Rotational Coherent Anti-Stokes Raman Spectroscopy: Experimental and Theoretical Developments in Gas-phase Thermometry

Abstract: Dual-broadband pure rotational coherent anti-Stokes Raman spectroscopy was investigated in terms of thermometry and species concentration measurement for studies of combustion. The modelling of rotational CARS spectra under conditions of high pressure at room temperature was investigated. Resonant terms of the Stokes (CSRS) type were found to have a significant effect on the intensity of the anti-Stokes (CARS) branch, the effect increasing in strength with an increase in pressure. Taking account of this inter-branch interference effect improved the accuracy of temperature measurement under conditions of high pressure. An experimental study of the statistical precision of temperature measurements showed the precision to improve with use of a single-mode rather than a multi-mode Nd:YAG laser, a decrease in standard deviation by a factor of two or more being observed. Further investigation linked the measurement precision to the intensity noise observed in the CARS spectra. Temperature measurements in the post-flame zone of ethylene/air flames were conducted with the aim of studying the impact on temperature measurements of the collisional line widths employed in the model. Taking account of the line broadening of the species CO, CO₂ and H₂O was found to increase the evaluated temperature by 20-47 K. Spectra recorded in ethylene/air flames revealed the spectral signatures of N₂, O₂, CO and CO₂, the spectrum of nitrogen always being strongest. The potential of rotational CARS for species concentration measurements under such conditions was also investigated. A model for binary mixtures of N₂ and CO that was developed was tested for concentration measurements in these flames. The measured CO concentrations agreed closely with theoretical calculations. The collisional line broadening of Raman spectral lines in mixtures of N₂ and CO was studied theoretically by means of semiclassical Robert-Bonamy calculations. The self-broadening of nitrogen (N₂-N₂) in particular was investigated, employing the ab initio intermolecular potential PES8. The major objective was to calculate CO-N₂ and N₂-CO line broadening. Use of the line widths obtained resulted in more accurate CO concentration measurements in binary mixtures of CO and N₂ by use of rotational CARS.