Femtosecond time-resolved spectroscopy of coherent oscillations in nanomaterials

The interaction of laser radiation with a material can excite coherent lattice vibration. The observation of such periodic motion of the atoms in the lattice provides information on the properties of the material. In the present work a femtosecond pump-probe technique was applied for studies of acoustic vibrations in nanoparticles and nanowires, and coherent optical phonons in thin films. The elastic properties of spherical Ag nanoparticles and Ag and Bi nanowires were studied in a dual-color femtosecond pump-probe experiment. The results of the period determinations of the acoustic vibrations, obtained from the time-domain measurements with low intensity pump pulses, together with the information about the size of the particles, were used to determine the elastic constants of the materials. Also changes in the measured acoustical response of the Ag nanowires under high intensity laser excitation were studied. In addition the coherent optical phonon excitation in a Bi film was studied in a femtosecond pump-probe experiment. A red-shift of the phonon frequency at the high photoexcitation density was observed. To separate the effect of the lattice softening and the lattice anharmonicity the excitation with two pump pulses was employed. Numerical simulations, which took into account the evolution of the spatial inhomogeneity of the excitation density, were carried out and compared to the experimental results

A ramsayite-type oxide, Ca2Sn2Al2O9

The title compound, dicalcium nonaoxidodistannate(IV)dialuminate, is the second example which crystallizes in the isotypic structure of a pyroxene silicate, Na2Ti2Si2O9 (ramsayite). ∞ 1[Sn2O8] chains and pyroxene-type ∞ 1[Al2O6] chains are formed along the b axis by sharing O atoms. The Ca atoms are situated in the resulting channels and exhibit a coordination number of 7

Estudos sobre a perspectiva histórico-cultural de Vigotski

Veresk é uma publicação internacional de Psicologia que se propõe a divulgar trabalhos que examinam e aprofundam o estudo do legado da teoria de Lev Semionovitch Vigotski. Os autores são especialmente convidados a apresentarem seus textos pelos Editores Associados ou pelo Conselho Editorial. Veresk é apresentada tanto no formato impresso, quanto no eletrônico, nas versões em russo, português e inglês. Sua publicação é patrocinada pelo Instituto de Psicologia L. S. Vigotski, da Universidade Estatal de Humanidades da Rússia, pela Faculdade de Educação da Universidade Federal Fluminense, Brasil e pela Faculdade de Ciências da Saúde, do Centro Universitário de Brasília, Brasil

Estudos sobre a perspectiva histórico-cultural de Vigotski

Veresk é uma publicação internacional de Psicologia que se propõe a divulgar trabalhos que examinam e aprofundam o estudo do legado da teoria de Lev Semionovitch Vigotski. Os autores são especialmente convidados a apresentarem seus textos pelos Editores Associados ou pelo Conselho Editorial. Veresk é apresentada tanto no formato impresso, quanto no eletrônico, nas versões em russo, português e inglês. Sua publicação é patrocinada pelo Instituto de Psicologia L. S. Vigotski, da Universidade Estatal de Humanidades da Rússia, pela Faculdade de Educação da Universidade Federal Fluminense, Brasil e pela Faculdade de Ciências da Saúde, do Centro Universitário de Brasília, Brasil.Série VERESK: Cadernos Acadêmicos Internacionais, volume

Spontaneous Raman spectra of dipicolinic acid in microcrystalline form

Abstract. Dipicolinic acid (DPA) is an important component of bacterial spores. The Raman spectrum of DPA in the form of compacted powder was measured in reflection at room temperature with excitation by a nanosecond laser at 532 nm. The spectrum presents a set of characteristic frequency bands in the region 700-3090 cm À1 that were identified with characteristic vibrational modes of the DPA molecule

Laser-excited acoustic oscillations in silver and bismuth nanowires

Journals published by the American Physical Society can be found at http://journals.aps.org/Coherent acoustic oscillations in Bi and Ag nanowire samples were studied with a femtosecond pump-probe technique and detection of the scattered light. The observed optical and acoustic properties reflect the nanostructure of these materials. The electronic and lattice contributions to the excitation of coherent acoustic phonons are described using a two-temperature model. The excitation is performed at different laser fluences, and the high density of optically induced excitations modifies the state of the lattice. A transient state with softening of the lattice was observed in Ag nanowire samples

The limitations of hibonite as a single-mineral oxybarometer for early solar system processes

The relationships between the composition of hibonite with the general formula CaAl12-2x-yMgxTi4 +xTi3 +yO19, the oxidation state of Ti (Ti3 +/ΣTi, where ΣTi = Ti3 + + Ti4 +), and oxygen fugacity (fO2) were investigated experimentally. It was found that hibonite can be synthesised with a range of Ti3 +/ΣTi values at constant fO2 and with a constant Ti3 +/ΣTi value for a range of fO2s. It was also found that if hibonite with the formula CaAl12-yTi3 +yO19 (Ti3 +/ΣTi = 1) is equilibrated with a melt of CAI composition at fO2s below the iron-wüstite buffer then the resulting hibonite contained Mg, with Mg per formula unit (pfu) ~ 0.8 Ti pfu, and Ti3 +/ΣTi ~ 0.2, irrespective of the fO2. These results suggest that the availability of Mg, rather than fO2, is the key factor that determines Ti3 +/ΣTi of hibonite. The structures of synthetic samples of hibonite with the general formula CaAl12-2xMgxTi4 +xO19, where 0 ≤ X < 1, were determined by Rietveld refinement of X-ray powder diffraction data. The predominant site occupied by Ti4 + was found to change from M2 to M4 with increasing Ti content. The range of Ti concentrations over which the site occupancy changed corresponds to that observed in meteoritic hibonite. This change in the Ti4 + site produces changes in the Ti K-edge XANES spectra, particularly in the intensity of the pre-edge feature, for constant Ti3 +/ΣTi. The observed dependence of the pre-edge on the Ti4 + site was reproduced by ab initio simulations of the XANES spectra. The XANES spectra of natural hibonite with variable Ti content from the Murchison carbonaceous chondrite closely match the spectra of the synthetic samples with similar Ti contents. These differences in the spectra of meteoritic hibonite could be misinterpreted as being due to changes in Ti3 +/ΣTi, but are instead due to differences in ΣTi, which relate to the petrogenetic history. Crystal chemistry exerts a first order control on the Ti site occupancy and Ti3 +/ΣTi value of hibonite. As a result, no simple relationship between Ti3 +/ΣTi and fO2 should be expected. It is unlikely that hibonite will be useful as an oxybarometer for solar processes without Ti3 +/ΣTi standards that are compositionally matched to the unknown

O papel das vivências da personalidade na instrução: no caso de estudos da psicologia

Neste artigo, propomos, inicialmente, a nossa compreensão da essência e das características das vivências da pessoa e, posteriormente, empreendemos uma tentativa de analisar o processo de instrução que contribui com o desenvolvimento de outra pessoa por meio da orientação em vivências o que, a nosso ver, permite pôr substancialmente em outra dimensão o processo pedagógico

Shock waves and acoustic signals from the optical breakdown and interaction with targets of ultrashort laser pulses

The study and comparison of sound waves produced by nanosecond and femtosecond laser pulses in the focal region as a result of the optical breakdown in air, water and near surfaces of targets were performed. Ultrashort laser pulses focused in the medium produced highly ionized plasma with light and sound emission. For time-resolved detection of sound waves a probe-beam-deflection setup was used. The amplitude of the shock wave was evaluated by measuring the propagation velocity. The sound wave produced by a femtosecond laser pulse in the air had a delayed compressional contribution in the profile that was attributed to the recombination of charges in the plasma region