Raman analysis of SF6 molecular beams excited by a cw CO2 laser

Abstract

In a molecular beam the effects of vibrational pumping of SF 6 (ν 3 = 948 cm −1 ) are studied, using a line-tunable cw CO 2 laser. Intracavity spontaneous Raman scattering is used for analysis. For excitation in the collision regime ( x E / D ≤ 1), a thermal redistribution of the ν 3 excitation over all vibrational modes is found, together with an average absorption up to six photons per molecule. The infrared absorption profile shows a red-shift of 6 cm −1 . For excitation in the relatively rare collision regime ( x E / D ⩾ 4), a structured non-thermal ν 1 Raman spectrum is observed, especially in the case of seeded molecular beams (10% in He). The observed hot-band peaks can be explained in terms of single-photon absorptions and collision-induced near-resonant V-V energy transfer, leading to single, double and triple excitations of the ν 3 mode. The value of T rot in the beam is found to influence sensitively the non-resonant energy-transfer rate [e.g. h ν 3 (948 cm −1 )+Δ E rot → h (ν 4 + ν 6 )(962 cm −1 ) relative to the near-resonant transfer rate ( h ν 3 + h ν 3 → 2 h ν 3 + 3.5 cm −1 )].