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Raman and the Diamond Anvil Cell

The Raman Effect Group Theory Lin et. al (2012), Am. Mineral. Light from a laser source is used to excite the vibrational, rotational, Group theory is based on mathematical reduction, which assigns Vibrational and elastic properties of and electronic states of a sample. Light emitted from the sample a point group to of the same fundamental symmetry. magnesian siderite across the electronic having same energy as the indicent light is said to be elastically Point groups hold important implications for many of the spin-pairing transition of iron scattered. Inelastic scattering occurs when the sample scatters the properties, including spectral, of molecules. Take for example A Raman and X-ray di raction study of the electronic and light in such a way that the di erence in energy between the indicent water: vibrational properties of magnesian siderite (Mg ,Fe )CO and emitted light is not zero. This is termed the Raman E ect, 0.35 0.65 3 shows a spin-pairing transition at around 45 GPa in a DAC. The whereas the elastic process is called Rayleigh scattering. Geometry: Bent observed Raman shifts result from an ~8% decrease in unit cell O Point group: C2V volume and a 10% increase in bulk modulus. # of normal modes: 3(3)-6=3 Raman Shift H H The interaction of the incoming light with the increases the 3 peaks in Raman spectrum T: translation; energy of the vibration to a certain virtual energy state. When the L: libration; ν : symmetric stretch; energy of the vibration falls back down, it can either reach an energy Figure 3. Symmetry operations of water, showing vertical mirror plane and rotation axis. 1 ν state that was lower or higher than the initial state. The di erence in 4 : in-plane bend; The point group of water is derived from its symmetry: one cyclic energy between the initial and nal states denes the Raman shift, F: possible Fe-O (C) 2-fold rotation, and one vertical mirror plane. Water has two and is expressed in units of wavenumber (cm -1 ). vibration. symmetric stretching modes and one asymmetric mode. Group

Inelastic scattering theory is used to predict the number of observed spectral bands. Elastic scattering Pressure increase causes C-O bond Instrumentation lengthening, which

Y uses a monochromatic laser source in the results in a frequency G R S near-UV to near-IR range (200-2500 nm) of the electromagnetic increase, as seen in E 2

N S spectrum. The set-up in the Mineral Physics lab at UT-Austin uses the Raman shift E 1 So a Verdi V2 532 nm laser and an Andor iCCD detector. (Figs. 6,7). Rayleigh Stokes shift Anti-Stokes shift Figure 1. Corresponding energetic transitions of Rayleigh, Stokes, anti-Stokes scattering. Diamond Anvil Cell Near-UV Minerals found in the deep Earth can Figure 6. Raman spectra of magnesian siderite over pressure increase. Raman shift is defined as: undergo changes under an increase Increases in unit cell stiffness and bulk modulus, and 400 nm 1 _ 1 in pressure and temperature. Some of decrease in unit cell volume contribute to frequency ν = these changes can be observed using increase in all modes except for the symmetric stretch.The λincident λscattered Raman spectroscopy in a diamond symmetric stretch shows splitting presumably due to the in- 2- anvil cell (DAC). phase and out-of-phase vibration of adjacent CO 3 units. Vibrational Modes 500 nm Light Linear molecules have 3N-5 normal modes (N= # of ); non-linear molecules have 3N-6. These modes include stretching and bending Visible between atoms as well as out-of-plane vibration. The following gure Anvil shows some of the Raman active modes of (Mg,Fe)CO . 3 600 nm

Gasket Y

G Sample Chamber

R 700 nm E N

E Near-IR Figure 2. Lattice and internal modes of ferromagnesite, as described in Lin et al. (2012). Figure 4. Near-UV to near-IR portion (200- Lattice vibrational modes involve molecular vibration relative to 2500 nm) of the . other molecules. Internal vibrational modes refer to atomic Figure 5. Diamond anvil cell schematic. movement relative to other atoms. Figure 7. Increase in Raman shift at the spin transition, except in symmetric stretch.