Yasuo Iida

Effects of atmosphere on laser vaporization and excitation processes of solid samples

Abstract Effects of atmosphere on the laser vaporization and excitation processes were investigated with spectral measurements and with the direct measurement of vaporized weight of samples. The samples, metals and ceramics, were positioned in three different atmospheres, i.e. air, argon and helium, from atmospheric pressure to a pressure reduced to a few torr. The time-resolved emission intensities of the Fe I lines and the Al I lines of the Al alloy and Al metal samples were measured in two time windows, i.e. 0–1 μs and 1 μs-1 ms. The excitation temperatures and the electron number densities of the plasmas were also estimated. The emission spectra, the excitation temperatures, and the electron densities were shown to be appreciably influenced by the ambient atmosphere. The amount of sample vaporized which was measured directly with an electric micro-balance after irradiation by 500 laser shots changed considerably with the atmosphere, e.g. from 12 ng pulse at atmospheric pressure to 330 ng pulse at 10 torr in argon. The results are discussed in the scope of the possibility of ambient gas breakdown before sample vaporization and a change in the laser radiation coupling to the solid surface. It is revealed that the control of the interaction between laser radiation and plasmas and the prevention of the preceding gas breakdown are important for effective laser vaporization and for the emission measurement of the plasmas. The sample characteristics also influence the initiation stage of the plasmas and the effects from the atmosphere.

Atomic emission characteristics of laser-induced plasmas in an argon atmosphere at reduced pressure

The emission characteristics of laser-induced plasma, with the use of a Q-switched ruby laser of 1.5 J, were studied in argon atmosphere at reduced pressure. The time- and spatially resolved emission profiles were measured. In argon atmosphere at reduced pressure, the emission period of plasma is elongated to over a hundred microseconds, and the emissive region expands to more than a few tens of millimeters above the sample surface. The emission intensities of atomic lines increase severalfold in an argon atmosphere, in comparison with those obtained in air at the same pressure. Moderate confinement of plasmas and a resultant increase of emission intensities are achieved at 50 Torr. These results are explained by the chemical inertness and the thermal characteristics of the argon atmosphere and the decrease in absorption of the laser pulse by the plasma plume. The re-excitation of emissive species by collisions with metastable argon atoms seems to be less important.

In situ Raman monitoring of low-temperature synthesis of YAG from different starting materials

Abstract Low-temperature formation of yttrium aluminum garnet (YAG) from different starting materials, i.e., alkoxide–acetate, nitrates, and hydroxides by homogeneous precipitation, has been monitored up to 1200°C using in situ Raman spectroscopy. YAG was formed at relatively low temperature around 800°C by the alkoxide method, but at 1100°C by the nitrate and the hydroxide methods. Formation of an intermediate hexagonal YAlO 3 phase was observed for the oxide powders prepared by the latter two methods at about 1000–1100°C, which hindered the lower temperature formation of YAG phase. Additional Raman and X-ray diffraction investigation revealed the presence of impurities and the difference of local homogeneity in the oxide powders prepared by the different methods. Highly homogeneous powders obtained by the alkoxide method enabled the direct and low temperature formation of YAG.

Laser vaporization of solid samples into a hollow-cathode discharge for atomic emission spectrometry

Abstract A hollow cathode discharge system has been developed for atomic emission spectrometry of solid samples. In this system, a solid sample is vaporized by a Q-switched Nd:YAG laser, and the vaporized material is introduced into a hollow cathode discharge by means of an argon gas flow. A photodiode array detector is used to acquire the spectral data and the spatial intensity profiles of atomic lines in the cathode bore. Experimental parameters which have been optimized include the pressure and flow rate of argon gas, the forms of the electrodes, and the discharge current. The laser is operated in either single pulse mode or multiple pulse mode, 20 pulses at 10 Hz repetition rate. The latter mode offers improved precision and detection limits, and quasi-continuous emission signals. Analytical curves obtained for standard samples of aluminum alloys are linear over 3 orders of magnitude with a relative standard deviation of 1.6–4.6%, and the estimated detection limits for the various elements range from 3 μg g −1 for Mg to 16 μg g −1 for Ni.

COMPARISON OF CAPILLARY AND SKIMMER INTERFACES IN EVOLVED GAS ANALYSIS-MASS SPECTROMETRY (EGA-MS) WITH REGARD TO IMPURITIES IN CERAMIC RAW MATERIALS

Capillary and skimmer interfaces in EGA-MS were compared by analysis of the carbon substances remaining in the hydrolysis products of aluminum alkoxides. Peaks with higher intensities were detected in the skimmer interface mode than those in the capillary interface mode. It was confirmed in particular that the skimmer interface is effective in increasing the sensitivity for species with higher mass, e.g. the evolved butyl group, as carbon substances remaining in the hydrolysis product of aluminum sec-butoxide.

In situ Anti-Stokes Raman Monitoring of Gel-to-Anatase Phase Transformation of Titania

The phase transformation from a gel of titanium alkoxide to crystalline anatase was observed in situ by anti-Stokes Raman scattering. Utilization of anti-Stokes scattering eliminates interference from the fluorescence of decomposing organic residuals during the pyrolysis process. The anti-Stokes Raman spectra suggest a highly disordered structure during the pyrolysis process. The structure of the titania gel, which is characterized by broad Raman peaks around 440 and 620 cm−1, was transformed into the amorphous structure by increasing the temperature up to 200–300 °C. This amorphous structure is supported by a flat Raman spectrum lacking specific features in the wavenumber region from 200 to 1000 cm−1. A partially crystallized gel also exhibits the further progress of amorphization during the pyrolysis process, which is explained by the disordering of the local structure of the gel due to the generation of pyrolyzed organic species.

Comparison of Detection Schemes for High-Temperature Raman Spectroscopy:

APPLIED SPECTROSCOPY 0003-7028 / 98 / 5109-1426

Pressure Dependence of Laser-Induced Fluorescence of Sm3+ in Al2O3—Y2O3 System Compounds

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Repetitive spectral subtraction method for the spectrophotometric determination of rare earth elements

The piezospectroscopic (PS) coefficients relating frequency shift to pressure are evaluated for Sm3+ in the Al2O3–Y2O3 system compounds, which include Al2O3, Y3Al5O12, YAlO3, Y4Al2O9, and Y2O3. The fluorescence peaks are inherently narrow for all the synthesized compounds, on the basis of the f–f transition nature of Sm3+ ion. Furthermore, the PS coefficients are ranged from −2.2 to −8.2 cm−1/GPa, which are similar to the reported value for the R1 line of ruby and suitable for the PS stress evaluation. PS stress distribution measurements are demonstrated with sintered Y3Al5O12 ceramics as a sample, with 50 μm intervals, under a microscope.

On the suppression of sonochemiluminescence reduction at high acoustic amplitudes by the addition of particles

SummaryA new multiwavelength data-analysis method for the determination of multicomponent mixtures, the repetitive spectral subtraction method (RSSM), is proposed. In RSSM, spectral data are obtained at evenly spaced wavelengths in the range of interest, and the analysis of the spectrum is performed as repetitive determination-subtraction cycle. In the determination step, the series of data in the narrow wavelength range is used, which is specified as to include the most characteristic or the largest peak for each component. Assuming a linear background, the target component, which can be a small group of components, is simultaneously determined together with co-existing components. In the next subtraction step, the calculated contribution of the component is subtracted from the spectral data in the whole measured wavelength range, and the component is assigned as determined and is excluded from the following analysis. This determination-subtraction cycle is repeated until all the components are determined.With RSSM, a mixture of many components can be precisely determined by the use of most information contained in the spectrum. The accuracy of determination can be improved by automatic estimation of the background and the gradual decrease of interferences. In order to demonstrate the usefulness of RSSM, the 8 rare earth elements (Pr, Nd, Sm, Eu, Dy, Ho, Er, Tm) in the model mixtures were determined by spectrophotometry, and the results were compared with those of the usual correction factor method and the derivative method. Furthermore, the 8 rare earth elements were determined in ores, monazite and xenotime.