Vibrational/Rotational Hybrid fs/ps Coherent Anti-Stokes Raman Scattering for Combustion Analysis

摘要

A method for simultaneous vibrational and rotational hybrid femtosecond/picosecond coherent anti-Stokes Raman scattering (fs/ps CARS) spectroscopy is presented for multi-species detection and improved temperature sensitivity over a wide range of flame conditions. N_2/CH_4 vibrational and N_2/O_2/H_2 rotational Raman coherences are excited simultaneously using fs pump/Stokes pulses at 675/799 nm and 799/799 nm, respectively, and a fourth narrowband ps pulse at 799 nm is used to probe all coherence states at a time delay that minimizes non-resonant background and the effects of collisions. The strength of these transitions is concentration dependent, while the distribution among observed transitions can be related to temperature through the Boltzmann distribution of energy states. The multiplexed signal covers the entire range of temperatures observed in combustion reactants and products (298-2300 K) with one measurement and can detect four species (N_2, O_2, CH_4, H_2) using broadband excitation. This approach is discussed in the context of the feasibility for simultaneous, high-speed, interference-free measurements of key combustion species and temperature in flames with widely varying conditions.