中研院地球科學研究所 - 礦岩物理實驗室
IES, Academia Sinica - Mineral Physics Lab


Facilities

Main Tools

Raman Spectrometer

Raman signals are caused by the scattering of photons with energies of the order of 2 to 4 eV by optical photons. When the spectrum of the radiation scattered by the crystal illuminated with monochromatic light of frequency ωL is analyzed, it is found that it consists of a very strong line at the frequency ωL, as well as of a series of much weaker lines with frequencies ωL ± ωj (q), where ωj (q) are optical phonon frequencies. The strong line centered at ωL is due to elastic scattering of photons and is known as Rayleigh scattering. The series of weak lines at ωL ± ωj (q) originates from inelastic scattering of photons by phonons and constitutes the Raman spectrum. The Raman bands at frequencies ωL - ωj (q) are called Stokes lines, those at frequencies ωL + ωj (q) are known as anti-Stokes lines. The intensities of the anti-Stokes lines are usually considerably weaker than those of the Stokes lines.

In first-order Raman scattering only optical phonons with q ≈ 0 are involved. This is a consequence of momentum conservation.  Since the selection rules in infrared and Raman scattering are different, the two techniques provide complementary information.  The intensity of the light is proportional to |M|2, where M = αE is the dipole moment induced by the electric field E of the light, and α is the electronic polarizability tensor with components αxx, αxy, αxz, etc.  The components αρσ depend on the normal coordinates of the vibrating sysem.

Reference: P. Bruesch (1986) Phonons: Theory and Experiments II – Experiments and Interpretation of Experimentalo Results. Springer-Verlag, Berlin.


Brillouin spectrometer

Infrared absorption, Raman scattering, and Brillouin (or Brillouin-Mandelshtam) scattering are three of the major events caused by the interaction between photons and phonons in a substance.  They all involve the vibration of chemical bonds.    However, Brillouin scattering can be observed only on the condensed matters.  The interaction between light and thermally excited acoustic excitations in a solid was first predicted early this century by L. Brillouin (1922) and independently by L. I. Mandelshtam (1926).  The Brillouin scattering was then experimentally confirmed by E. F. Gross in 1930.  However, it was still hard to detect acoustic as well as spin waves in an opaque solid until the advent of a Fabry-Pérot interferometer designed by J. R. Sandercock in 1970.  Today, a Brillouin scattering system with a multipass Fabry-Pérot interferometer has became a powerful tool for the observation of light scattering from acoustic wave in a material.

The main difference between Raman and Brillouin scatterings is the phonons involved: optical phonons in Raman scattering and acoustical phonons in Brillouin scattering.  The intensity of the scattered light depends on the change in the electronic polarizability (or susceptibility) of the material which is induced by phonons.  The signals from the interaction of incident beam with acoustic transverse and longitudinal phonons have enabled us to determine the elastic constants and moduli of a solid.  The difference among IR, Raman and Brillouin spectroscopy is shown below.

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In a crystal, the elastic moduli (Cijkl),sound velocities (V) and crystallographic directions are correlated by Christoffel's equation:

det│Γrs  –  ρV2δrs│= 0

where ρ is the density, δrs is a delta function (δrs = 1 when r = s; δrs = 0 when r≠s), and Γrs is the Christoffel matrix.  Based on the measured velocities of a crystal, the adiabatic elastic contants and moduli of an aggregate of crystals can then be estimated by solving Christoffel’s equation.

Brillouin spectroscopy is superior to the ultrasonic-related techniques in the measurement of elasticity of a small crystal.  In this lab, we have set up a six-pass JRS tandem Fabry-Pérot interferometer system for single-crystal Brillouin scattering.  The elastic properties of the mantle materials and minerals (including crystalline and non-crystalline) can be determined by Brillouin spectroscopy at various pressures or temperatures.  The data obtained from this technique can be compared with the seismic data.  The possible contribution of a specific mineral and/or its high-pressure polymorphs to the elastic properties of the Earth’s interior can be explored.

References:
L. Brillouin (1922) Ann. Phys. (paris) 17, 88.
E. F. Gross (1930) Nature 126, 201.
L. I. Mandelshtam (1926) Zh. Russ. Fiz. Khim. Ova. 58, 381.
J. R. Sandercock (1970) Optical Commun. 2, 73.

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Auxiliary Tools

(1) Polarizing microscope

 

(2) Diamond anvil cells

 

(3) Others:
heating stages (-190 ~1500 oC), temperature furnaces (up to 1700 oC), slow cutter, polishing machine, stereomicroscopes,electric discharging machine