OBSERVATION OF RAMAN SELF-CONVERSION OF FS-PULSE FREQUENCY DUE TO IMPULSIVE EXCITATION OF MOLECULAR VIBRATIONS

We report on the observation of a Raman frequency conversion mechanism different from the usual Stokes (anti-Stokes) stimulated process. A c ontinuous down-shift of laser pulse frequency was measured for suffici ently short (less than or equal to 40 fs) pulses propagating in gaseou s SF6. We attribute this behavior to the extremely nonstationary nucle ar response of SF6 molecules to the light held with a duration shorter than the period of the Raman-active vibration. [S0031-9007(98)06773-8 ].