Self-Induced Transparency of Degenerate Transitions with Thermally Equilibrated Levels

Pulse propagation in S${\mathrm{F}}_6$ has been investigated using 200-nsec pulses of a C${\mathrm{O}}_2$ laser. Operating at $\lambda =10.53\mathrm{}$ μm ($P14\mathrm{}$ transition) we have observed a breakup of the pulse envelope, an oscillatory energy transmission, and an energy-dependent pulse delay. We interpret our data in terms of the existing theory of self-induced transparency; the corresponding Bloch-type equations were derived assuming thermal equilibrium between rotational $M$ sublevels.