Raman Spectroscopy

>April 2001

All substances have characteristic spectroscopic features, fingerprints which allow them to be uniquely identified. Raman microscopy offers a unique analysis and identification that is chemically sensitive and spatially resolved.

Raman spectroscopy provides information on the vibrational frequencies of molecules. These frequencies depend on the masses of atoms in these molecules and on the strength of interatomic bonds. Thus each of the different bonds (for example C-H, C-C etc) is characterised by specific frequencies. These frequencies also depend on geometrical arrangement of atoms in molecules. Raman spectra are measured by illuminating a chemical with a laser and looking at light emerging from the specimen. The spectrum of this light is generally composed of several sharp peaks, and the energy shift between these peaks and the laser line is equal to the vibrational frequency. These frequencies can be, in principle, calculated, but in complex molecules this would be very difficult as the various peaks may merge forming complex bands. However the shape of these bands can be used as a chemically sensitive signature of the specimen. In the case if the specimen contains a mixture of various chemicals, the relative intensity of the peaks reflects the abundance of the components. With Raman microscopy a 1 cubic micrometer volume of a pure chemical can usually be easily identified.

Raman spectroscopy is based on a quantum-mechanical effect of emission of light at frequencies other than that of the exciting laser. In solids the emitted radiation reflects various vibrational modes present in the crystal, such as phonons, but also electronic transitions. It provides information about their energy, and in some circumstances about the density of electrons and/or defect centres. The Raman effect is weak, but it may be significantly enhanced using the technique of Resonant Raman Scattering. The RIEF-funded Raman system available at MU is designed to provide resonant enhancement for GaN with its He-Cd laser excitation at 325 nm, close to the bandgap of GaN, and thus we anticipate a vastly improved signal to noise ratio compared to the systems used elsewhere. The Renishaw Raman system at MU is based in a single grating spectrograph, a holographic notch filter and a CCD camera. Confocal operation using two intersecting slits allows imaging and mapping with in-plane resolution of 1 micrometer and in-depth resolution of 2 micrometers. The spectral resolution of 1 cm^-1 is adequate for high precision studies.

The system is designed to have a mapping capability. This is achieved by employing a computer-controlled x-y-z translation stage. This stage is located at the microscope stage where it steps the specimen according to the program (for example 1 micrometer at a time in a raster scan). The raster scan covers all single square micrometre sections of a designated area under investigation (for example 30 micrometers by 30 micrometers). At each step the stage stops for a designated time and a spectrum is taken. After all (900) spectra were taken, the data are processed. For example we may wish to evaluate an area under one of the observed peaks at a particular wavelength and plot its intensity over the whole 30 by 30 micrometer area, representing it using a gray scale(white: high intensity, black: low intensity). Thus we obtain a spatial map of the distribution of a given molecular species characterised by this particular peak. This option may help identify the spatial extent of chemical variations in the specimen.