On the Use of Rotational CARS on Polyatomics and in Time Domain

This thesis work concerns the further developments of dual-broadband rotational coherent anti-Stokes Raman spectroscopy (RCARS) through experimental and theoretical investigations. Rotational CARS is an established technique for gas-phase thermometry and concentration measurement with major applications in combustion diagnostics. The technique has previously mainly been applied on species with linear molecular symmetry, for which the calculations of their spectra are relatively straightforward.The developments in the present thesis can be divided in two main categories. The first part of the thesis deals with the extension of nanosecond (ns) RCARS diagnostics towards two hydrocarbon fuels with complex molecular symmetries. The first is the symmetric-top molecule ethane (C2H6), and the second is the asymmetric top ethylene (C2H4).In the ethane case, RCARS spectra were recorded in pure ethane, as well as in different mixtures of ethane/nitrogen (N2), at atmospheric pressure and at temperatures between 300 K and 650 K. It was demonstrated that the rigid-rotor assumption was not accurate for successful calculations of the ethane spectra at the higher temperatures, and that the effects of internal rotation should be taken into account. A novel method based on Fourier analysis was developed for evaluation of RCARS spectra of ethane for thermometry and for concentration measurements in binary mixtures with nitrogen. The method was successfully demonstrated for thermometry and concentration measurements in an ethane diffusion flame.RCARS spectra of ethylene were recorded at atmospheric pressure and temperatures between 300 K and 800 K. A theoretical model was developed that could reproduce spectra in good agreement with the experimentally recorded data. Furthermore, linewidth parameters of ethylene were estimated for the first time through comparison of the experimental and theoretically calculated spectra.In the second part of the thesis, a novel technique was developed for time-domain studies of molecular coherences using femtosecond (fs)/ns RCARS. All the rotational Raman coherences of molecular species were prepared by two Fourier-limited fs pulses and probed by a single-mode ns pulse. The generated CARS signal was dispersed in a spectrometer and recorded by a streak camera. This combination resulted in simultaneous resolving of the coherences in spectral and temporal domain. Through this novel approach we demonstrated single-shot Raman linewidth measurements of N2 in mixtures with C2H4 and C2H6. Moreover, a theoretical model for time-resolved RCARS of N2 was developed that exploits the measured Raman linewidth data for single-shot thermometry using spectral fitting. This is an essential improvement in N2 RCARS thermometry, since it removes the limitations regarding the necessary pre-knowledge about the number density of the species in the mixtures and their impact on N2 linewidths