No description available.

<p>The mechanism for defect related green emission from zinc (ZnO:Zn) and sulfur doped ZnO (ZnO:S) are determined through optical characterization of the green and UV emission bands. ZnO:Zn is prepared by heating ZnO in a slightly reducing atmosphere for 1 hour and sulfur doped ZnO is similarly obtained with a small amount of sulfur added. Photoluminescence (PL), photoluminescence excitation spectra (PLE), and quantum efficiency measurements are analyzed to determine the mechanism of the green defect emission. Low temperature PL and PLE measurements are used to assign activation energies to the emission processes and connect them with donor bound excitons in ZnO. It was determined that both ZnO:Zn and ZnO:S have a similar green emission mechanism. This common mechanism involves the formation of donor bound excitons <italic>I_3a</italic> and <italic>I₉</italic>, which were determined to be the mediators between photoexcitation of excitons and the transfer of energy to the defect responsible for green emission. The most efficient excitation processes for both the green and band edge emissions at low temperatures is through direct excitation of the neutral donor bound exciton <italic>I₉</italic> or by ionizing the neutral donor bound exciton <italic>I_3a</italic>. The ionization of <italic>I_3a</italic> eliminates this exciton localization site and simultaneously creates a bound exciton at <italic>I₉</italic>. The <italic>I₉</italic> bound exciton can then either transfer energy to the defect responsible for the green emission or contribute to the free exciton population through a phonon assisted transition. At room temperature a resonant absorption peak associated with <italic>I₉</italic> is still present in the absorption band for ZnO:Zn suggesting partial localization at <italic>I_3a</italic> and <italic>I₉</italic> of free excitons with low kinetic energy (excitations below the band gap) continues to be the intermediate between excitons and the energy transfer to the green emitting defect. </p><p>In ZnO:S, the addition of sulfur creates ZnS domains within the lattice leading to a type II band alignment at the interface of ZnO and ZnS domains. This band alignment at the interface increases the density of free electrons in ZnO, which may then encounter an ionized <italic>I_3a</italic> site converting it to its neutral form. Increasing the density of free electrons, a result of the type II band alignment, increases the chances of returning an ionized <italic>I_3a</italic> to its neutral form and thus increases the green emission. These results can lead to informed optimization of ZnO:S as a potential white light emitting phosphor.</p> / Dissertation

Entanglement is one of the most technologically important quantum phenomena and its con-trolled creation brings us a step closer to the realisation of a quantum computer. Hybrid electron and nuclear spin systems which combine long nuclear decoherence times with the high polarisation and rapid processing times of electron spins are considered reliable candidates for the representation of the fundamental building block of a quantum computer, the qubit. In the literature electron spins quite often play the role of a mediator which can access, manipulate and couple states with long coherence times, beneficial for storing quantum information. Despite the fact that an electron spin can be a useful resource for nuclear spin systems, its permanent presence can be a source of decoherence. The use of transient photoexcited electron spins provide an additional advantage and once the operations which involve the electron spin are completed, the electron spin can decay and not interfere further with the evolution of the system. In this thesis we report magnetic resonance experiments and density functional theory calculations for the demonstration of nuclear - nuclear entanglement using photoexcited triplet states. We study homonuclear and heteronuclear fullerene derivatives and we identify in each case the relevant parameters that can lead to high fidelity entangling operations. The hyperfine interaction in a homonuclear system is the key parameter which determines the degree of entanglement between the nucelar spins according to a recent theoretical proposal. We measure and calculate the hyperfine interaction in homonuclear systems with ¹³C nuclear spins in order to prove the feasibility of this scheme. Further experiments on a fullerene system with two nuclear spins a ³¹P and a ¹H show that entangling operations of high fidelity which involve the demonstration of CNOT gates, are possible within the lifetime of the triplet state.

Ce document présente une étude des mécanismes de relaxation de fullerènes induits par photoexcitation. Nous avons exploré les processus d'ionisation et de fragmentation de ces systèmes modèles suite à une irradiation par différentes sources de lumière : impulsion femtoseconde, laser à électron libre (IR lointain) et Synchrotron (XUV). Afin d'étudier ces mécanismes, nous utilisons l’imagerie par cartographie de vitesse qui nous permet de suivre l'évolution de la distribution angulaire de vitesse des particules émises. Suivant la durée, l'intensité et la longueur d'onde de l'excitation utilisée, la nature des mécanismes change drastiquement (mécanismes statistiques, ionisation cohérente en champ fort, autoionization de plasmons, etc…). Plusieurs approches théoriques sont utilisées allant du modèle Weisskopf pour les processus statistiques à la résolution de l'équation de Schrödinger dépendante du temps pour les processus cohérents. Cette étude permet de relier les mécanismes de relaxation intervenant à différentes échelles de temps / This document presents a study of relaxation mechanisms of fullerenes induced by photoexcitation. We have investigated ionization and fragmentation processes of these model systems under irradiation by various light sources: femtosecond pulse, free electron laser (far IR) and Synchrotron (XUV). To study these mechanisms, we use the velocity map imaging technique which allows us to follow the evolution of the angular distribution and velocity of the emitted particles. The nature of the relaxation mechanisms changes drastically (statistical mechanisms, coherent ionization in strong field, autoionization of plasmons, etc.) depending on the pulse duration, its intensity and the wavelength. Several theoretical approaches have been used including a Weisskopf modelling for the statistical processes and direct treatment of the time dependant Schrödinger equation for the coherent processes. This study allows us building bridges between the relaxation mechanisms occurring on various scales of time

No description available.

Single-walled carbon nanotubes (SWNTs) are one of the leading candidate materials to realize novel nanoscale photonic devices. In order to assess their performance characteristics as optoelectronic materials, it is crucial to examine their optical properties in highly non-equilibrium situations such as high magnetic fields, low temperatures, and under high photoexcitation. Therefore, we present our latest result on the magnetic susceptibility anisotropy of metallic carbon nanotubes due to the Aharonov-Bohm effect. Here, we performed magnetic linear dichroism on a metallic-enriched HiPco SWNT sample utilizing a 35 T Hybrid Magnet to measure absorption with light polarization both perpendicular and parallel to the magnetic field. By relating these values with the nematic order parameter for alignment, we found that the metallic carbon nanotubes do not follow a strict diameter dependence across the 7 chiralities present in our sample. In addition to the studying the absorption properties exhibited at high magnetic field, we performed temperature-dependent (300 K to 11 K) photoluminescence (PL) on HiPco SWNTs embedded in an ι -carrageenan matrix utilizing intense fs pulses from a wavelength-tunable optical parametric amplifier. We found that for each temperature the PL intensity saturates as a function of pump fluence and the saturation intensity increases from 300 K to a moderate temperature around 100-150 K. Within the framework of diffusion-limited exciton-exciton annihilation (EEA), we successfully estimated the density of 1D excitons in SWNTs as a function of temperature and chirality. These results coupled with our results of magnetic brightening, or an increase in PL intensity as a function of magnetic flux through each SWNT due to the Aharonov-Bohm effect, yield great promise that in the presence of a high magnetic field the density of excitons can be further increased.

Here, for the first time, the formation of adenine, hypoxanthine, and guanine from formamide solutions with heating only to 130 degrees C and UV-irradiation in the absence of minerals or inorganic salts is shown using LC-MS/MS as the analysis technique. The thesis goes on to demonstrate that the product distributions change drastically when the temperature is increased to 160 degrees C from 130 degrees C, specifically that the amount of hypoxanthine increases with the addition of UV light, and the amount of adenine increases with an increase in temperature. Along with showing the formation of purines in these reactions, the identification of pyrimidines was also achieved by GCxGC-MS. GCxGC-MS was also used to study additional samples, specifically bio-oils, where the type of compounds in the samples were easily identifiable, which allowed for a direct comparison between different types of bio-oils (e.g. Douglas-fir bark, Southern pine bark, and a Southern pine bark-wood mixture).

Šiame darbe, remiantis skaičiavimais kvazireliatyvistiniu dvikonfigūraciniu artiniu, atliktas vieno iš svarbių koreliacinių efektų atomuose – konfigūracijų 3s3pN+1 +3s23pN–13d maišymosi ir jo įtakos fotosužadinimo spektrams – teorinis tyrimas. Buvo nagrinėti sužadinimai iš pagrindinio lygmens, tai atitinka retą plazmą, būdingą astrofizikinių ir laboratorinių tyrimų objektą. Naudojantis Cowano programų kompleksu, atlikti banginių funkcijų, energijos ir fotosužadinimo spektrų skaičiavimai izoelektronėms sekoms esant įvairiam pN sluoksnio elektronų skaičiui, nuo pirmųjų jonų iki keturiasdešimt kartų jonizuotų atomų. Fotosužadinimo spektrai, skaičiuoti vienkonfigūraciniu ir dvikonfigūraciniu artiniais, palyginti su pateiktais literatūroje tikslesnių pusiauempirinių skaičiavimų atsižvelgiant į didesnį konfigūracijų skaičių duomenimis, liudija, kad vienkonfigūraciniu artiniu gaunamas labai netikslus rezultatas. Tačiau dvikonfigūracinis artinys duoda kokybiškai ir net kiekybiškai teisingą vaizdą. Tai liudija, kad spektro struktūrą iš esmės lemia konfigūracijų su simetrišku simetrijos pasikeitimu maišymasis ir dvikonfigūracinis artinys yra pakankamas bendriems spektro dėsningumams tirti. Pastebėta, kad intensyvumo pasiskirstymą fotosužadinimo spektruose lemia maišymasis tų lygmenų, į kuriuos galimas stiprus sužadinimas. Tie konfigūracijos 3s23pN–13d lygmenys visose izoelektronėse sekose turi didesnę energiją negu su jais besimaišantys atitinkami 3s3pN+1 lygmenys. Atsižvelgus į šių... [toliau žr. visą tekstą] / In this work the strong mixing of configurations 3s3pN+1 +3s23pN–13d and its influence on the photoexcitation spectra is investigated theoretically in quasirelativistic two-configuration approximation. The excitations from the ground level of 3s23pN configuration were considered, it corresponds to the plasma of low density – the important object of astrophysical and laboratory plasma. The calculations of wave functions, energy level and photoexcitation spectra were performed using Cowan code for isoelectronic sequences at various numbers of electrons in the pN shell from first ions till the ionization degree q = 40. The photoexcitation spectra calculated in the single-configuration and two-configuration approximations have been compared with the semiempirical ones obtained using wider configuration basis. The single-configuration approximation gives very inaccurate result, but the two-configuration approximation describes fairly well the main features of spectrum. It indicates that the structure of considered photoexcitation spectra is mainly determined by the mixing of two configurations with symmetric exchange of symmetry and this approximation is satisfactory for the investigation of general regularities of spectra. It is shown that the distribution of intensity in the photoexcitation spectra depends on the mixing of mainly populated excited levels. In all isoelectronic sequences such levels of 3s23pN–13d configuration are always lying above the mixed with them 3s3pN+1... [to full text]

Nous étudions dans ce mémoire de thèse les états diexcités 1po de l'atome d'hélium, situés au voisinage de son seuil de double ionisation (79 eV), atteints par absorption d'un photon. Ces états diexcités sont soit les états du double continuum situés au-dessus du seuil de double ionisation soit les états doublements excités situés en dessous. Ces deux types d'états diexcités sont dégénérés aux états du continuum simple avec ou sans excitation de l'ion résiduel He+ et sont donc fortement couplés à ces derniers. Dans un processus de photoabsorption à un photon, ces états ne peuvent être accessibles que grâce aux corrélations électroniques du système. Le travail que nous avons réalisé s'inscrit dans une démarche qui vise à étudier tous ces états par une seule et même méthode contrairement aux travaux antérieurs où les méthodes se spécialisaient dans le traitement d'un seul type d'état.<br />Nous utilisons pour cela la méthode HRM-SOW (Hyperspherical R-Matrix with Semiclassical Outgoing Waves), initialement dédiée à l'étude du double continuum de He.<br />Dans ce travail, elle est étendue à la description du continuum simple avec ou sans excitation. Ainsi, nous avons pu calculer les sections efficaces de photoionisation simple avec ou sans excitation jusqu'à n=50 de He à 100meV seulement au dessous de son seuil de double ionisation. Une analyse détaillée en ondes partielles a permis de mettre en évidence le rôle des corrélation angulaires et radiales dans les états du simple continuum de He. Nous avons ensuite établi une relation de continuité entre la photoionisation double et la photoionisation simple avec une excitation infinie de l'ion résiduel He+. la méthode HRM-SOW permet également de calculer la section efficace intégrée de double photoionisation avec une grande précision y compris à très basse énergie. Nous l'avons donc appliquée pour différentes énergies de photon au voisinage du seuil afin de tester la loi de seuil de Wannier. Les résultats obtenus sont en accord d'une part avec les prédictions théoriques de Wannier, d'autre part avec les résultats expérimentaux. La méthode HRM-SOW apparaît donc comme une méthode complète au dessus du seuil de double ionisation. <br />Nous avons poursuivi ce travail en amorçant l'étude des états doublement excités autoionisants. Ceux ci se manifestent comme des résonances dans les sections efficaces d'ionisation sous le seuil de double ionisation. Nos calculs de sections efficaces partielles intégrées et différentielles nous ont permis d'identifier ce type d'états dans la région allant jusqu'au cinquième d'ionisation simple. Certains ont été observés pour la première fois.

Excessive CO2 emissions can negatively impact society and our planet. Reduction of CO2 is one potential avenue for its abatement. One of the most significant challenges to reducing CO2 is its extremely stable linear form. Experimentally, Cu/TiO2 has shown promise for CO2 photocatalytic reduction. Dispersed atomic catalysts can achieve high catalytic efficiency on a per atom basis. Active sites also typically having lower coordination number, and therefore may be more reactive. Using density functional theory and experimental techniques, we have investigated the role of surface oxygen vacancies (Ov) and photoexcited electrons on supported single atom catalysts and CO2 reduction. Cu atoms with Ov have shown to aid in the process of bent, anionic CO2 formation. In the first step involving CO2 dissociation (CO2* --> CO* + O*), a single Cu atom in Ov lowered the activation barrier to 0.10 - 0.19 eV, which could enable fast reduction of CO2 even at room temperature, in agreement with experimental findings. A photoexcited electron model was shown to readily promote Cu binding to the surface vacancy, and CO2 adsorption and direct dissociation. Finally, we briefly compare our results to calculations of supported single Pt atoms to determine how metals besides Cu may behave as photocatalysts for CO2 reduction, and we found a single Pt with Ov can promote CO2 dissociation. Our results show that tailoring TiO2 surfaces with defects in conjunction with atomic catalysts may lead to useful catalysts in the photoreduction of CO2.