Coherent anti-Stokes Raman spectroscopy in the presence of strong resonant signal from background molecules

Abstract

Optical spectroscopy with broadband femtosecond laser pulses often involves simultaneous excitation of multiple molecular species with close resonance frequencies. Interpreting the collective optical response from molecular mixtures typically requires Fourier analysis of the detected timeresolved signal. We propose an alternative method of separating coherent optical responses from two molecular species with neighboring excitation resonances (here, vibrational modes of oxygen and carbon dioxide). We utilize ro-vibrational coupling as a mechanism of suppressing the strong vibrational response from the dominating molecular species (O2). Coherent ro-vibrational dynamics lead to long "silence windows" of zero signal from oxygen molecules. In these silence windows, the detected signal stems solely from the minority species (CO2) enabling background-free detection and characterization of the O2/CO2 mixing ratio. In comparison to a Fourier analysis, our technique does not require femtosecond time resolution or time-delay scanning.

Vibrational femtosecond/picosecond coherent anti‐Stokes Raman scattering with enhanced temperature sensitivity for flame thermometry from 300 to 2400 K

“…Unlike CPP CARS, there is no nonresonant contribution; unlike pure‐rotational fs/ps CARS, there is no complication from overlapping species; and unlike dual‐pump vibrational/rotational CARS, only one pump pulse and one detection scheme are required . Furthermore, if interfering species are present in the measurement volume, so‐called ‘silence windows’ between the molecular response of each species can be exploited to minimize contributions from the overlapping species …”

Recent advances in ultrafast-laser-based spectroscopy and imaging for reacting plasmas and flames

“…Similarly, the presence of CO 2 on O 2 fs-VCARS thermometry has also been investigated in a heated cell and it has been observed that the thermometric measurement is affected when CO 2 concentration reaches over 10%; it is concluded that careful modeling of fs-CARS is required to achieve the thermometric accuracy for such mixtures [211]. Bitter and Milner have presented another strategy for single-shot measurement of species concentration by separating the VCARS signals from a mixture of O 2 and CO 2 by exploiting a 'silence window' of one of the species in time domain [212]. The collisional effects on N 2 fs-VCARS is studied at high pressures (up to 50 bars) in the presence of different collisional partners [46], as discussed earlier in section 4.4.…”