Rotational Coherent Anti-Stokes Raman Spectroscopy - Development and Applications

Sammanfattning: During the last decades, coherent anti-Stokes Raman spectroscopy (CARS) has been an important tool in combustion research. The focus of this thesis is the improvement of the capability of the pure rotational CARS (RCARS) technique for this type of applications. The technique is used to measure temperatures and relative species concentrations in the gas phase and thermometry is performed by fitting theoretically modelled spectra to measured ones. In a first approximation, the thermometric information in the RCARS signal lies in the relative RCARS peaks intensities, reflecting the relative populations of the rotational states of the probed molecules. However, for accurate RCARS thermometry, a number of additional factors must be taken into account. The most important of these is the Raman linewidths. This parameter, which is governed by collisional energy transfer between the probed molecules and the surrounding gas, can have large impact on evaluated temperature if it is not properly included in the RCARS modelling. Therefore, for many of the articles on which this thesis is based, much effort has been made to investigate the thermometric impact of the Raman linewidths. There are also a number of works based on reporting Raman linewidths and developing a new method of measuring them. The RCARS technique can also be sensitive to different kinds of spectral interference, either in the form of scattered light from the laser sources used to generate the RCARS signal, or from unwanted contributions to the coherent RCARS signal itself. For several of the included papers, the application of polarization techniques was used to reduce this type of interference. This enabled more accurate thermometry and in some cases detection of the resonant signal was entirely dependent on this technique. The application of the RCARS technique in the papers included in this thesis has mainly been flame measurements. Both laminar and turbulent flames have been studied in collaborations with other experimental research groups and theoretical modelling groups. Spatial temperature scans in two dimensions have been performed in turbulent flames and the RCARS evaluations have been used to improve large eddy simulations of turbulent flames. RCARS measurements have also been performed in a heavily sooting flame in order to characterize how the laser induced incandescence (LII) technique influences the flame during measurement.