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

Abstract

The hybrid femtosecond/picosecond coherent anti-Stokes Raman scattering (fs/ps CARS) technique presents a promising alternative to either fs time-resolved or ps frequency-resolved CARS in both gas-phase thermometry and condensed-phase excited-state dynamics applications. A theoretical description of timedependent CARS is used to examine this recently developed probe technique, and quantitative comparisons of the full time-frequency evolution show excellent accuracy in predicting the experimental vibrational CARS spectra obtained for two model systems. The interrelated timeand frequency-domain spectral signatures of gas-phase species produced by hybrid fs/ps CARS are explored with a focus on gas-phase N2 vibrational CARS, which is commonly used as a thermometric diagnostic of combusting flows. In particular, we discuss the merits of the simple top-hat spectral filter typically used to generate the ps-duration hybrid fs/ps CARS probe pulse, including strong discrimination against non-resonant background that often contaminates CARS signal. It is further demonstrated, via comparison with vibrational CARS results on a time-evolving solvated organic chromophore, that this top-hat probe-pulse configuration can provide improved spectral resolution, although the degree of improvement depends on the dephasing timescales of the observed molecular modes and the duration and timing of the narrowband final pulse. Additionally, we discuss the virtues of a frequencydomain Lorentzian probe-pulse lineshape and its potential for improving the hybrid fs/ps CARS technique as a diagnostic in high-pressure gas-phase thermometry applications. The hybrid femtosecond/picosecond coherent anti-Stokes Raman scattering (fs/ps CARS) technique presents a promising alternative to either fs time-resolved or ps frequency-resolved CARS in both gas-phase thermometry and condensed-phase excited-state dynamics applications. A theoretical description of time-dependent CARS is used to examine this recently developed probe technique, and quantitative comparisons of the full time-frequency evolution show excellent accuracy in predicting the experimental vibrational CARS spectra obtained for two model systems. The interrelated timeand frequency-domain spectral signatures of gas-phase species produced by hybrid fs/ps CARS are explored with a focus on gas-phase N 2 vibrational CARS, which is commonly used as a thermometric diagnostic of combusting flows. In particular, we discuss the merits of the simple top-hat spectral filter typically used to generate the ps-duration hybrid fs/ps CARS probe pulse, including strong discrimination against non-resonant background that often contaminates CARS signal. It is further demonstrated, via comparison with vibrational CARS results on a time-evolving solvated organic chromophore, that this top-hat probe-pulse configuration can provide improved spectral resolution, although the degree of improvement depends on the dephasing timescales of the observed molecular modes and the duration and timing of the narrowba...

Hybrid fs/ps rotational CARS temperature and oxygen measurements in the product gases of canonical flat flames

“…In contrast to nanosecond CARS, which is treated as a steady process and modeled in the frequency domain, femtosecond CARS is modeled as a transient in the time domain [14,36,37,58]. Broadband femtosecond-duration pump and Stokes pulses arrive nearly simultaneously and prepare a rotational polarization that decays in time as a result of collisional dephasing over $100 ps at atmospheric pressure.…”

Quantitative interpretation of time‐resolved coherent anti‐Stokes Raman spectroscopy with all Gaussian pulses

“…The efficient methods for FWM elimination have been recently demonstrated with probe pulses shaped in time‐domain and frequency‐domain as in sinc‐square, square‐sinc, exponential–Lorentzian and Gaussian–Gaussian forms. In particular, the experiments on the impulsive CARS have been reported in Refs.…”

Hybrid fs/ps coherent anti-Stokes Raman scattering for multiparameter measurements of combustion and nonequilibrium

“…This solution matches the molecular response used previously to model fs/ps CARS spectra [2,78,74]. This statement of R(t) is valid for well-separated CARS transitions, as is the case for pure-rotational transitions and overlapping, rovibrational CARS transitions at low gas-density conditions.…”

“…Much of the early development of fs/ps CARS for study of gas-phase combustion species was performed at Iowa State University, Wright-Patterson Air Force Base, and Sandia National Laboratories, and an overview of the important advances are covered here. Much of the fs/ps CARS theory is described by Stauffer et al for gas-and condensed-phase samples [74].…”

“…Traditionally, the treatment of fs/ps CARS starts with the third-order polarization in the time-domain as described by a molecular response function consistent with the treatment of nonlinear spectroscopy by that presented by Mukamel [131,78,74]. The time-domain polarization is directly linked to the frequency-domain through the Fourier transform, and can be written as…”