Raman Spectroscopy Suite

The Raman Spectrometer within the CAPS group consists of Jobin Yvon HR 640 microRaman module which is attached to an Olympus microscope (BX40). The same spectrometer can also be connected to larger macroscopic system for the analysis of much larger samples or those requiring special atmospheres or heated/cooled enviroments. The light source comes from a HeNe laser of wavelength 632.8 nm. The spectrograph has a 1200 gr/mm grating, projecting onto a liquid nitrogen cooled CCD (1024 x 256 pixels). Our spectrometer has the advantage of being fitted with holographic notch filter, this enables us to look at both Stokes and Anti-Stokes for samples

The basic theory of Raman Spectroscopy comes from the principle of inelastic light scattering (Raman Scattering) as opposed to elastic scattering of light (Rayleigh Scattering). When an incident photon interacts with the electron cloud around a molecule, the incident photon excites an electron into a virtual energy state. If a molecule is excited from the ground state to a virtual energy state, and then relaxes into a vibrational excited state, this generates Stokes Raman scattering. If the molecule was already in an elevated vibrational energy state, the Raman scattering is then called anti-Stokes Raman scattering. The diagram below demonstrates the change in energy levels.
Unlike Energy Dispersive X-Ray spectrocsopy Raman Spectroscopy does not provide direct elemental composition of a sample. What the Raman spectra of a sample contains is a set peaks that are characteristic of the chemical structure. Thus a Raman spectra of a given mineral e.g. Olivine, will be unique to its structure and different to another mineral such as Enstatite in a similar way that finger prints are unique to individual humans. By incorporating a microscope into the spectrometer the laser can be focussed onto very small areas within a sample, enabling us to carry out detailed microanalysis. This provides us with a system sensitive enough to tell the difference in composition of mineral such as olivine (i.e. it's Mg/Fe ratios). The technique requires very little sample preparation and is non-destructive. This makes it an attractive precursor to other forms of more destructive chemical analysis

Current Raman Research Projects

  • In depth study of mineralogy and chemical composition of cometary samples returned from NASA's Stardust Mission to visit Comet 81P/Wild 2.
  • Compositional changes of minerals impacting aluminium foils & ultra low density aerogels from hypervelocities.
  • Alterartion of organic materials being capture in aerogel.
  • Development of in-situ Raman spectroscopy of particles captured in aerogel.