Detection of atomic oxygen and its electronic coherence decays using time-resolved ultrafast coherent Raman scattering.

Abstract

We report the detection of atomic oxygen and quantitative measurements of its electronic Raman coherence decays in flames and low-temperature plasmas using time-resolved hybrid femtosecond/picosecond coherent anti-Stokes Raman scattering (CARS). Atomic oxygen was detected using the Raman transitions between the spin-orbit coupled triplet ground states. Atomic oxygen was generated in an H2/O2/Ar diffusion flame and an O2/Ar pulsed plasma discharge. Single exponential decays were observed for the O(3P2)-O(3P1) Raman transition at 158.3 cm-1 and the O(3P2)-O(3P0) Raman transition at 227 cm-1. From the decay measurements, the atomic O Raman linewidths were obtained from 25 to 150 Torr in non-equilibrium plasma and at 760 Torr in a flame. Enhanced signal-to-noise ratios (SNRs) of atomic oxygen and atomic to molecular oxygen signal contrasts were obtained by taking advantage of electronic triplet coherence beating. Enhancement of up to seven times in the atomic O SNR was observed. We also found that the dephasing rates of O2(v = 0-3, N = 37) were similar, which provides evidence for the assumption that vibrational excitation does not influence the dephasing of diatomic molecular rotational CARS transitions.