Interference-free gas-phase thermometry at elevated pressure using hybrid femtosecond/picosecond rotational coherent anti-Stokes Raman scattering

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. 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

Time- and frequency-dependent model of time-resolved coherent anti-Stokes Raman scattering (CARS) with a picosecond-duration probe pulse

“…Thus, these 3-ps-delay data represent an optimal compromise that allows detection of spectra essentially free of NR background while exhibiting no collision-dependent behavior at pressures as high as 20 bars. 9,10,53 Still, the changes in spectral profiles observed in the 39-ps-and 68-ps-delay data emphasize that much of the spectral information observed within the hybrid fs/ps CARS signal is contained in the time-dependent nature of this signal. Whereas these longer delay spectra contain coarse peaks associated with the same vibrational transitions observed in the 3-ps data, a unique fine structure is observed within the spectra at each of these delays, with the 39-ps data emphasizing mid-range rotational Q-branch frequencies and the appearance of bimodal rotational structure with peaks near J ≈ 22 and J ≈ 28.…”

“…[5][6][7][8][9][10][11][12] This probe technique has been developed as a variant of fs time-resolved CARS (tr-CARS), [13][14][15][16][17][18][19][20][21][22][23][24][25] the history of which has been described thoroughly in recent reviews. 26,27 Hybrid fs/ps CARS utilizes the same four-wave-mixing excitation scheme as does fs tr-CARS; however, the use of a time-delayed, narrowband, ps-duration probe pulse-rather than a broadband, fs-duration probe pulse-allows direct frequency-domain detection of Raman-active molecular states.…”

“…4 The spectral resolution of hybrid fs/ps CARS signal, furthermore, allows single-shot thermometry and multispecies measurements in reacting flows at repetition rates in the kHz range, 5,11,12,28 and several hybrid fs/ps CARS schemes have been used to probe gas-phase vibrational 5,6 and a) Author to whom correspondence should be addressed. Electronic mail: hans.stauffer@gmail.com rotational [7][8][9][10][11][12][29][30][31] CARS spectra. In addition to the multiplex nature of this newer probe technique, hybrid fs/ps CARS also provides the additional feature that Raman-active rotational/vibrational frequencies can be directly recovered; 1 they do not need to be deduced from time-dependent coherent oscillations observed in the spectrally dispersed CARS signal beam, as is the case in fs tr-CARS.…”

“…The general benefits associated with a time-delayed probe have also been explored theoretically, 4,48 although the emphasis of that work is on a probe pulse with a Gaussian temporal profile rather than with temporal profiles matching those used experimentally. More recent applications of the hybrid fs/ps CARS technique to gas-phase thermometry [5][6][7][8][9][10][11][12]31 have generally highlighted the agreement between experiment and simulation at single probe delays with an emphasis on the corresponding thermometric accuracy and precision afforded by reasonable simulation. The emphasis of this current work is on providing a thorough theoretical description of the hybrid fs/ps CARS probe technique, accompanied by full simulations of the time evolution of experimental spectra and expected lineshapes and intensities, in order to explore further the inherent advantages and limitations of this probe technique.…”

Dual-pump vibrational/rotational femtosecond/picosecond coherent anti-Stokes Raman scattering temperature and species measurements

“…To avoid complex coherent interferences that occur with broadband femtosecond excitation/detection of multiple species, an approach that combines broadband, femtosecond excitation with narrowband, picosecond detection has been employed. This approach, hybrid femtosecond/picosecond (fs/ps) CARS, can be used to enhance chemical specificity [12,13], simplify spectral analysis, suppress nonresonant background, and reduce the effects of collisions for accurate thermometry from low to high pressures [14].…”