Elisa Buffagni

Evidences of Rare-Earth Nanophases Embedded in Silica Using Vibrational Spectroscopy

Transmission electron microscopy, Raman scattering, Fourier transform infrared spectroscopy, and radio-luminescence are employed to investigate rare-earth (RE) incorporation and aggregate formation in silica glasses prepared by the Sol-Gel method and doped with Ce3+, or Tb3+, Gd3+, Yb3+ with concentrations up to several mol%. The results demonstrate that rare-earth aggregates with a mean diameter extending up to several tens of nanometers occur, further increasing their size after post-densification high temperature treatments. Rare-earth segregation causes a reduction of the OH content of glasses. Nanoclusters are amorphous, possibly close to a (RE)2SiO5 stoichiometry. Room temperature radio-luminescence measurements reveal that the emission spectra are dominated by RE3+ emissions and no bands due to silica matrix defects are detected.

Hyperfine structure of Ho3+ levels and electron-phonon coupling in YPO4 single crystals

High resolution spectroscopy (the finest being 0.01 cm(-1)) was applied in the 75-25,000 cm(-1) and 9-300 K ranges to a 1 mol% holmium doped Y PO(4) single crystal with two purposes: (1) to study the hyperfine splitting of Ho(3+) energy levels of interest for possible quantum manipulation media and (2) to analyze the electron-phonon interaction. The hyperfine structure was clearly revealed for a high number of lines in a wide wavenumber range (up to ~21,500 cm(-1)) and for a large number of multiplets. Several hyperfine patterns were monitored, differing in the number of components (a maximum of 16 could be easily distinguished in a single beautiful pattern), in their separation, and in their relative statistical weight. These features were all understood in terms of a crystal-field model, whose results are in good agreement with experiments and account for the involved level symmetry, the type of transitions (electric and magnetic dipole allowed), and the contribution of a second-order (pseudoquadrupolar) hyperfine coupling between close levels. The electron-phonon interaction, investigated through the thermally induced line shift, was critically discussed in the framework of single phonon coupling and of two phonon Raman scattering models.

Crystal field fine spectroscopy of trivalent terbium in yttrium aluminum borate single crystals

Tb3+ doped single crystals of huntite-type YAl3(BO3)4 (YAB) have been characterized by means of Fourier transform absorption spectroscopy in the wave number range 500-25000 cm−1 and in the temperature range 9-300 K, and by means of 9 K linear dichroism measurements. Crystal-field splitting of the fundamental 7F6 and of the excited 7F5, 7F4, 7F3, 7F2, 7F1, 7F0, and 5D4 manifolds of Tb3+ have been analyzed and fitted within a single ion Hamiltonian model, to obtain free-ion and crystal-field parameters. The electron-phonon interaction has been put into evidence by thermally induced line shift and discussed in the framework of a single phonon coupling model.

Vibrational spectroscopy of silica glasses doped with Eu3+ ions

In the present work the effects produced by Eu3+ incorporation into sol-gel silica (doping level range: 0.001-10 mol%) are monitored by means of Fourier transform vibrational spectroscopy in the wave number range 200-3000 cm−1. Complementary microreflectance, TEM, and Raman spectroscopy measurements are also performed. By increasing the Eu3+ concentration up to 3 mol%, the intrinsic absorption/reflectance bands related to vibrational modes of O–Si–O groups are gradually modified for what concerns the peak position and intensity, and absorption shoulders attributable to the Eu3+ doping appear. At 10 mol% sharp, new, Eu-related peaks grow at the expenses of the intrinsic absorptions. The results can be explained in terms of Eu-rich cluster formation, as also supported by Raman spectra analysis and TEM images. The cluster sizes increase by increasing the Eu3+ concentration. Their structure, in which tetrahedral (SiO4)4- groups are still present, remains amorphous for Eu3+ concentration up to 3 mol%, turning into a nearly ordered arrangement for the higher one. The spectral analysis in the regions of the weaker absorptions due to combination and overtone modes further supports the results and allows to associate the fundamental and overtone frequencies for two modes in Eu3+ 10 mol% doped silica. In the framework of the Morse anharmonic oscillator model, the related anharmonicity parameters and binding energies were calculated and found close to those reported for (SiO4)4- groups.

Rare earth crystal field spectra as a probe of librational motions in BaY2F8 solid state laser crystals

The fine structure (FS) accompanying a few lines, originated by crystal field (CF) transitions of rare earths (RE), as Er3+ and Tm3+, in BaY2F8 single crystals, is analyzed as a function of the RE3+ concentration (0.5÷20 at%) and temperature (9-300 K), by using high resolution (as fine as 0.02 cm−1) Fourier transform spectroscopy and linear dichroism measurements. The 9 K absorption spectra show that FS includes weak, narrow, and closely spaced (0.4÷0.8 cm−1) lines, covering a few cm−1 range on both sides of the narrowest among the CF lines. The FS increases by increasing the RE3+ concentration and vanishes at rather low temperature (40 and 60 K for Er3+ and Tm3+, respectively). The polarized light spectra confirm the association of a given set of FS lines to a specific CF line. The FS is ascribed to the simultaneous excitation of an electronic CF transition and of a local librational (or hindered rotation) mode of the RE3+-F− group. The attribution is supported 1) by specific features of the host matrix and guest rare earths, which allow some mobility of F− ions, and 2) by the spacing of the FS lines, which is in excellent agreement with the calculated RE3+-F− group rotational constant.

Correction to “Evidences of Rare-Earth Nanophases Embedded in Silica Using Vibrational Spectroscopy” [Jun 10 1361-1369]

In the above titled paper (ibid., vol. 57, no. 3, pp. 1361-1369, Jun. 10), the authors recognized some errors in Fig. 3 (Panel A) following the publication of the paper. The correct figure is presented here.