Communication: Hybrid femtosecond/picosecond rotational coherent anti-Stokes Raman scattering thermometry using a narrowband time-asymmetric probe pulse

Abstract

A narrowband, time-asymmetric probe pulse is introduced into the hybrid femtosecond/picosecond rotational coherent anti-Stokes Raman scattering (fs/ps RCARS) technique to provide accurate and precise single-shot, high-repetition-rate gas-phase thermometric measurements. This narrowband pulse—generated by inserting a Fabry-Pérot étalon into the probe-pulse beam path—enables frequency-domain detection of pure-rotational transitions. The unique time-asymmetric nature of this pulse, in turn, allows for detection of resonant Raman-active rotational transitions free of signal contamination by nonresonant four-wave-mixing processes while still allowing detection at short probe-pulse delays, where collisional dephasing processes are negligible. We demonstrate that this approach provides excellent single-shot thermometric accuracy (<1% error) and precision (∼2.5%) in gas-phase environments.

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.…”

“…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.…”

“…[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.…”

“…The interdependence between the time-and frequency-domain signals is particularly significant in cases where molecular dephasing timescales are comparable to or longer than the probe-pulse duration; under these conditions, both the transition frequencies and lineshapes of the observed CARS signal can exhibit a notable temporal dependence. 5,10 As such, direct comparison to computational simulations is imperative for quantitative interpretation of the experimental results. The hybrid fs/ps CARS technique has been described previously via experimental comparison with traditional fs tr-CARS, 1,2 and some computational simulations of tr-CARS were used in prior work 1,3 that emphasize the similarities and differences between these two related optical techniques.…”

“…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.…”