Concentration measurements in molecular gas mixtures with a two-pump pulse femtosecond polarization spectroscopy technique

Abstract

Recently, we have demonstrated the ability of the Raman-induced polarization spectroscopy (RIPS) technique to accurately determine concentration or polarizability anisotropy ratio in low-pressure binary molecular mixtures [E. Hertz, B. Lavorel, O. Faucher, and R. Chaux, J. Chem. Phys. 113, 6629 (2000)]. It has been also pointed out that macroscopic interference, occurring when two revivals associated to different molecules time overlap, can be used to achieve measurements with picosecond time resolution. The applicability of the technique is intrinsically limited to a concentration range where the signals of both molecules are of the same magnitude. In this paper, a two-pump pulse sequence with different intensities is used to overcome this limitation. The relative molecular responses are weighted by the relative laser pump intensities to give comparable signals. Furthermore, by tuning the time delay between the two-pump pulses, macroscopic interference can be produced regardless of the accidental coincidences between the two molecular temporal responses. The study is performed in a CO2–N2O gas mixture and the concentration is measured with and without macroscopic interference. Applications of the method in the field of noninvasive diagnostics of combustion media are envisaged.