Rotational-level-dependent dephasing rates and nonresonant background can lead to significant uncertainties in coherent anti-Stokes Raman scattering (CARS) thermometry under high-pressure, lowtemperature conditions if the gas composition is unknown. Hybrid femtosecond/picosecond rotational CARS is employed to minimize or eliminate the influence of collisions and nonresonant background for accurate, frequency-domain thermometry at elevated pressure. The ability to ignore these interferences and achieve thermometric errors of <5% is demonstrated for N 2 and O 2 at pressures up to 15 atm. Beyond 15 atm, the effects of collisions cannot be ignored but can be minimized using a short probe delay (~6.5 ps) after Raman excitation, thereby improving thermometric accuracy with a time-and frequency-resolved theoretical model. KeywordsFriedrich-Alexander University Erlangen, Raman scattering, Raman spectroscopy, dephasing rates, elevated pressure, femtoseconds, frequency domains, gas compositions, gasphase, low temperatures, nonresonant, Raman excitation, chemistry, computer aided design, phase transition, radiation scattering, thermography, computer-aided design, equipment design Disciplines Chemistry | Mechanical Engineering CommentsThis article is from Optics Express 15 (2012): 5003, doi: 10.1364/OE.20.005003. Posted with permission. RightsThis paper was published in Optics Express and is made available as an electronic reprint with the permission of OSA. The paper can be found at the following URL on tahe OSA website: https://www.osapublishing.org/ oe/abstract.cfm?uri=oe-20-5-5003. Systematic or multiple reproduction or distribution to multiple locations via electronic or other means is prohibited and is subject to penalties under law. Abstract: Rotational-level-dependent dephasing rates and nonresonant background can lead to significant uncertainties in coherent anti-Stokes Raman scattering (CARS) thermometry under high-pressure, lowtemperature conditions if the gas composition is unknown. Hybrid femtosecond/picosecond rotational CARS is employed to minimize or eliminate the influence of collisions and nonresonant background for accurate, frequency-domain thermometry at elevated pressure. The ability to ignore these interferences and achieve thermometric errors of <5% is demonstrated for N 2 and O 2 at pressures up to 15 atm. Beyond 15 atm, the effects of collisions cannot be ignored but can be minimized using a short probe delay (~6.5 ps) after Raman excitation, thereby improving thermometric accuracy with a time-and frequency-resolved theoretical model. ©2012 Optical Society of America

Hybrid femtosecond/picosecond coherent anti-Stokes Raman scattering (fs/ps CARS) of N 2 has recently been demonstrated for gas-phase thermometry in reacting flows, enabling frequency-domain detection at high repetition rates with excellent chemical specificity and independence from the effects of collisions and nonresonant background. In this work, we overcome the limited sensitivity of vibrational fs/ps CARS thermometry of N 2 below 1200 K by spectrally resolving J-dependent rovibrational coherence revivals that occur 32 ps after initial excitation. The N 2 rovibrational coherence is excited using broadband, 100-fs pump and Stokes pulses and probed as a function of time using a narrowband, 5.8-ps probe pulse (bandwidth of 2.5 cm À1 ). The rovibrational features exhibit sufficient temperature sensitivity below 1200 K for accurate thermometry using a simple, time-dependent phenomenological model. Specifically, three distinct spectral features at a single probe delay of 32.5 ps are analyzed, corresponding to rovibrational revivals with mean rotational quantum numbers of = 3.5, 14, and 28. Good agreement is found between simulated and measured fs/ps CARS spectra in an adiabatic flat-flame burner from 298 to 2400 K.

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Hybrid femtosecond/picosecond (fs/ps) vibrational coherent anti‐Stokes Raman scattering was demonstrated for high‐speed, in‐situ measurements of CH4/N2 mole fractions in binary gas mixtures with temporal nonresonant suppression. A single broadband, 100‐fs laser source was used to simultaneously excite CH4 and N2 vibrational Raman transitions, characterized by a spectral separation of ~584 cm−1, followed by frequency‐domain detection with a time‐delayed ps probe pulse. In this manner, the temporal evolution of the nonresonant background and vibrational wavepackets of CH4 and N2 could be evaluated to maximize signal and minimize interferences. A typical accuracy of ~95% was achieved for experimental CH4 mole fractions between 0.02 and 0.70, and a precision of ~4% was achieved by maximizing relative excitation energy through induced chirp in the pump and Stokes pulses. It was further shown that mole‐fraction measurements were not affected by variations in bi‐molecular broadened linewidths over a wide range of mixture fractions. Copyright © 2013 John Wiley & Sons, Ltd.