Abstract
The direct measurement of self-broadened linewidths using the time decay of pure-rotational hybrid femtosecond/picosecond coherent anti-Stokes Raman scattering (fs/ps RCARS) signals is demonstrated in gas-phase N(2) and O(2) from 1-20 atm. Using fs pump and Stokes pulses and a spectrally narrowed ps probe pulse, collisional dephasing rates with time constants as short as 2.5 ps are captured with high accuracy for individual rotational transitions. S-branch linewidths of N(2) and O(2) from ~0.06 to 2.2 cm(-1) and the line separation of O(2) triplet states are obtained from the measured dephasing rates and compared with high-resolution, frequency-domain measurements and S-branch approximations using the modified exponential gap model. The accuracy of the current measurements suggests that the fs/ps RCARS approach is well suited for tracking the collisional dynamics of gas-phase mixtures over a wide range of pressures.
Highlights
In ps coherent anti-Stokes Raman scattering (CARS), the ability to measure short time decays is limited by relatively long pump, Stokes, and probe pulse widths and by the need to avoid nonresonant background by temporally separating the probe pulse from the pump and Stokes pulses
The hybrid fs/ps CARS approach offers the ability to temporally discriminate against nonresonant background, similar to either fs or ps CARS, but has the unique capability to resolve gas-phase rotational spectra within several hundred fs after Raman excitation
Few direct measurements of selfbroadened S-branch linewidths are reported in the literature, and many ps and ns rotational CARS models rely on S-branch approximations from Q-branch data which can lead to errors at high pressure
Summary
The use of femtosecond (fs) and picosecond (ps) laser sources for gas-phase coherent anti-Stokes Raman scattering (CARS) thermometry has grown significantly in recent years due to the high peak powers of these laser sources and interest in resolving the time dynamics of the molecular response.1 In fs CARS, the temporally short and spectrally broad (transform-limited) pump and Stokes pulses allow highly efficient, in-phase, and impulsive excitation of the entire rovibrational or pure-rotational manifold.1–3 In this case, the molecular response is resolved in time using the spectrally integrated signal, but the dephasing rate of individual (J-level) transitions cannot be measured directly.2,3 In ps CARS, the ability to measure short time decays is limited by relatively long pump, Stokes, and probe pulse widths (typically ∼100 ps) and by the need to avoid nonresonant background by temporally separating the probe pulse from the pump and Stokes pulses.4–6 For measurements of collisional dephasing rates, especially at high pressure, the ideal approach is to utilize broad (fs) pump and Stokes pulses and a spectrally narrowed (ps) probe pulse that can simultaneously isolate each molecular transition while still resolving short ∼ps decay times. Communication: Time-domain measurement of high-pressure N2 and O2 self-broadened linewidths using hybrid femtosecond/picosecond coherent anti-Stokes Raman scattering
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