Time-domain modelling and thermometry of the CH₄ ν₁ Q-branch using hybrid femtosecond/picosecond coherent anti-Stokes Raman scattering

We present time-domain measurements and modelling of the CH₄ν₁ vibrational Q-branch, as well as simultaneous temperature acquisition from the CH₄ and N₂ vibrational Q-branches using broadband hybrid femtosecond/picosecond coherent anti-Stokes Raman scattering (fs/ps CARS). Accurate measurements with fs/ps CARS require knowledge of the time-domain response of the probed molecule at different temperatures and pressures. In this work, a fs/ps CARS model was developed for the CH₄ν₁ Q-branch and validated using time-resolved probe delay scans at pressures between 70 and 600 Torr and temperatures between 295 and 1000 K. A simplified modified exponential energy gap model was used to determine the linewidths of the Raman transitions within the Q-branch. Total Q-branch linewidths for the entire explored parameter space were also determined from the probe delay scans. This enabled quantitative measurements of CH₄ with fs/ps CARS as well as the temperature through the hot bands of the CH₄ν₁ Q-branch. With a broadband fs laser pulse in a two-beam phase matching scheme, the N₂ Q-branch and CH₄ν₁ Q-branch were simultaneously measured in a heated cell, and the temperatures from each molecule's spectra was evaluated. The fitted temperatures corresponded well with differences ranging from 4 to 17%. It was also shown that exchanging 90% of CH₄ for N₂ or Ar at 295 K and 500 Torr did not significantly impact the normalized time-domain signal prior to a 100 picosecond probe delay. Spatially resolved one-dimensional (1-D) thermometry using the CH₄ Q-branch was also demonstrated with an imaged spatial resolution of 40 µm. With the developed model, 1-D quantitative measurements of CH₄ concentration and temperature can be measured with time resolution of tens of picoseconds.