Pressure-dependent N2 Q-branch fs-CARS measurements

We demonstrate the feasibility of the time-delayed femtosecond CARS (fs-CARS) technique in a dense gas environment. The experiments were performed on N-2 molecules between 0.2 and 5 bar pressure. The method provides several advantages compared with its wavenumber-resolved counterpart, e.g. the possible separation of the non-resonant contributions to the signal, and is more suitable when the isolated lines approximations collapse and complex line mixing and energy transfer models have to be implemented in the signal description. The experimental results are in good agreement with theoretical considerations based on collision-induced rotational energy transfer (RET) models such as the common energy-corrected sudden approximation (ECS). Moreover, it is shown that the structure of the time-delayed signals is very sensitive to the applied pressures. Therefore, fs-CARS is an ideal supplementary tool for investigations of collision-induced spectroscopic effects, which are decisive for temperature and concentration determination in combustion processes. Copyright (C) 2002 John Wiley Sons, Ltd.