Abstract

We have applied a picosecond time-domain coherent active (anti-Stokes) Raman spectroscopy (CARS) technique to the study of the dephasing kinetics in a free supersonic jet of N₂. We have directly detected the oscillatory decay of free optical polarization induced in molecules in a jet by a pair of picosecond pulses with properly chosen frequencies ω₁ and ω₂ (so that ω ₁- ω₂≃Ω_Q, the mean frequency of ro-vibrational transitions in the Q-brach) probed by a delayed picosecond pulse of frequency ω. The pulses at ω₁ and ω₂ were provided by a mode-locked Nd:YAG laser (λ₁ = 1.06μm, τ_p = 20 ps), and LiNbO₃ optical parametric oscillator (λ₂ =1.4 μm, fwhm Δv≃20 cm^–1) pumped with the second harmonic (SH) of the main oscillator, the portion of the last SH pulse after a variable delay being used as a probe ω pulse.

© 1984 Optical Society of America