Thomas F. Cooney

Comparative Study of Some Fiber-Optic Remote Raman Probe Designs. Part I: Model for Liquids and Transparent Solids

We have developed models describing the sensitivity and sampling volume of various remote fiber-optic Raman probes—single-fiber, lensed, dual-fiber beveled-tip, dual-fiber flat-tipped, and multi-fiber flat-tipped. The models assume clear samples and incorporate radii, separation, bevel angle, and numerical aperture of the fibers; overlap geometry of illumination and excitation light cones; and refractive index of immersion medium. For the Raman spectra of solid samples in air, single-fiber and lensed probes are predicted to yield the highest Raman signal. Beveled probes should provide greater Raman signal strength than do flat-tipped probes because beveled probes can collect light from a restricted volume closer to the probe end. Although multiple collection fibers improve Raman signal strength, progressively distant concentric fiber rings contribute less and sample material further from the probe.

Comparative Study of Some Fiber-Optic Remote Raman Probe Designs. Part II: Tests of Single-Fiber, Lensed, and Flat- and Bevel-Tip Multi-Fiber Probes

We compare relative performances of flat-tipped, beveled (two-fiber and six-around-one), and single-lensed focused fiber-optic Raman probes and, where feasible, evaluate the utility of optical filters for reducing fiber background. The sensitivity profile of each probe is determined by measuring the relative intensity of light backscattered off a flat surface as a function of distance from the probe tip. The experimental results are compared with a simple light-cone-overlap model incorporating fiber numerical aperture, fiber and immersion medium refractive indices, separation between excitation and collection fibers, number of fibers, and fiber bevel angle and/or lens focal length. The model and sensitivity profiles are used to interpret the sampling regions for Raman spectra obtained by using each of the probes with a clear, transparent sample (single-crystal sparry calcite), a white, partially transparent sample (acetaminophen tablet), and a set of organic liquids of varying refractive index. The sensitivity of the tested commercial lensed probe drops off symmetrically about the focal point. For both solid samples, the intensity of fiber background follows a profile determined primarily by laser backscattering off the surface, whereas the sample Raman signal follows a profile dependent upon sampling depth.

Long fiber-optic remote Raman probe for detection and identification of weak scatterers

A long-length fiber-optic remote Raman probe that can detect materials (e.g., Fe(2)O(3)) that are weak Raman scatterers is described. The 100-m-long probe consists of one excitation and one collection fiber and accessory optics and is <3 mm in diameter. We used special filters to solve the problems of Raman scattering and luminescence generated in the silica excitation and collection fibers and a liquid-nitrogencooled charge-coupled detector to analyze weak Raman signals.

Structure of glasses in the systems Mg2SiO4Fe2SiO4, Mn2SiO4Fe2SiO4, Mg2SiO4CaMgSiO4, and Mn2SiO4CaMnSiO4

Abstract We have measured polarized micro-Raman spectra for twelve orthosilicate glasses containing varying proportions of Ca2+, Fe2+, Mg2+, and Mn2+, and one FeOSiO2 composition slightly enriched in silica relative to fayalite. We have also obtained selected area electron diffraction data for glasses having the compositions of forsterite (Mg2SiO4), fayalite (Fe2SiO4), and tephroite (Mn2SiO4). Our Raman data indicate that: (1) CaMg olivine glasses all have similar structures consisting of isolated SiO4 tetrahedra with subordinate short-chain polymers; (2) addition of Ca2+ causes reduction in the effective SiO vibrational force constants; (3) in the absence of iron, a proportion of manganese occurs in tetrahedral MnO4 complexes which are not network formers; (4) silicon-oxygen bonds are shorter in glasses than in the isocompositional crystals; (5) significant polymerization occurs in fayalite-bearing glasses. The diffraction results for forsterite glass show evidence for cryptocrystalline periclase regions no larger than 40 A, probably resulting from a melt free oxygen fraction which cannot be quenched to glass. Fayalite and tephroite glass diffraction patterns show no such evidence for a free oxygen fraction. Therefore, it is proposed that the high degree of polymerization of fayalite-bearing glass results from network-forming Fe2+ ions.

Vibrational Spectra of Olivine Composition Glasses: The Mg- Mn Join

The vibrational frequencies of a series of splatquenched, olivine glasses spanning the compositional range from Mg2SiO4 to Mn2SiO4 have been determined using both infrared and Raman spectroscopies. The spectra of all glasses show evidence of tetrahedral coordination of silicon (possibly with some slight distortions), and largely octahedral coordination of magnesium. Spectra of Mn-rich glasses indicate that there is some manganese in 4 or 5-fold coordination. The frequencies observed for the fundamental vibrations of the silica tetrahedra are similar to those previously observed for SiO4 groups in both crystalline and glassy orthosilicates. Additionally, there is evidence for a small amount of silicate polymerization in all glasses characterized: vibrations attributable to Si2O7 groups are visible in both infrared and Raman spectra.

IN SITU STRUCTURAL INVESTIGATION OF IRON-CONTAINING SILICATE LIQUIDS AND GLASSES

In situ high-temperature Raman spectra are presented below and above the glass transition temperature (Tg) of the 0.5Na2O·0.5Fe2O3·3SiO2 (Fe-albite, NFSO), 1.5Na20·0.5Fe2O3·3SiO2 (NFS2), K2O·0.5Fe2O3·3SiO2 (KFS), and reduced K2O·FeO·3SiO2 (KFS-R) compositions. For the oxidized sample, there is a close structural relationship between the glass and the super-cooled melt above Tg. With increasing temperature, no coordination changes of Fe3+ and no new anionic species are observed in the oxidized melts. In the spectrum of fully polymerized Fe-albite (FeAb) glass, the increase in linewidth of the low frequency Raman band at 452 cm−1 and shift in the position of the band to higher frequency at high temperature indicate an increase in the extent of disorder and decrease of average T-O-T angle, where T = Si, Fe3+ in tetrahedral coordination. In the depolymerized glasses, (NFSI ) and KFS, the increase in intensities of the Raman bands associated with Q3 species, containing three bridging oxygen atoms per T cation, result from either a change in relative cross sections of Raman modes or from a net increase in Q3 species with increasing temperature. On the basis of observed changes in the Raman spectra of the reduced sample (KFS-R), it is proposed that most of the Fe2+ ions act as network modifier. This suggestion is justified by the observation of the polarization character of the spectra and the appearance of new Raman bands.

High temperature structural investigation of Na2O·0.5Fe2O3·3SiO2 and Na2O·FeO·3SiO2 melts and glasses

The structure of glasses and melts of Na2O· 0.5Fe2O3·3SiO2 and Na2O·FeO·3SiO2 compositions have been measured using high temperature Raman spectroscopy. For the oxidized sample it has been demonstrated that there is a close structural relationship between melt and glass. No coordination changes of Fe3+ with temperature and no new anionic species have been observed in the oxidized melt. The Raman spectra of the reduced sample clearly show a decrease in the degree of polymerization, as determined by the observation of the polarization character of the spectra and the details of the change of the Raman intensities during heating in hydrogen. Mössbauer spectra suggest that Fe3+ is tetrahedrally coordinated in the oxidized glass and part of the Fe2+ is tetrahedrally coordinated in the reduced glass.

Rare-Earth-Doped Glass Fiber for Background Rejection in Remote Fiber-Optic Raman Probes: Theory and Analysis of Holmium-Bearing Glass

We have investigated the feasibility of using rare-earth-doped glasses in “self-filtering” optical fibers used for remote Raman spectral collection. We have derived a theoretical treatment and have used the measured relevant sample and glass parameters (e.g., optical absorption of the doped glass and ratio of the intensities of inelastic scattering to elastic scattering plus reflection) to evaluate the usefulness of such fibers. With the use of these parameters, the optical absorption bands of Ho-doped glass, in particular, are found to be sufficiently intense and sharp to enable this glass to be used in the collection fiber of a remote Raman probe. Ho-doped glass fiber of as little as 2 cm in length is sufficient to filter undesirable laser radiation while permitting a high proportion of the sample Raman signal to pass. Use of the 488.0-nm Ar+ laser line or green or red laser wavelengths from a “tunable” laser can ensure that the excitation is within an absorption band and close to the long-wavelength transmission cut-on for the doped glass.

Electron paramagnetic resonance study of iron(III) and manganese(II) in the glassy and crystalline environments of synthetic fayalite and tephroite

Crystals of the olivine minerals, tephroite (Mn2SiO4) and fayalite (Fe2SiO4) containing manganese(II) and iron (II and trace of III), respectively, were synthesized. Glasses were prepared from these crystalline materials by a splat-quench technique. Measurement of electron paramagnetic resonance (EPR) of all these powdered samples at room temperature show that the g-factors of Mn2+ in both glassy and crystalline environments (geff = 2.004) are the same, although the EPR linewidths (for glass, ΔHpp = 200 G; for crystals ΔHpp = 287 G) suggest less clustering of paramagnetic Mn2+ ions in the glass. Mn2+ probably occupies a distorted octahedral site in the tephroite crystal structure, although a four-fold coordination is suggested from other spectroscopic investigation on this glass. The EPR parameters of Fe3+ in synthetic fayalite glass (geff = 2.01 and 6.00; ΔHpp=150 and 1375 G, respectively, for the high and low field resonances) and powdered crystals (geff = 3.31 and ΔHpp = 900 G) indicated that Fe3+ ion in the crystals, is probably located in a distorted tetragonal site M2 and an axial environment has been proposed in the glassy system.