ROTATIONAL COHERENT ANTI-STOKES RAMAN SCATTERING FOR COMBUSTION RESEARCH

Abstract
Coherent Anti-Stokes Raman Scattering (CARS) has been used extensively as a diagnostic technique for temperature measurements in combustion system, in general by employing the relative intensities of the vibrational band of the investigated gas species (mostly N₂). Pure rotational CARS is superior to vibrational CARS for thermometry near room temperature, since the spectra are easily resolvable compared with the congestion of the rotational lines in the vibrational bands of the Q-branch spectra. The rotational Raman cross section for rotational CARS is larger than tlie vibrational CARS cross section and the Raman linewidth is more narrow for rotational CARS than for vibrational CARS, both tending to give a higher rotational CARS intensity. On the other hand, rotational CARS intensities are substantially reduced at higher temperatures, and in technical flames an interference of CARS contributions in the rotational spectra of different gas molecules present may complicate a temperature and concentration evaluation. The difficulty in separating these different contributions may be overcome by using Fourier analysis, which also can be used simultaneously for data reduction. Applying this technique to calculated pure and noisy rotational CARS spectra and also to experimentally obtained ones, temperature and concentration, determination is demonstrated for simple flames.