Photoconductivity and Photoemission of Diamond Under Femtosecond Vuv Irradiation

In order to gain some insight on the electronic relaxation mechanisms occuring in diamond under high intensity laser excitation and/or VUV excitation, we studied experimentally the pulsed conductivity induced by femtosecond VUV pulses, as well as the energy spectra of the photoelectrons released by the same irradiation. The source of irradiation consists in highly coherent VUV pulses obtained through high order harmonic generation of a high intensity femtosecond pulse at a 1.55 eV photon energy (titanium-doped sapphire laser). Harmonics H9 to H17 have been used for photoconductivity (PC) and harmonics H13 to H27 for photoemission experiments (PES). As the photon energy is increased, it is expected that the high energy photoelectrons will generate secondary e-h pairs, thus increasing the excitation density and consequently the PC signal. This is not what we observe : the PC signal first increases for H9 to H13, but then saturates and even decreases. Production of low energy secondary e-h pairs should also be observed in the PES spectrum. In fact we observe very few low energy electrons in the PES spectrum obtained with H13 and H15, despite the sufficient energy of the generated free carriers. At the other end (H21 and above), a very intense low energy secondary electron peak is observed. As a help to interprete such data, we realized the first ab initio calculations of the electronic lifetime of quasiparticles, in the GW approximation in a number of dielectrics including diamond. We find that the results are quite close to a simple "Fermi-liquid" estimation using the electronic density of diamond. We propose that a quite efficient mechanism could be the excitation of plasmons by high energy electrons, followed by the relaxation of plasmons into individual e-h pairs

Luminescence résolue en temps de solides cristallins et de nano particules excités par des impulsions IR, UV et VUV femtosecondes d'intensité variable

Mon travail pendant cette thèse a d abord été le développement d une source de génération d harmoniques d ordre élevé basée sur une chaîne laser femtoseconde amplifiée (Saphir-Titane) fonctionnant à une cadence de 1 kHz (AURORE). Une ligne de lumière construite au CELIA permet de fournir un faisceau focalisé VUV-XUV femtoseconde, monochromatique dans une région spectrale comprise entre 10 nm et 73 nm environ (17 eV à 120 eV). Cette installation expérimentale est en fonctionnement et est parmi les toutes premières lignes à être mise en service pour la communauté scientifique française et étrangère. J ai également mis en place une installation d'étude des cinétiques de luminescence avec résolution temporelle sub-picoseconde (450 fs) par mélange de fréquences. Le thème général de ce travail est l étude des processus de relaxation et d'interaction entre les excitations électroniques créées par des impulsions ultra brèves femtosecondes de photons IR, UV et VUV-XUV dans les solides diélectriques massifs et des nano particules. L observable principale utilisée est la luminescence émise par ces systèmes, résolue spectralement et en temps sur des échelles allant de la s à des temps sub picosecondes. Ce travail a abouti à une avancée sensible de la description des processus principaux de formation et d évolution des excitations électroniques. La comparaison et l interprétation des données expérimentales obtenues pour des nano particules et des cristaux ont permis d élucider certaines propriétés spécifiques de ces systèmes.The work during this Ph.D. was a development of a source of high order harmonics generation based on amplified Ti:Sapphire femtosecond laser with repetition rate 1kHz (AURORE). The beam line constructed in CELIA has on its exit a VUV-XUV focalized beam; it may has wide spectrum or monochromatic in spectral range from 10nm up to 73nm (17-120eV). This beam line is in operation and is using for experiments for solid state VUV spectroscopy, photoelectron spectroscopy etc. Also it was installed a system for detection of luminescence with sub-picosecond time resolution (450fs) based on the nonlinear effect generation of sum of two light frequencies. The main subject of this work was the study of processes of relaxation and interaction of electronic excitations, created by ultra-short pulse of IR, UV or XUV in dielectric crystals and nanoparticles. Out method is based on observation of luminescence with spectral and time resolution up to sub-picosecond temporal resolution. This study has given new experimental results for description of fundamental processes of creation and evolution of electronic excitations. Comparison and interpretation of experimental data of semiconductor nano-particles and monocrystals gave some interpretations of extra-fast luminescence of these systems.BORDEAUX1-Bib.electronique (335229901) / SudocSudocFranceF

Channels of Energy Losses and Relaxation in CsI:A Scintillators (A = Tl, In)

Radiative relaxation channels and energy losses in In and Tl doped CsI scintillation crystals have been investigated as a function of temperature and excitation conditions to evaluate scintillation efficiency of the activator channel. Two activator concentration series of crystals were grown by the Bridgman method. Temperature dependence of excitation and luminescence spectra were measured under VUV and X-ray excitation; thermostimulated luminescence was also studied. The observed drop of radioluminescence yield of doped CsI crystals at room temperature relative to the pure crystal is explained by the migration losses caused by charge carrier trapping on the activator centers. The energy losses in CsI:A at low temperatures are due to the trapping of charge carriers on different centers: self-trapping of holes and capture of electrons by the activator centers. We suppose that migration energy losses are the main reason for significantly lower luminescence yield of CsI:A at room temperature than that of self-trapped excitons in pure CsI crysta

Scintillation Efficiency Improvement by Mixed Crystal Use

International audienc

Emission centers in Ca1–xPrxF2+x(x=0.35)Ca_{1–x}Pr_xF_{2+x} (x = 0.35) solid solutions

Luminescence spectra of Ca0.65Pr0.35F2.35 solid solutions are studied. It is found that, depending on the excitation energy, different kinds of emission centers appear in these spectra. An interconfigurational 4f 15d1 →4f2 luminescence is typical for single Pr3+ ions in tetragonal sites. Data on the structure of the solid solutions show that the emission centers involved in 1S0 → 4f2 transitions can be attributed to Pr3+ ions contained in clusters

Deep traps can reduce memory effects of shallower ones in scintillators.

International audienceX-ray induced luminescence sensitization results have been obtained on three commercially relevant scintillators, namely CsI:Tl, YAG:Ce and LSO:Ce. The obtained curves have been used to validate a model based on the competition among trapping and recombination of free charge carriers. The model was able to accurately describe the complex phenomenology of the detected sensitization curves. We also used the model to predict the role of a high temperature and concentration trap in shaping the sensitization curves. Based on these modelling results we also proposed a novel, and rather counterintuitive, strategy to deal with the sensitization phenomenon based on the deliberate introduction of deep traps which can significantly reduce the bright burn effect

Nonlinear behavior of structural and luminescent properties in Gd(NbxTa1-x)O-4 mixed crystals

International audienceCeramic samples of gadolinium tantalo-niobate mixed crystals were obtained by the solid-state technique. The dependence of luminescence properties on the Nb/Ta ratio in the Gd(NbxTa1-x)O4 system is studied in the 5–450 K temperature range, including thermostimulated luminescence curves in the series of solid solutions. The relation of nonlinear behavior of light output with x variation to non-homogeneous distribution of Nb and Ta in solid solutions is discussed

Decay Kinetics of CeF3 under VUV and X-ray Synchrotron Radiation

Characteristic dimensions and evolution times of regions of secondary electronic excitations created by the interaction of ionizing radiation with matter cannot be measured directly. At the same time these are essential parameters both for engineering of nanostructured composite materials defining optimal layer thickness and nanoparticles radii and for the development of optimized scintillators. The paper demonstrates how such spatial and temporal data can be extracted from luminescence decay kinetics excited by vacuum ultraviolet (VUV) and X-ray photons at modern sources of synchrotron radiation MAX IV and PETRA III. Specific features of energy-band structure of self-activated crystal CeF 3 are discussed, and its potential for a super-fast detection of ionizing radiation evaluated. Diffusion-controlled dipole-dipole interaction of Frenkel excitons is demonstrated to account well for the luminescence non-exponential decay kinetics providing information on the scales of excited regions created by photons of different energy. For 20 eV photons the radius of excited regions is estimated to be 10 nm, and for 200 eV photons it increases to 18 nm. Effective radius of excited regions of complicated shape created by 19 keV is as large as 80 nm and the diffusion length of Frenkel excitons over radiative time is 14 nm