The transient optical Kerr effect of simple liquids studied with an ultrashort laser with variable pulsewidth

The coherent transient responses of two simple molecular liquids, CS/sub 2/ and CHBr/sub 3/, are studied at room temperature with the technique of time-resolved pump/probe optical Kerr effect. Use is made of an ultrashort laser with pulsewidth varying from /spl ges/100 to 28 femtoseconds (fs). For both liquids the instantaneous electronic response is found to increase markedly relative to the delayed nuclear response as input pulse is shortened. The contribution of the coherent coupling effect to the Kerr signal near delay zero is found to be significant, and is attributed to a retarded nuclear response. The latter, along with the nonvanishing response of nuclear motion to the shortest (28 fs) input pulse, strongly suggests that nuclei are intrinsically coupled to the valence electrons. The intrinsic electron-nucleus coupling in combination with molecular symmetry satisfactorily accounts for the observation that the ultrafast nuclear Kerr response of liquid CS/sub 2/ is much stronger than the electronic one, whereas the opposite is true with liquid CHBr/sub 3/. The ultrafast response of liquid CHBr/sub 3/ also features the occurrence of vibrational quantum beats, involving as many as three normal vibrations of the CHBr/sub 3/ molecule. The normal vibrations identified from the beats are the CBr/sub 3/ symmetric deformation (/spl nu//spl tilde//sub 3/=222 cm/sup -1/) and the CBr/sub 3/ symmetric stretch (/spl nu//spl tilde//sub 6/=540 cm/sup -1/), in addition to the CBr/sub 3/ degenerate deformation (/spl nu//spl tilde//sub 6/=154 cm/sup -1/) observed in temporal domain heretofore. Both liquids also exhibit identical ultrafast intermolecular dynamics.