Search results for "PHOTOCHEMISTRY"
showing 10 items of 2034 documents
The nature of the 4.8 eV optical absorption band induced by vacuum-ultraviolet irradiation of glassy SiO2
2000
Abstract The controversial optical absorption band centered at 4.8 eV, which is present in nearly all irradiated silicas, was investigated. It is caused by at least two different defects: non-bridging oxygen hole center (NBOHC) and interstitial ozone (O3). Both species have absorption bands at 4.8 eV, the O3-related band is identified by its susceptibility to bleaching by 4 to 5 eV photons, by a smaller halfwidth and by its independence from the NBOHC-associated 1.9 eV photoluminescence (PL) band. The contribution of NBOHC to the 4.8 eV band is dominant in most cases, while O3 is important in F2 excimer laser-irradiated samples of oxygen-rich glassy SiO2.
The behavior of interstitial oxygen atoms induced by F2 laser irradiation of oxygen-rich glassy SiO2
2002
Abstract Interstitial oxygen atoms in glassy silicon dioxide were created by photolysis of pre-existing interstitial oxygen molecules O 2 with a fluorine excimer laser (7.9 eV). The concentration of atomic oxygen interstitials was indirectly monitored by the disappearance and subsequent recovery of interstitial molecules which were monitored by their 1272 nm photoluminescence band. Most of the oxygen interstitials (>95%) are immobile at room temperature. The onset of their mobility occurs between 200 and 400 °C where around 95% of them recombine to form O 2 molecules. The high stability of interstitial oxygen atoms is consistent with the theoretical prediction that they are incorporated int…
Luminescence Properties and Energy Transfer Processes in Nanosized Cerium Doped YAG
2008
Luminescence properties of cerium doped Y3Al5O12 (YAG) nanocrystals in form of nanopowders and nanoceramics have been studied. The comparative analysis of luminescence characteristics for nano- and single-crystals has been done. It was detected that an excitonic mechanism of the energy transfer from the host lattice to cerium ions does not work in nanopowders and nanoceramics. It was also shown that antisite-related and self-trapped exciton-related luminescence bands are suppressed strongly in nanopowders, and it can be excited only under certain circumstances. These bands practically disappeared in the nanoceramic samples. It was suggested that nanoparticle's surface, which is an efficient…
Oxygen-related defects and energy accumulation in aluminum nitride ceramics
2001
Abstract Features of oxygen-related defects in the AlN crystalline lattice were studied. Spectral characteristics of photoluminescence and photostimulated luminescence under the UV light irradiation of AlN ceramics were examined. The results obtained allow us to propose the mechanisms of luminescence and radiation-induced energy accumulation in AlN.
Cathodoluminescence and IR absorption of oxygen deficient silica – influence of hydrogen treatment
1999
Abstract The cathodoluminescence (CL) and IR absorption of silica samples with normal stoichiometry as well as with an extremely high level of oxygen deficit were studied. Additionally, some samples have been treated in hydrogen at 800°C. Crystalline quartz was used for reference measurements and the CL data have been compared with those of X-ray excited luminescence (XL). The luminescence spectra of silica have a band at 1.85 eV due to non-bridging oxygens and the two bands at 2.7 and 4.4 eV due to twofold-coordinated silicons. The energetic yield for CL is about 0.1%, for XL it approaches 0.15%. Cathodoluminescence of quartz at temperatures >130 K exhibits the self-trapped exciton lumines…
Iron-related luminescence centers in ZnWO 4 :Fe
2002
A systematic spectroscopic study of single ZnWO4 :Fe crystals with different iron concentrations has been performed under excitation by ultraviolet light, by synchrotron radiation or under photostimulation by near-infrared light. The luminescence of Fe3+-related centres has been studied. It is shown that iron centres of different types efficiently promote the formation of crystal defects at low temperatures. Electrons and holes can be trapped near Fe2+ or Fe3+ ions, which is further revealed in phosphorescence, thermostimulated or photostimulated luminescence. At room temperature the main effect of iron impurity is to reduce the light yield of a ZnWO4 scintillator.
Luminescence mechanisms of oxygen-related defects in AlN
2002
Spectral characteristics of native oxygen-related defects existing in the crystalline lattice of AlN were studied. Features of photoluminescence observed under exposure to ultraviolet light together with those of the photostimulated luminescence testify the recombination character of luminescence. The mechanism of luminescence of oxygen-related defects is proposed.
Irradiation induced defects in fluorine doped silica
2008
International audience; The role of fluorine doping in the response to UV pulsed laser and c radiation of silica preforms and fibers was studied using electron spin resonance (ESR) spectroscopy. Exposure to radiation mainly generates E0 centers, with the same effectiveness in fibers and in preforms. The E'concentration in F-doped silica fibers is found to increase with UV energy fluence till a saturation value, consistently with a precursor conversion process. These results show the fluorine role in reducing the strained Si–O bonds thus improving the radiation hardness of silica, also after drawing process.
Extension of the Squaraine Chromophore in Symmetrical Bis(stilbenyl)squaraines
1997
The bis(stilbenyl)squaraines 6d−j,d‘,j‘ represent a novel class of NIR pigments. Their synthesis was performed by the regioselective 2-fold condensation of highly nucleophilic 3,5-dihydroxystilbene...
Understanding the polar mechanism of the ene reaction. A dft study
2014
The molecular mechanism of ene reactions has been characterised by DFT methods at the MPWB1K/6-311G(d,p) level of theory. Most reactions take place along a two-stage one-step mechanism in which the C-C bond formation takes place before the hydrogen transfer process. A very good correlation between the polar character of the reaction measured by the global electron density transfer at the transition state and the activation energy has been found. This behaviour allows establishing a useful classification of ene reactions in N-ene having a very high activation energy, P-ene reactions having activation energies between 35 and 20 kcal mol(-1), and H-ene reactions having activation energies belo…