Observation of Raman Self-Conversion of fs-Pulse Frequency due to Impulsive Excitation of Molecular Vibrations

Abstract

We report on the observation of a Raman frequency conversion mechanism different from the usual Stokes (anti-Stokes) stimulated process. A continuous down-shift of laser pulse frequency was measured for sufficiently short $(\ensuremath{\le}40\mathrm{fs})$ pulses propagating in gaseous ${\mathrm{SF}}_{6}$. We attribute this behavior to the extremely nonstationary nuclear response of ${\mathrm{SF}}_{6}$ molecules to the light field with a duration shorter than the period of the Raman-active vibration.