970 resultados para Time-resolved spectroscopy


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Previously reported results on deep level optical spectroscopy, optical absorption, deep level transient spectroscopy, photoluminescence excitation, and time resolved photoluminescence are reviewed and discussed in order to know which are the mechanisms involved in electron capture and emission of the Ti acceptor level in GaP. First, the analysis indicates that the 3T1(F) crystal¿field excited state is not in resonance with the conduction band states. Second, it is shown that both the 3T2 and 3T1(F) excited states do not play any significant role in the process of electron emission and capture.

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The structural and optical properties of three different kinds of GaAs nanowires with 100% zinc-blende structure and with an average of 30% and 70% wurtzite are presented. A variety of shorter and longer segments of zinc-blende or wurtzite crystal phases are observed by transmission electron microscopy in the nanowires. Sharp photoluminescence lines are observed with emission energies tuned from 1.515 eV down to 1.43 eV when the percentage of wurtzite is increased. The downward shift of the emission peaks can be understood by carrier confinement at the interfaces, in quantum wells and in random short period superlattices existent in these nanowires, assuming a staggered band offset between wurtzite and zinc-blende GaAs. The latter is confirmed also by time-resolved measurements. The extremely local nature of these optical transitions is evidenced also by cathodoluminescence measurements. Raman spectroscopy on single wires shows different strain conditions, depending on the wurtzite content which affects also the band alignments. Finally, the occurrence of the two crystallographic phases is discussed in thermodynamic terms.

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The preparation of [FeIV(O)(MePy2tacn)]2+ (2, MePy2tacn = N-methyl-N,N-bis(2-picolyl)-1,4,7-triazacyclononane) by reaction of [FeII(MePy2tacn)(solvent)]2+ (1) and PhIO in CH3CN and its full characterization are described. This compound can also be prepared photochemically from its iron(II) precursor by irradiation at 447 nm in the presence of catalytic amounts of [Ru II(bpy)3]2+ as photosensitizer and a sacrificial electron acceptor (Na2S2O8). Remarkably, the rate of the reaction of the photochemically prepared compound 2 toward sulfides increases 150-fold under irradiation, and 2 is partially regenerated after the sulfide has been consumed; hence, the process can be repeated several times. The origin of this rate enhancement has been established by studying the reaction of chemically generated compound 2 with sulfides under different conditions, which demonstrated that both light and [Ru II(bpy)3]2+ are necessary for the observed increase in the reaction rate. A combination of nanosecond time-resolved absorption spectroscopy with laser pulse excitation and other mechanistic studies has led to the conclusion that an electron transfer mechanism is the most plausible explanation for the observed rate enhancement. According to this mechanism, the in-situ-generated [RuIII(bpy)3] 3+ oxidizes the sulfide to form the corresponding radical cation, which is eventually oxidized by 2 to the corresponding sulfoxide

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Poikilohydric organisms have developed mechanisms to protect their photosynthetic machinery during times of desiccation. In hydrated conditions nonphotochemical quenching (NPQ) mechanisms are able to safely dissipate excess excitation energy as heat, but mechanisms of NPQ associated with desiccation tolerance are still largely unclear. In the lichen Parmelia sulcata, photosystem protection has been associated with an energy quenching energetically coupled to PSII and characterized by a fast-fluorescence decay lifetime, and long-wavelength emission. The present study compares the relative ability of green algae and lichens to recover photosynthetic activity after periods of desiccation using steady state fluorescence emission spectroscopy, and picosecond time-resolved fluorescence decay measurements. It was determined that desiccation induced quenching involves an antenna quenching mechanism with similar characteristics appearing in both P. sulcata and green algae. Algae isolated from lichens suggest symbiosis in the lichen appears to enhance this naturally occurring phenomenon and provide greater protection during desiccation.

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L’étude du cerveau humain est un domaine en plein essor et les techniques non-invasives de l’étudier sont très prometteuses. Afin de l’étudier de manière non-invasive, notre laboratoire utilise principalement l’imagerie par résonance magnétique fonctionnelle (IRMf) et l’imagerie optique diffuse (IOD) continue pour mesurer et localiser l’activité cérébrale induite par une tâche visuelle, cognitive ou motrice. Le signal de ces deux techniques repose, entre autres, sur les concentrations d’hémoglobine cérébrale à cause du couplage qui existe entre l’activité neuronale et le flux sanguin local dans le cerveau. Pour être en mesure de comparer les deux signaux (et éventuellement calibrer le signal d’IRMf par l’IOD), où chaque signal est relatif à son propre niveau de base physiologique inconnu, une nouvelle technique ayant la capacité de mesurer le niveau de base physiologique est nécessaire. Cette nouvelle technique est l’IOD résolue temporellement qui permet d’estimer les concentrations d’hémoglobine cérébrale. Ce nouveau système permet donc de quantifier le niveau de base physiologique en termes de concentrations d’hémoglobine cérébrale absolue. L’objectif général de ma maîtrise était de développer un tel système afin de l’utiliser dans une large étude portant sur la condition cardiovasculaire, le vieillissement, la neuroimagerie ainsi que les performances cognitives. Il a fallu tout d’abord construire le système, le caractériser puis valider les résultats avant de pouvoir l’utiliser sur les sujets de recherche. La validation s’est premièrement réalisée sur des fantômes homogènes ainsi qu’hétérogènes (deux couches) qui ont été développés. La validation des concentrations d’hémoglobine cérébrale a été réalisée via une tâche cognitive et appuyée par les tests sanguins des sujets de recherche. Finalement, on présente les résultats obtenus dans une large étude employant le système d’IOD résolue temporellement en se concentrant sur les différences reliées au vieillissement.

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Une compréhension profonde de la séparation de charge à l’hétérojonction de semi-con- ducteurs organiques est nécessaire pour le développement de diodes photovoltaïques organiques plus efficaces, ce qui serait une grande avancée pour répondre aux besoins mondiaux en énergie durable. L’objectif de cette thèse est de décrire les processus impliqués dans la séparation de charges à hétérojonctions de semi-conducteurs organiques, en prenant en exemple le cas particulier du PCDTBT: PCBM. Nous sondons les excitations d’interface à l’aide de méthodes spectroscopiques résolues en temps couvrant des échelles de temps de 100 femto- secondes à 1 milliseconde. Ces principales méthodes spectroscopiques sont la spectroscopie Raman stimulée femtoseconde, la fluorescence résolue en temps et l’absorption transitoire. Nos résultats montrent clairement que le transfert de charge du PCDTBT au PCBM a lieu avant que l’exciton ne soit relaxé et localisé, un fait expérimental irréconciliable avec la théorie de Marcus semi-classique. La paire de charges qui est créée se divise en deux catégories : les paires de polarons géminales non piégées et les paires profondément piégées. Les premiers se relaxent rapidement vers l’exciton à transfert de charge, qui se recombine radiativement avec une constante de temps de 1– 2 nanoseconde, alors que les seconds se relaxent sur de plus longues échelles de temps via l’effet tunnel. Notre modèle photophysique quantitatif démontre que 2 % de l’excitation créée ne peut jamais se dissocier en porteurs de charge libre, un chiffre qui est en accord avec les rendements élevés rapportés pour ce type de système.

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This thesis is entitled “OPTICAL EMISSION DIAGNOSTICS OF LASER PRODUCED PLASMA FROM GRAPHITE AND YBa2Cu3O7. The work presented in this thesis covers the experimental results on the plasma produced with moderately high power laser with irradiance range in between 10 GW cm 2 to 100 GW cm -2. The characterization of laser produced plasma from solid targets viz. graphite and high temperature superconducting material like YBa2Cu3O7 have been carried out. The fundamental frequency from a Q - switched Nd: YAG laser with 9 ns pulse duration is used for the present studies. Various optical emission emission diagnostic techniques were employed for the the characterization of the LPP which include emission spectroscopy, time resolved studies, line broadening method etc. In order to understand the physical nature of the LPP like recombination, collisional excitation and the laser interaction with plasma, the time resolved studies offer the most logical approach

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Laser radiation at 1.06 µm from a pulsed Nd:YAG laser was focused onto a multielement YBa2Cu3O7 target in vacuum and the plasma thus generated was studied using time-resolved spectroscopic techniques. Line broadening of the Ba I emission line at 553.5 nm was monitored as a function of time elapsed after the incidence of a laser pulse on the target. Measured line profiles of barium species were used to infer the electron density and temperature, and the time evolution of these important plasma parameters has been worked out.

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The laser produced plasma from the multi-component target YBa2CU3O7 was analyzed using Michelson interferometry and time resolved emission spectroscopy. The interaction of 10 ns pulses of 1.06 mum radiation from a Q-switched Nd:YAG laser at laser power densities ranging from 0.55 GW cm-2 to 1.5 GW cm-2 has been studied. Time resolved spectral measurements of the plasma evolution show distinct features at different points in its temporal history. For a time duration of less than 55 ns after the laser pulse (for a typical laser power density of 0.8 GW cm-2, the emission spectrum is dominated by black-body radiation. During cooling after 55 ns the spectral emission consists mainly of neutral and ionic species. Line averaged electron densities were deduced from interferometric line intensity measurements at various laser power densities. Plasma electron densities are of the order of 1017 cm-3 and the plasma temperature at the core region is about 1 eV. The measurement of plasma emission line intensities of various ions inside the plasma gave evidence of multiphoton ionization of the elements constituting the target at low laser power densities. At higher laser power densities the ionization mechanism is collision dominated. For elements such as nitrogen present outside the target, ionization is due to collisions only.

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The emission features of laser ablated graphite plume generated in a helium ambient atmosphere have been investigated with time and space resolved plasma diagnostic technique. Time resolved optical emission spectroscopy is employed to reveal the velocity distribution of different species ejected during ablation. At lower values of laser fluences only a slowly propagating component of C2 is seen. At high fluences emission from C2 shows a twin peak distribution in time. The formation of an emission peak with diminished time delay giving an energetic peak at higher laser fluences is attributed to many body recombination. It is also observed that these double peaks get modified into triple peak time of flight distribution at distances greater than 16 mm from the target. The occurrence of multiple peaks in the C2 emission is mainly due to the delays caused from the different formation mechanism of C2 species. The velocity distribution of the faster peak exhibits an oscillating character with distance from the target surface.

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Femtosecond time-resolved techniques with KETOF (kinetic energy time-of-flight) detection in a molecular beam are developed for studies of the vectorial dynamics of transition states. Application to the dissociation reaction of IHgI is presented. For this system, the complex [I---Hg---I](++)* is unstable and, through the symmetric and asymmetric stretch motions, yields different product fragments: [I---Hg---I](++)* -> HgI(X^2/sigma^+) + I(^2P_3/2) [or I*(^2P_l/2)] (1a); [I---Hg---I](++)* -> Hg(^1S_0) + I(^2P_3/2) + I(^2P_3/2) [or I* (^2P_1/2)] (1 b). These two channels, (1a) and (1b), lead to different kinetic energy distributions in the products. It is shown that the motion of the wave packet in the transition-state region can be observed by MPI mass detection; the transient time ranges from 120 to 300 fs depending on the available energy. With polarized pulses, the vectorial properties (transition moments alignment relative to recoil direction) are studied for fragment separations on the femtosecond time scale. The results indicate the nature of the structure (symmetry properties) and the correlation to final products. For 311-nm excitation, no evidence of crossing between the I and I* potentials is found at the internuclear separations studied. (Results for 287-nm excitation are also presented.) Molecular dynamics simulations and studies by laser-induced fluorescence support these findings.

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The real-time dynamics of molecular (Na_2 . Na_3) and cluster Na_n (n=4-2l) multiphoton ionization and -fragmentation has been studied in beam experiments applying femtosecond pump-probe techniques in combination with ion and electron spectroscopy. Wave packet motion in the dimer Na_2 reveals two independent multiphoton ionization processes while the higher dimensional motion in the trimer Na_3 reflects the chaotic vibrational motion in this floppy system. The first studies of cluster properties (energy, bandwidth and lifetime of intermediate resonances Na^*_n) ) with femtosecond laser pulses give a striking illustration of the transition from "molecule-like" excitations to "surfaceplasma"-like resonances for increasing cluster sizes. Time-resolved fragmentation of cluster ions Na_n^* indicate that direct photo-induced fragmentation processes are more important at short times than the statistical unimolecular decay.

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Two Multifunctional photoactive complexes [Re(Cl)(CO)(3)-(MeDpe(+))(2)](2+) and [Re(MeDpe(+))(CO)(3)(bpy)](2+) (MeDpe(+) = N-methyl-4-[trans-2-(4-pyridyl)ethenyl]pyridinium, bpy = 2,2'-bipyridine) were synthesized. characterized. and their redox and photonic properties were investigated by cyclic voltammetry: ultraviolet-visible-infrared (UV/Vis/IR) spectroelectrochemistry, stationary UV/Vis and resonance Raman spectroscopy; photolysis; picosecond time-resolved absorption spectroscopy in the visible and infrared regions: and time-resolved resonance Raman spectroscopy. The first reduction step of either complex Occurs at about -1.1 V versus Fc/Fc(+) and is localized at MeDpe(+). Reduction alone does not induce a trans -> cis isomerization of MeDpe(+). [Re(Cl)(CO)(3)(MeDPe(+))(2)](2+) is photostable, while [Re(MeDpe(+))(CO)(3)(bpy)](2+) and free MeDpe(+) isomerize under near-UV irradiation. The lowest excited state of [Re(Cl)(CO)(3)(MeDPe(+))(2)](2+) has been identified as the Re(Cl)(CO)(3) -> MeDpe(+) (MLCT)-M-3 (MLCT = metal-to-ligand charge transfer), decaying directly to the ground state with lifetimes of approximate to 42 (73%) and approximate to 430ps (27%). Optical excitation of [Re(MeDpe(+))(CO)(3)(bpy)](2+) leads to population of Re(CO)(3) -> MeDpe(+) and Re(CO)(3) -> bpy (MLCT)-M-3 states, from which a MeDpe(+) localized intraligand 3 pi pi* excited state ((IL)-I-3) is populated with lifetimes of approximate to 0.6 and approximate to 10 ps, respectively. The 3IL state undergoes a approximate to 21 ps internal rotation, which eventually produces the cis isomer on a much longer timescale. The different excited-state behavior of the two complexes and the absence of thermodynamically favorable interligand electron transfer in excited [Re(MeDpe(+))(CO)(3)(bpy)](2+) reflect the fine energetic balance between excited states of different orbital origin, which can be tuned by subtle Structural variations. The complex [Re(MeDpe+)(CO)(3)(bpy)](2+) emerges as a prototypical, multifunctional species with complementary redox and photonic behavior.

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The chemical composition and dissociation behaviour of water adsorbed on clean and oxygen pre-covered Pd{111} was studied using high-resolution time-resolved and temperature-programmed X-ray photoelectron spectroscopy. We find that water remains intact at all temperatures up to desorption on the clean surface and at high oxygen coverage(0.69 ML) when a surface oxide is formed. The highest desorption peaks occur at 163 K from the clean surface and at 172 K from the surface oxide. At the intermediate coverage of 0.20 ML oxygen reacts with coadsorbed water at 155 K, to generate a mixed H2O/OH layer exhibiting a (root 3- x root 3)R30 degrees diffraction pattern, which is stable up to 177 K. The measured ratio between intact water and the hydroxyl species in this layer varies between 1.5 and 2 depending on temperature. (C) 2008 Elsevier B.V. All rights reserved.

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Solvent influences on the crystallization of polymorph and hydrate forms of the nootropic drug piracetam (2-oxo-pyrrolidineacetamide) were investigated from water, methanol, 2-propanol, isobutanol, and nitromethane. Crystal growth profiles of piracetam polymorphs were constructed using time-resolved diffraction snapshots collected for each solvent system. Measurements were performed by in situ energy dispersive X-ray diffraction recorded in Station 16.4 at the synchrotron radiation source (SRS) at Daresbury Laboratory, CCLRC UK. Crystallizations from methanol, 2-propanol, isobutanol, and nitromethane progressed in a similar fashion with the initial formation of form I which then converted relatively quickly to form II with form III being generated upon further cooling. However, considerable differences were observed for the polymorphs lifetime and both the rate and temperature of conversion using the different solvents. The thermodynamically unstable form I was kinetically favored in isobutanol and nitromethane where traces of this polymorph were observed below 10 degrees C. In contrast, the transformation of form II and subsequent growth of form III were inhibited in 2-propanol and nitromethane solutions. Aqueous solutions produced hydrate forms of piracetam which are different from the reported monohydrate; this crystallization evolved through successive generation of transient structures which transformed upon exchange of intramolecular water between the liquid and crystalline phases. (c) 2007 Wiley-Liss, Inc. and the American Pharmacists Association J Pharm Sci 96:1069-1078, 2007.