. Total absorption of the VUV light by CaF2 in the 115-126 nm range is
observed. With the narrow bandwidth tunable laser light, absorption spectra were obtained
in the range of 143-146.7 nm of all our CaF2 samples. No significance peaks which can be
related to the STE states in CaF2 were observed in the VUV laser absorption spectra, but
the results are valuable to improve the technique. The conclusion is that either a different
spectral range or fluorescence detection can be investigated in future. / AFRIKAANSE OPSOMMING: Die groot bandgaping (ongeveer 11.5-12.1 eV) en hoë transmissie van kalsiumfluoried
(CaF2) kristal in die ultraviolet (UV) en die vakuum ultraviolet (VUV) gebied maak dit
’n belangrike materiaal vir optika vir laser toepassings in die UV. CaF2 degradeer egter
gedurende langdurige blootstelling aan UV lig as gevolg van die generering van defekte.
Die vorming van ’n elektron-holte paar wat deur die kristalstuktuur gestabiliseer word teen
rekombinasie (self-trapped excitons, afgekort STE) word beskou as die eerste stap in defek
generering. In hierdie projek word die moontlikheid ondersoek om STE toestande in CaF2
waar te neem deur die gebruik van ’n afstembare VUV laserbron met emissie in ’n smal
spektrale band. Dit is die eerste spektroskopiese studie van ’n aardalkali-fluoried deur die
gebruik van afstembare VUV laserlig in plaas van ’n vaste golflengte laser. Die gebruik
van ’n afstembare VUV laserbron het potensiaal vir die bepaling van die energieë van die
STE teostande, wat onbekend is. Ons hoofdoel word aangespreek deur die ontwikkeling
van tegnieke vir die meet van absorpsie spektra van suiwer en gedoteerde CaF2 monsters
met behulp van ’n VUV skanderende monochromator en ’n afstembare VUV laser, en
deur ’n literatuurstudie. Die resultate wat behaal is met die skanderende monochromator
toon die absorpsieprofiele van al ons monsters in die 126-180 nm spektrale gebied. Die
absopsieprofiele varieer vir die verskillende monsters en korreleer met die inligting van die
verskaffer oor die fluoressensie spektra van die monsters. Totale absorpsie van die VUV lig
deur CaF2 in die 115-126 nm gebied is waargeneem. Met die smalband afstembare laserlig
is absorpsie spektra in die 143-146.7 nm gebied vir al ons CaF2 monsters verkry. Geen
beduidende pieke wat verband hou met die STE toestande in CaF2 is waargeneem in die
VUV laser absorpsie spektra nie, maar die resultate is waardevol vir die verbetering van
die tegniek. Die gevolgtrekking is dat of ’n ander spektraalgebied of fluoressensiedeteksie
in die toekoms ondersoek kan word.
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Introducing organic molecular crystals into ultrafast electron diffractionRohwer, Andrea Berenike 12 1900 (has links)
Thesis (MSc)--Stellenbosch University, 2014. / ENGLISH ABSTRACT: Organic molecular salts have a wide range of physical properties which can be chemically tailored
by minor variations of their substituents. These characteristics include high degrees of anisotropy,
electrical conductivity ranging from superconducting to insulating, and structural changes in the
crystal lattice during first order phase transitions brought about by minimal changes in temperature,
effective pressure, and in some cases even light. Hence, these materials are particularly interesting
for the development of molecular electronics and also as study materials in solid state physics.
The family of copper-dimethyl-dicyanoquinone-diimine (Cu(DMe-DCNQI)2) salts forms part of the
radical anion salt subclass of organic molecular crystals and is of particular interest due to its extraordinarily
high conductivity compared to other quasi one-dimensional organic conductors. Its
metal-to-insulator phase transition is characterised by conductivity jumps across several orders of
magnitude within a few kelvin. Over the past three decades the metallic and insulating phases,
as well as the transition behaviour have been investigated extensively utilising a broad spectrum
of methods amongst others electrical conductivity, electron spin resonance, and re
ectivity measurements,
x-ray photoelectron and infrared spectroscopy, x-ray diffraction, and dilatometry. Fast
light-switching between phases has been observed in partially deuterated forms of Cu(DCNQI)2 on
sub-100-ps time scales. Furthermore, the phase transition is believed to be induced by a deformation
of the crystalline lattice and a charge density wave formation which are detectable in diffraction
images. Therefore we want to investigate this metal-to-insulator phase transition structurally and
temporally via ultrafast electron diffraction. The technique of ultrafast electron diffraction employs
the fundamentals of pump-probe spectroscopy: One of the two femtosecond pulsed laser beams
excites the thin, crystalline sample, while the other - after being converted into a pulsed electron
beam via the photoelectric effect - forms a diffraction image of the sample's lattice structure. The
arrival time of the two pulses at the sample can be varied by a few femtoseconds with respect to
each other enabling the resolution of ultrafast structural dynamics of the crystal's atomic lattice via electron diffraction. During the work presented in this thesis the sample preparation and characterisation
leading to a successful introduction of Cu(DCNQI)2 into our ultrafast electron diffraction
setup is presented. A diffraction pattern of comparable quality to that of a commercially available
transmission electron microscope was recorded of the metallic state of partially deuterated d6
Cu(DCNQI)2. Subsequent analysis of the obtained diffraction data and further studies of the low temperature state { including simulations as well as experiments { have narrowed down the factors
still making the diffraction pattern
evasive. Possible solutions to experimental challenges are proposed to make the documentation of
structural ultrafast dynamics in these organic molecular salts an attainable goal in the future. / AFRIKAANSE OPSOMMING: Organiese molekulêre soute het `n wye verskeidenheid van fisiese eienskappe wat chemies verander
kan word deur geringe variasie in die samestelling van die sout. Hierdie eienskappe sluit in `n hoë
graad van anisotropie, elektriese geleidingsvermoë wat strek van supergeleiding tot elektriese isolasie,
en strukturele veranderinge in die kristalstruktuur tydens eerste orde fase-oorgange wat veroorsaak
word deur geringe veranderinge in temperature, effektiewe druk en in sommige gevalle selfs lig.
Gevolglik is hierdie material besonder interessant vir die ontwikkeling van molekulêre elektronika
en ook as studiemateriaal in vastetoestandfisika. Die familie van koperdimetieldisianokinoondiimien
(Cu(DMe-DCNQI)2) soute vorm `n deel van die radikaal-anioon-sout subklas van organiese
molekulêre kristalle en is van besondere belang as gevolg van hulle buitengewone hoë elektriese
geleidingsvermoë in vergelyking met ander kwasi-eendimensionele organiese geleiers. Die metaal-na-isolator fase-oorgang van hierdie kristal word gekenmerk deur die verandering van die geleidingsvermoë met verskeie ordegroottes binne `n paar kelvin. Gedurende die laaste drie dekades is
die metaal en isolator fases, asook die oorgangsgedrag deeglik ondersoek met behulp van `n wye
verskeidenheid van metodes wat onder andere elektriese geleidingsvermoë, elektron-spin resonans
en reeksiemetings, x-straal fotoelektron en infrarooi spektroskopie, x-straal diffraksie en dilatometrie
insluit. Vinnige skakeling tussen fases is waargeneem in gedeeltelik gedeuteerde vorms van
Cu(DCNQI)2 op `n sub-100-ps tydskaal. Daar word verder geglo dat die fase-oorgang geïnduseer
word deur `n deformasie van die kristalstruktuur en die vorming van `n ladingsdigtheidgolf wat
meetbaar is in elektrondiffraksiebeelde. Om hierdie rede wil ons die metaal-na-isolator fase-oorgang
se struktuur- en tydafhanklikheid ondersoek deur gebruik te maak van ultra-vinnige elektron diffraksie.
Die tegniek van ultra-vinnige elektron diffraksie maak gebruik van die beginsels van pomp-toets
spektroskopie: Een van die twee femtosekonde laserpulse wek die dun kristallyne monster op, terwyl
die ander na omskakeling in `n elektronpuls via die foto-elektriese effek `n diffraksiebeeld van die
monster se kristalstruktuur vorm. Die aankomtyd van die twee pulse by die monster kan met `n
paar femtosekondes ten opsigte van mekaar verander word om die tydresolusie van die ultra-vinnige
strukturele dinamika van die kristal se atoomstruktuur deur elektrondiffraksie moontlik te maak. In
hierdie tesis word die monstervoorbereiding en karakterisering wat gelei het tot suksesvolle eksperimente
op Cu(DCNQI)2 in ons ultra-vinnige elektron diffraksie opstelling behandel. `n Diffraksie
patroon waarvan die kwaliteit vergelykbaar is met die van `n kommersiëel beskikbare transmissie
elektron mikroskoop is gemeet vir die metaalfase van gedeeltelik gedeuteerde d6 Cu(DCNQI)2.
Daaropvolgende analiese van die gemete diffraksiedata en verdere studies van die lae temperatuur
toestand wat simulasies sowel as eksperimente insluit het `n klein aantal faktore uitgewys wat
steeds die deteksie van die isolatorfase se ladingsdigtheidgolf se kenmerkende diffraksiepatroon verhoed.
Moontlike oplossings tot eksperimentele uitdagings word voorgestel om die dokumentering
van strukturele ultra-vinnige dinamika in hierdie organiese molekulêre soute `n haalbare toekomstige
doelwit te maak.
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Structural dynamics of 1T-TiSe2 using femtosecond electron diffractionSuleiman, Aminat Oyiza 12 1900 (has links)
Thesis (MSc)--Stellenbosch University, 2014. / ENGLISH ABSTRACT: Trilayered transition metal dichalcogenides such as our sample (1T-TiSe2) have been studied
for many years as systems with strong electron-electron and electron-phonon correlations.
The main attraction to this family of compound is its potential to exhibit ground state phenomena
known as charge density waves whose detailed physical origin has been controversially
determined.
In this study, we have used an ultrafast femtosecond laser based on a pump-probe technique,
namely ultrafast electron diffraction, to investigate these exotic features associated with the
crystal. A pump laser pulse photo-excites the crystal from its ground state and the probe
pulse (ultrashort electron pulse) takes the snapshot of the evolution of the lattice generating
an electron diffraction pattern of the crystal. Hence the dynamical structural behaviour can
be observed in time with a subpicosecond temporal resolution. As a hexagonal close-packed
structure, its signature is expected to be seen in the diffraction pattern in both a steady-state
and electron time-resolved femtosecond electron diffraction.
In addition, simulations of electron diffractions pattern for room and low temperature structural
data via a software called Simulation and Analysis of Electron diffraction (SAED) have
been carried out. Clear signatures of charge density waves were seen at low temperature. / AFRIKAANSE OPSOMMING: Drie-laag oorgangsmetaal dikhalkogeniedes soos ons voorbeeld (1T-TiSe2), word reeds vir
baie jare bestudeer as sisteme met sterk elektron-elektron en elektron-fonon korrelasies. Die
hoof aantrekkingskrag van hierdie sisteme is die verskynsel van ladingdigtheidsgolwe in
die grondtoestand. Die fisiese oorsprong van hierdie ladingdigtheidsgolwe was bepaal te
midde van verskeie teenstrydighede.
In hierdie studie, maak ons gebruik van die ultravinnige femtosekonde laser gebaseerde
aktiveer-interogeer tegniek, genaamd ultravinnige elektron diffraksie (UED) om unicke eienskappe
wat met die kristal geassosieer is te bestudeer. In UED wek ’n ultravinnige laserpuls
(aktivering) die kristal op vanaf die grondtoestand waartydens n ultravinnige elektronpuls
(interogering) ’n foto neem van die evolusie van die elektron diffraksiepatroon wat deur
die kristalrooster gegenereer word. Hierdie wisselwerking van die interogerings elektronpuls
en die sisteem kan gevolglik teen verskeie vasgetelde tye toegelaat word. Dus kan die
dinamiese strukturele gedrag waargeneem word met ’n tydresolusie in die orde van die
elektronpuls (sub-pikosekondes). Siende dat die kristal ’n diggepakte-heksagonale struktuur
vorm, behoort die kenmerkende diffraksiepatroon daarvan waarneembaar te wees in
beide die bestendige diffraksie en femtosekonde elektron diffraksie tegnieke. In hierdie konteks
was duidelike tekens van ladingdigtheidsgolwe waargeneem.
Benewens was daar ook simulasies uitgevoer om die elektron diffraksiepatrone asook die
strukturele data by kamer en lae temperature vas te pen. Die sagteware wat hiervoor gebruik
word is genaamd Simulasie en Ontleding van Elektronendiffraksie (SAED) - Simulation and
Analysis of Electron Diffraction (SAED).
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Vacuum polarisation energies of two interacting scalar fields with a mass gap in (1 + 1) dimensionsCapraro, Martin Horst 12 1900 (has links)
Thesis (MSc)--Stellenbosch University, 2014. / ENGLISH ABSTRACT: The tools of perturbative Quantum Field Theory are by now a standard part of the theoretical
physicist's arsenal. In this thesis we investigate the spectral method, an approach that uses tools from
the quantum theory of scattering to calculate the O(~) corrections to fields. Speci cally, we investigate
whether the approach can be extended to deal with two interacting scalar elds with a mass gap in
(1+1) dimensions. To achieve this we need to verify the analyticity of the appropriate Jost function.
All the machinery to do so is introduced during the course of the thesis. This includes the eld theoretic
formalism which describes such a system, and the derivation of a number of di erential equations
from which the density of states can be constructed. The numerical method is also outlined. Concrete
results are presented to verify that the approach reproduces known results. Arguments related to
Levinson's theorem are then presented that suggest that the Jost function is indeed analytic, with
some caveats. / AFRIKAANSE OPSOMMING: Die gereedskap van perturbatiewe Kwantumveld-teorie is teen di e tyd al `n standaard-deel van die
teoretiese sikus se werkskis. In hierdie tesis ondersoek ons die spektrale metode, `n benadering wat
gereedskap van die kwantumteorie van verstrooing gebruik om die O(~) korreksie van velde te bereken.
Ons ondersoek spesi ek of die metode uitgebrei kan word om toepaslik te wees in die geval van twee
wisselwerkende skalaarvelde met `n massa-gaping in (1+1) dimensies. Vir hierdie doel moet ons bepaal
of die toepaslike Jost-funksie analities is of nie. Al die masjinerie om dit te bepaal word deur die loop
van hierdie tesis ingevoer. Dit sluit in die veld-teorietiese formalisme wat so `n stelsel beskryf, en die
a
eiding van `n aantal di erensiaal vergelykings wat gebruik kan word om die digtheid van toestande
te konstrueer. Die numeriese metode word ook beskryf. Konkrete resultate wat bevestig dat die
metode die korrekte antwoorde in `n analities bekende geval weergee, word verskaf. Argumente wat
verband hou met Levinson se stelling word gebruik om te bevestig dat die Jost-funksie inderdaad
analities is, met sekere voorbehoude.
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A new femtosecond electron diffractometer for structural dynamics experiments at cryogenic temperaturesSmit, Albert Bart 12 1900 (has links)
Thesis (MSc)--Stellenbosch University, 2014. / ENGLISH ABSTRACT: In this thesis, a femtosecond electron diffraction (FED) set-up that is capable of investigating
the photo-induced switching of Cu(DCNQI)2 from being an insulator to being a conductor
is presented. Movies of atomic structural changes with temporal resolution within the typical
photo-switching transition timescales (sub-picoseconds) are obtainable with this set-up
by employing a femtosecond laser. The experimental technique and the design of a crucial
instrument of the machine, the electron gun, are extensively described and characterised both
numerically and experimentally.
The interest in observing atomic structural changes of Cu(DCNQI)2 in real time is because
of the rich variety of the radical salts available that show alloy-specific Charge Density Wave
(CDW) transitions. Valuable insights about the driving mechanisms behind these structural
changes that are responsible for a change in conductivity are obtainable, as well as the relation
between crystal alloys and their transition characteristics. Electron diffraction patterns
of crystals in their metallic phase (room temperature) are shown in this thesis, but diffraction
patterns of cryo-cooled Cu(DCNQI)2 in its insulating phase are still to be acquired.
The temporal resolution of the atomic movie can be improved by recompression of electron
pulses that are debunched due to Coulomb repulsion and electron energy spread within a
pulse. Numerical and preliminary experimental results presented in this work expose the potential
of a simple compression technique. In this way, more electrons in a single electron pulse
can be afforded which allows to perform experiments at shorter integration time or lower repetition
rate. / AFRIKAANSE OPSOMMING: In hierdie tesis word ’n femtosekonde elektron diffraksie opstelling aangebied wat daartoe
in staat is om die foto-geïnduseerde omskakeling in Cu(DCNQI)2 van nie-geleier tot geleier
te ondersoek. Deur gebruik te maak van ’n femtosekonde laser in hierdie opstelling, is ’rolprente’
van strukturele veranderinge op atoomskaal met ’n tyd resolusie beter as die tipiese
foto-omskakelings tydskaal (sub-pikosekonde) verkrygbaar. Die eksperimentele tegniek en die
ontwerp van ’n noodsaaklike instrument van die masjien, die elektron geweer, word breedvoerig
beskryf en numeries en eksperimenteel gekenmerk.
Die belangstelling om strukturele veranderinge in Cu(DCNQI)2 op atoom skaal in reële tyd
waar te kan neem is as gevolg van die ryke verskeidenheid van radikale soute, wat allooispesifieke
ladings digtheid golf (CDW) oorgange toon, wat beskikbaar is. Waardevolle insigte
oor die meganismes wat hierdie strukturele veranderinge wat ’n verandering in geleiding veroorsaak
dryf is verkrygbaar, sowel as die verwantskap tussen die kristal allooi en die oorgang
kenmerke. Diffraksie patrone van kristalle in die metaalagtige fase (kamer temperatuur) word
in hierdie tesis getoon, maar diffraksie patrone van cryo-verkoelde Cu(DCNQI)2 in die niegeleier
fase moet nog verkry word.
Die tyd resolusie van die atomiese rolprent kan verbeter word deur die elektron puls — wat
deur Coulomb afstoting en elektron energie spreiding versprei is — weer saam te pers. Numeriese
en voorlopige eksperimentele resultate toon die potensiaal van ’n eenvoudige kompressie
tegniek. Hierdeur kan meer elektrone in ’n elektron puls gegun word en so die integrasie tyd
of die herhalingstempo van die eksperimente verkort kan word.
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Spectral characterization of materials using terahertz time domain spectroscopy (THz-TDS)Hissen, Huzifa Zain Alabdeen Abdarahman 12 1900 (has links)
Thesis (MSc)--Stellenbosch University, 2014. / ENGLISH ABSTRACT: Terahertz (THz) radiation is often used in many promising applications such as information
and communication technology and airport security. Optimized and adapted terahertz fields
hold huge promise for leading this technology further. This study is focused on terahertz time
domain spectroscopy (THz-TDS). In THz-TDS the electric field is measured, therefore both
amplitude and phase information of the THz pulse can be obtained. We used the pump-probe
technique in order to measure a THz pulse from the photoconductive antenna. A pulsed fiber
laser with FWHM of ' 100 fs was used for this. The frequency spectrum of the measured THz
pulse was obtained via a fast Fourier transform. We studied the principles of the THz pulse
generation as well as detection, with a photoconductive antenna as emitter and detector. In this
study terahertz spectroscopy has been used to investigate the refractive index and absorption
coefficient of different types of materials in the terahertz region. The last part of this study deals
with a simple process for material parameter extraction of a polymer sample using commercial
software called Teramat1.0. It uses the sample thickness, the reference THz pulse and the
transmitted THz pulse to retrieve the complex refractive index of the sample. / AFRIKAANSE OPSOMMING: Terahertz (THz) straling word gereeld gebruik vir belowende toepassings soos inligting en kommunikasie
tegnologie en lughawe sekuriteit. Geoptimeerde en aangepaste terahertz velde dra by
tot die bevordering van die tegnologie. Hierdie studie fokus op terahertz tyd domein spektroskopie
(THz-TDS). In THz-TDS word die elektriese veld gemeet en dus word beide amplitude
en fase inligting van die THz puls verkry. Ons gebruik ’n pomp en toets tegniek om die THz
puls deur ’n fotogeleidende antenna te bepaal. ’n Gepulseerde vesel laser met FWHM van 100
fs word hiervoor benut. Die frekwensie spektrum van die gemete THz puls word bereken deur
’n vinnige Fourier transvorm te bereken. Die beginsels van die generering en deteksie van THz
pulse is bestudeer met ’n fotogeleidende antenna as sender en ontvanger. In die studie is terahertz
spektroskopie gebruik om die brekingsindekse en die absorpsie koeffisiënte van verskillende
materiale in die terahertz gebied te bepaal. Die laaste gedeelte van die studie handel oor ’n
eenvoudige proses om die materiaal parameters van ’n polimeer te bepaal deur gebruik te maak
van kommersiële sagteware Teramat 1.0. Die monster dikte, die THz verwysingspuls en die
deurgelate puls word gebruik om die komplekse brekingsindeks van die materiaal te bereken.
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Laser spectroscopy of the Fourth Positive System of carbon monoxide isotopomersDu Plessis, Anton 03 1900 (has links)
Thesis (PhD (Physics))--University of Stellenbosch, 2006. / Carbon monoxide (CO) is a diatomic molecule of particular interest in astrophysics, due to
its high abundance in interstellar space. The Fourth Positive System A1Π−X1Σ+ of CO is an
important feature in the vacuum ultraviolet (VUV) region of the electromagnetic spectrum
in astronomical observations, especially in high-resolution spectra recorded by satellite-based
spectrographs. The interpretation of these astronomically detected spectra requires accurate
laboratory wavelengths to serve as rest wavelengths and to resolve possible Doppler-shifts.
Such rest wavelengths are known for the 12C16O, 13C16O and 12C18O isotopomers for all
astronomically observed spectral lines of the Fourth Positive System. The only laboratory
wavelengths currently available for the Fourth Positive System of the 12C17O isotopomer
have been determined in a previous work carried out in our laboratory for the vibronic band
A1Π(v0 = 3)−X1Σ+(v00 = 0). The present study continues this work for the other vibronic
bands which have been detected astronomically, namely A1Π(v0 = 2 − 5)−X1Σ+(v00 = 0).
The A1Π(v0 = 0− 1)−X1Σ+(v00 = 0) vibronic bands have also been investigated due to their
probability for future astronomical detection. Rotationally-resolved spectra of these six vibronic
bands were obtained by selective rovibronic laser excitation, and subsequent detection
of the undispersed fluorescence, observed as a function of the excitation wavelength in the
VUV. A tunable narrow-bandwidth VUV laser source is used for excitation, and the CO gas
sample is introduced by supersonic expansion. Flow-cooling in the supersonic expansion to
rotational temperatures roughly corresponding to temperatures in the interstellar medium
simplifies and aids the spectral analysis of the spectral lines of interest. The cold conditions
in the supersonic expansion facilitates a high sensitivity for detection of the low-J lines, and
allows the detection of rare isotopomers of CO in natural abundance. The experimental
setup has been improved in the present study by the addition of a vacuum monochromator,
facilitating an improved characterisation of the VUV source. Furthermore, a number
of experimental conditions have been optimised for the detection of rare CO isotopomers,
significantly increasing the signals of these lines in the spectra. The combination of this
increase in sensitivity and the addition of the vacuum monochromator to the experimental
setup, allowed the simultaneous detection of absorption spectra with the fluorescence spectra
as an additional source of information in spectral analysis. The increased sensitivity
also contributed to the detection of a large number of spectral lines of interest, with some
additional lines identified in the previously studied vibronic band. Spectral lines of 12C16O,
13C16O, 12C18O and 12C17O were detected in each vibronic band, allowing accurate calibration
of the spectra. A total of 29 new lines of 12C17O were recorded in the six vibronic
bands investigated. Additionally, 10 new singlet-triplet lines of 12C16O were recorded in
the wavelength regions investigated. The new wavelengths of 12C17O have been applied to
calculate consistent heliocentric velocities of a gas cloud toward the star X Persei, obtained
from spectra of the different CO isotopomers taken by the Hubble space telescope.
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Near-target and other heavy residues in the interaction of ¹²C and ¹⁶O with ¹⁰³RhButhelezi, E. Z.(Edith Zinhle) 04 1900 (has links)
Dissertation (PhD)--University of Stellenbosch, 2004. / 175 Leaves printed single pages, preliminary pages i-xiii and 147 numberd pages. Includes bibliography. List of figures, List of tables. / ENGLISH ABSTRACT: This study forms part of a larger investigation which has as a primary objective the development of a
comprehensive theoretical description of all the processes which contribute to the continuum in the
interaction of 12C and 16O with nuclei. Previous investigations of 12C and 16O induced reactions on
targets with mass close to A = 100 have shown that the experimental excitation functions and recoil
range distributions of heavy residues can be reproduced satisfactorily by means of a theoretical model
which takes relatively few dominant reaction mechanisms into account. These include the complete
fusion of the projectile with the target, the incomplete fusion of break-up α-type fragments (i.e. single
α particles, 8Be fragments and for the 16O induced reactions also 12C fragments) with the target and
single-nucleon transfer at incident energies above about 15 MeV/nucleon. The mean-field interaction
is mainly responsible for these interactions. The thermalization of the intermediate excited nuclei
produced in this first stage of the reaction is described by an intranuclear interaction cascade, during
which pre-equilibrium emission of particles and clusters may occur, followed by evaporation after
statistical equilibrium has been attained. The model also included the probability that break-up α
particles may escape with a large fraction of their initial energy after only a few interactions with
individual target nucleons following their initial incomplete fusion. The theory also predicted an
enhanced isobaric yield for residues with mass similar or near to that of the target.
The subsequent analysis of the emission spectra of intermediate mass fragments in these
reactions, however, indicated that two additional aspects need to be considered as well in order to
reproduce the experimental data. The first is that the projectile may lose a substantial amount of
energy in an initial-state interaction before breaking up, which can be described as a friction
dissipative process. The second is that several other incomplete fusion channels of “non-α-cluster”-
type fragments should also be included in a more complete description of these reactions as their
contributions are not negligible.
The present study has two main objectives. Firstly, to investigate the isobaric yield in the neartarget
mass region by measuring production cross sections for 103Pd, 103mRh and 103Ru. Previous
studies only provided data for 103Ag, which constitute only a few percent of the A = 103 isobaric yield.
The new data constitute more than 80% of the A = 103 isobaric yield, which provide experimental
confirmation of the enhanced isobaric yield in the near-target mass region. The second objective is to
perform extensive new calculations of the excitation functions and recoil ranges in order to investigate
the predictive power of the extended model in a priori calculations for the entire available data set. / AFRIKAANSE OPSOMMING: Hierdie studie maak deel uit van ‘n meer omvattende ondersoek wat as ‘n primêre doelwit die
beskrywing van al die bydraende prosesse tot die kontinuum in die interaksie van 12C en 16O met
kerne behels. In vorige ondersoeke van 12C en 16O geïnduseerde reaksies op skywe met massa naby
A = 100 kon die eksperimentele opwekkrommes van swaar reskerne en reikwydte distribusies van
terugslagkerne bevredigend gereproduseer word met behulp van ‘n teoretiese model wat slegs enkele
dominante reaksiemeganismes in berekening bring. Hierdie sluit in die volledige versmelting van die
projektiel met die skyfkern, die onvolledige versmelting van opbreek α-tipe fragmente (d.w.s. α-
deeltjies, 8Be fragmente, en in die geval van 16O geïnduseerde reaksies ook 12C fragmente) met die
skyfkern, en enkel-nukleon oordrag by invalsenergië wat hoër is as ongeveer 15 MeV/nukleon. Die
gemiddelde-veld interaksie is hoofsaaklik verantwoordelik vir bogenoemde reaksie meganismes. Die
oorgang na termiese ewewig van die opgewekte tussenkerne wat in hierdie eerste stadium van die
reaksie gevorm word, word beskryf deur ‘n intrakern interaksie kaskade wat gekenmerk word deur die
voorewewigs emissie van deeltjies en klonte van deeltjies, gevolg deur verdamping nadat statistiese
ewewig bereik is. Dié model sluit ook die waarskynlikheid in dat opbreek α-deeltjies kan ontsnap met
‘n betekenisvolle fraksie van hul aanvanklike energie na slegs enkele interaksies met individuele
skyfnukleone nadat hulle aanvanklik onvolledig versmelt het.
In latere studies van die emissiespektra van intermediêre massa fragmente in hierdie reaksies het
dit egter geblyk dat twee addisionele aspekte ook in berekening geneem moet word om die
eksperimentele data te reproduseer. Eerstens kan die projektiel ‘n substansiële hoeveelheid energie
verloor in ‘n aanvangstoestand interaksie voordat dit opbreek, wat beskryf kan word as ‘n wrywingdissipatiewe
proses. Tweedens kan verskeie ander onvolledige versmeltingskanale van fragmente met
‘n nié-α-karakter ook betekenisvol bydra en kan hulle dus nie verwaarloos word in ‘n meer volledige
beskrywing van hierdie reaksies nie.
Die huidige studie het twee hoofdoelwitte. Eerstens word die isobariese opbrengs in die nabyskyfgebied
ondersoek deur die produksie kansvlakke van 103Pd, 103mRh en 103Ru te meet. In vorige
studies is slegs data verkry vir 103Ag, wat net ‘n klein persentasie van die A = 103 isobariese opbrengs
verteenwoordig. Die nuwe data verteenwoordig meer as 80% van die A = 103 isobariese opbrengs,
wat eksperimetele bevestiging verleen dat ‘n verhoging in die isobariese opbrengs in die nabyskyfgebied
bestaan. Die tweede doelwit is om ‘n volledige stel nuwe a priori berekeninge te doen vir
al die opwekkrommes van reskerne en reikwydte distribusies van terugslagkerne wat tans beskikbaar
is om sodoende die voorspellings van die nuwe uitgebreide teoretiese model te toets.
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