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Department of Earth Science

RAMAN LAB

Analytical facility for Raman spectroscopy and photoluminescence detection in the visual and near-infrared range.

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Instruments

 

Confocal Laser Raman Microscope

We are currently building the Raman spectroscopy analytical facility. It will consist of a confocal Raman spectrometer attached to a petrographic microscope with motorized mapping stage. This setup contains three different lasers and two detectors, which allows for Raman spectroscopy and Photoluminescence detection in the visual (VIS) and near-infrared (NIR) range. Possibly our scope of analysis will be extended in the future with detection in the ultraviolet range (UV).

The instrument will be installed during the summer of 2009. It will consist of the following components. Updates with pictures and further details will follow soon.

 

  • Raman spectrometer with VIS-CCD camera
  • Petrographic microscope and motorized mapping stage
  • Resolution x,y better than 1 um
  • Resolution z better than 2.5 um
  • High speed mapping and depth profiling capability
  • Argon laser 488 nm and 514 nm wavelength
  • Diode-laser 785 nm wavelength
  • Statistical software for background subtraction and peak fitting
  • Statistical software for PCA and map construction
  • Peak identification software
  • Spectral databases for minerals, inorganic and organic substances

 

Raman Spectroscopy - Theory:

The Raman effect, named after Noble price winner Chandrasekhara Venkata Raman, can be described as an inelastic light scattering process. When a strong light source (laser) is focused on a substance most of this energy will be scattered elastically. In this case the molecules of the substance are excited to a virtual electronic state and immediately fall back to their original state by releasing a photon (see figure 1). The photon energy of this scattered light is equal to that of the incoming light. This process is called Rayleigh scattering. A molecule may also fall back from an excited electronic state to an energy state that is higher (Stokes type scattering) or lower (anti-Stokes type scattering) than the original state. The difference in energy between the incoming and scattered photon (Raman shift) corresponds to the energy difference between vibrational energy levels of the molecule. The different vibrational modes of a molecule can therefore be identified by recognizing Raman shifts (or ‘bands') in the inelastically scattered light spectrum.

Fig1-energy-levels

Figure 1. (See figures at the top of the page) Simplified energy level diagram. The shift in wavelength between the excitation light (λe) and the scattered light (λs) is related to Raman shift (ΔV in cm-1) according to: ΔV = (1/ λe) + (1/ λs).

A wide variety of substances - minerals, organic molecules, fluids, gases - can be identified directly from their Raman spectrum. It should be noted that Raman scattering is effective for covalent bonds, and very weak for ionic bonds. The covalent bonding environment can be influenced by cation substitution in a mineral structure. An example for the mineral calcite is shown in figure 2, where Raman bands are generated by vibrational modes of the carbonate anion.

Fig2-calcite-spec

Figure 2. The Raman spectrum of Calcite and some of the associated normal vibrations of the crystal structure are shown.

The substitution of Mg for Ca in the carbonate structure, e.g. dolomite, leads to an altered crystal structure and a shift of the main Raman band from 1085 to 1091 cm-1. This difference in Raman shift can be mapped in a rock thinsection, as shown in figure 3.


Fig3-carbonate-mapping

Figure 3. Example of a Raman map using fast x,y-Raman scanning. This map consists of thousands of spectra, and was generated within a few minutes.

As vibrational energy is directly related to molecular bond strength, many additional molecular characteristics can be studied by Raman spectroscopy. These include:

- Stress patterns in minerals

- Qualitative recognition of stable isotopes (e.g. 13C-labelling)

- Breathing-modes of carbon nanotubes

- Degree of structural order in kerogen and graphite.


For further reading see:

Nasdala, L., Smith, D. C., Kaindl, R., and Ziemann, M. A., 2004. Raman spectroscopy: Analytical perspectives in mineralogical research. In: Beran, A. and Libowitzky, E. (Eds.), Spectroscopic Methods in Mineralogy. Eötvös University Press, Budapest. European Mineralogical Union, Notes in Mineralogy, Vol. 6.

 

Geological samples:

 

Laser Raman spectroscopy is used in our group primarily as a rapid tool for identification of minerals and characterization of microfossils in geological samples. We typically work with normal 30 um thinsections. For fluid inclusion studies 100 um thinsections are preferred. A typical analysis is entirely non-destructive, requires no further sample preparation, and takes place in a matter of seconds. Raman maps (i.e. chemical or mineralogical maps) and depth profiles can be constructed as well by obtaining thousands of spectra in a x,y- x,z- or y,z-grid. Such maps can be constructed in a matter of minutes to hours, depending on resolution (down to 0.3 um) and area (up to centimetres).

 

In geological and biological studies samples often need to be prepared for a range of dedicated in-situ techniques such as LA-ICP-MS, SIMS, NanoSIMS, SEM, TEM, EMP, and Synchrotron-based micro-analytical tools such as XRF and XANES. The rapid non-destructive character of Raman spectroscopy makes it the perfect tool for a first mineralogic characterization and recognition of areas of interest. Some examples of Raman applications are:

 

  • General mineral identification in rock slabs or thinsections
  • Organic microfossils (2D- and potentially 3D-mapping)
  • Opaque minerals (e.g. magnetite-series, sulfide-series)
  • Carbonates (siderite-magnesite-dolomite-rhodocrosite-calcite)
  • Carbonates in shells and microfossils (calcite-aragonite)
  • Carbonaceous matter (graphite, kerogen, shungite)
  • Mineral assemblages in hot spring sinters, hydrothermal vents
  • High-pressure minerals at impact sites
  • Mineral inclusions in e.g. diamonds
  • Fluid-inclusions (semi-quantitative CO2-CH4-N2-H2O etc.)

Other samples:
Although emphasis in our group is on geological and biological samples, Raman spectroscopy can be applied to a wide variety of samples. The switching between different laser wavelengths (488, 514, 785 nm) allows optimization for certain types of materials. In addition, it allows in certain specific cases for Resonant Raman spectroscopy. Some interesting examples are:

 

  • Dye in paintings
  • Archaeological artefacts
  • Material sciences: carbon nanotubes, silicon wafers
  • Stress/strain studies: solar panels, diamonds
  • Chemical processes: real time reaction analysis in micro-reactors
  • Cryogenic samples: gas hydrates in ice

 

 

Scientific responsible:

Rolf Birger Pedersen office Geobio 4. floor

Nicola McLoughlin office 4130, 4. floor

Technical responsible/contact:

Ole Tumyr office 3115 3. floor

 

The Raman lab is located in the second floor of the Department of Science building (Realfagbygget), the door number 2155