Rotational coherence beating in molecular oxygen: Coupling between electronic spin and nuclear angular momenta.

Abstract

Time-resolved pure-rotational hybrid femtosecond/picosecond coherent anti-Stokes Raman spectroscopy (fs/ps RCARS) of oxygen (O2) is performed at pressures from ∼0.04 to 0.4 atm. As the RCARS spectra evolve with probe delay, they exhibit coherence beating between unresolved S-branch triplet transitions (ΔN = 2, ΔJ = 2). The time-domain fitting of the RCARS signal intensity enables the determination of these transition frequency separations, which are as low as 480 MHz (0.016 cm-1). Additionally, we study the underlying pressure-dependent dynamics and the signatures of the time-domain triplet signals compared to the simple decays associated with the O2 self-broadened linewidths. Pressure- and N-dependent O2 linewidths are compared to literature coefficients obtained from experiments and models that have not incorporated the triplet splitting. Our findings are incorporated into a time-domain model for rotational CARS thermometry of O2 and have significant impact for spectral evaluations at probe delays greater than 100 ps for temperature or species concentration determination. The time- and frequency-resolved experiments presented in this work provide insight into the spectroscopic complexities introduced by the electronic ground state of O2 for accurate evaluation of time-resolved coherent Raman spectra.