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.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/96015 |
| Date | 12 1900 |
| Creators | Smit, Albert Bart |
| Contributors | Schwoerer, Heinrich, Stellenbosch University. Faculty of Science. Dept. of Physics. |
| Publisher | Stellenbosch : Stellenbosch University |
| Source Sets | South African National ETD Portal |
| Language | en_ZA |
| Detected Language | English |
| Type | Thesis |
| Format | x, 73 p. : col. ill. |
| Rights | Stellenbosch University |
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