Hybrid rotational femtosecond/picosecond coherent anti-Stokes Raman spectroscopy of nitrogen at high pressures and temperatures

Abstract

In this thesis, the use of two-beam hybrid rotational femtosecond/picosecond coherent anti-Stokes Raman spectroscopy (HR-CARS) for temperature measurements in nitrogen gas at high pressures (1-70 atm) and temperatures (300-1000 K) is demonstrated.

An experimental setup for 1 kHz measurements of well-resolved frequency-domain HR-CARS spectra across all investigated pressures and temperatures was built. To achieve the required spectral pump/Stokes excitation bandwidth, a pulse shaper was used to create an almost transform limited 42 fs pump/Stokes pulse at the interaction volume, after having passed through a 28 mm fused silica window of a high pressure cell. To obtain nonresonant background free spectra at the required spectral resolution, a narrow-bandwidth, frequency-upconverted 5.5 ps probe pulse was created in a beta barium borate (BBO) crystal (Type 1) via sum frequency generation (SFG) using second harmonic bandwidth compression (SHBC).

A computational code has been developed to model S-branch HR-CARS spectra and fit to experimental results to obtain best-fit temperatures. The model spectra are based on transition frequencies calculated from a non-rigid rotor approximation, taking rotational-vibrational interaction into account. Linewidths are taken from published measurements and interpolated to the required temperature. The model assumes impulsive pump/Stokes excitation and the probe pulse is modelled with chirp, as this was observed experimentally. The fitting is done through a nonlinear least-squares algorithm.

Good qualitative fits, including good accuracy and precision between thermocouple measured and best-fit temperatures over all the explored pressure and temperature regimes are shown. Across all experimental spectra, the average percentage temperature difference between best-fit and thermocouple measured temperatures (as a percentage of the thermocouple measurement), is -0.3% with a standard deviation of 7.1%.

Overall, accurate nitrogen thermometry is demonstrated, showing the suitability of HR-CARS for characterising high pressure and temperature environments.