Raman scattering is observed in MgO microcrystals with three different particle sizes (105, 103, and 102 Å). The second-order Raman lines in the bulk crystals of MgO disappear in all the microcrystals, while new peaks emerge at 280, 446, and 1088 cm−1 except for specimen prepared by grinding a large single crystal. Raman lines with lower frequencies, which are enhanced with decrease of the particle size, are discussed as due to the first-order Raman scattering forbidden in the bulk crystals. The assignment is confirmed in terms of photon density of states in microcrystals. Another line with the highest frequency is coincident with a previously reported Raman line whose origin has been attributed to a surface phonon mode.
Measurements ofRaman scattering from the vibronic origin in MnCO, (rhodochrosite) are reported. The observed five Raman lines (184, 289, 719, 1086 and 1415cm-') are identified with and classified into the relevant phonon modes. Those lines show the theoretically expected polarization characteristics. INTRODUCIIONManganese carbonate (rhodochrosite) is isomorphic with calcite (CaC03) and ferric borate (FeB03), which belong to a rhombic structure with DGrR3c space group. Phonon modes of these crystals are easily identified because of the simple crystal structure. On the latter two crystals phonon energies have been measured and analysed by infrared absorption and Raman scattering experiment^.'-^ However, no experiments have been reported on Raman scattering in MnC03. In this paper we present the exact values of phonon energy expected in MnC03 and discuss the polarization characteristics of phonon Raman spectra. EXPERIMENTALThe Raman experiments were performed on natural single crystals attached to the cold finger of a liquid helium flow-type cryostat. The temperature of the crystals was controlled and maintained constant to better than 1 K. The samples were illuminated by linearly polarized light (ca. 300mW at maximum for a single mode) at 488.0 nm and at 514.5 nm with an argon ion laser. A Jobin-Yvon HG-2S Ramanor double monochromator and standard photon counting equipment with a microcomputer control system were employed for detection. A quartz wedge in front of the entrance slit of the monochromator spatially scrambled the polarization characteristics of the detection system. Five configurations were examined for the observation of the polarization characteristics of Raman lines. In every case the observed scattered radiation lies in the direction orthogonal to the incident beam.The MnC03 crystal was cut in the form of a rectangular parallelpiped, the x, y and z axes being normal to the faces. These orthogonal axes taken for convenience are represented in terms of the primitive vectors in the rhombohedra1 system as follows: the x, y (binary axis) and z (optic axis) directions may be designated as [ii2], [IT01 and [lll], respectively.
Spectroscopic investigations are carried out on the radiation emitted from laser-induced plasma on a NaCl crystal surface. It is found that all atomic lines emitted from plasma produced by laser irradiation in air are broadened and some of them are self-reversed, while the self-reversal is not observed for plasma induced in vacuum. Self-reversal occurs for the pressure of ambient gas above 1.3 kPa. It is found from line broadening, mainly caused by resonance and the Stark effect, that the Na atom concentration in the plasma is 5.3 x loz4 m-3. Observed time-resolved spectra agree with a model that the plasma generated by laser irradiation is composed of a hot core and a low-temperature periphery, both behaving, respectively, as emission and reabsorption centers and expanding at a speed determined by the ambient gas pressure. I) 3-5-1 Johoku, Hamamatsu 432, Japan.
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