Line Raman Scattering

Spontaneous Raman Scattering (SRS) is an experimental technique to measure the densities of chemical species. Fundamentally, this phenomenon is the inelastic scattering of light from vibrationally active molecules. The scattered light is collected using a spectrometer, and provides signals to measure the number-densities of chemical species. Unfortunately, due to the weak signal strength this technique only permits the measurement of major species. However, temperature can then be inferred with knowledge of the local pressure and an appropriate equation of state.

Experimentally, the scattered light is collected at a right angle to the laser beam path, and processed with a spectrometer. When alignment of the spectrometer slit coincides with the laser beam direction, concentration measurements may be performed along a 1D spatial domain—hence the term “line measurement.”

An example experimental schematic is shown to the right, adapted from Eckbreth (1996). The laser beam is focused through the desired spatial domain, and the signal is collected and focused on a spectrometer.

The Raman signals are observed as discrete spectral lines, that are characteristic to each gas species being considered. These signals are proportional to the species density, and are quantitatively determined using a calibration procedure. An example measurement is shown below, where the spectrum is measured using two camera exposures (to cover the full wavelength range of the Raman lines). A small flame exists near the spatial origin, as observed by the sharp decrease in Raman signals (due to lower species densities from the high temperature).

Line Raman measurement of a small diameter premixed flame (~3 mm). The reagents are a mixture of Carbon Dioxide, Oxygen, and Hydrogen. The signal peaks are labeled except for Oxygen, which lies to the right of the two CO₂ peaks.

 

Reference:
Eckbreth, Alan C. Laser Diagnostics for Combustion Temperature and Species, 2nd ed. CRC Press, 1996.