Time-frequency analysis of femtosecond CARS spectroscopy of N2 and O2 using the superlet transform

Molecular dynamics plays a crucial role in understanding molecular interactions, rovibrational coupling mechanisms, and energy transfer processes. Femtosecond time-resolved coherent anti-Stokes Raman scattering spectroscopy was employed to study the molecular dynamics of N2 and O2 in air at room temperature. To reveal hidden spectral features, we have for the first time applied an analytical method that balances time resolution and frequency resolution, namely, the superlet transform (SLT), to perform time-frequency resolved spectral analysis of the complex molecular dynamics of N2 and O2 in air. A distinct evolution of the partial rotational modes of N2 and O2 outside the selective excitation region was observed, which is related to energy transfer collisions between N2 and O2 molecules during the rotational energy relaxation process in air. The SLT results accord well with the S-branch rotational spectra of N2 and O2 obtained from theoretical calculations, confirming the validity of SLT analysis. This method provides a valuable experimental analysis technique to deepen the understanding of the microscopic dynamic processes in molecular dynamics.