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, CS₂and CHBr₃, 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 ≥ 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 CS₂ is much stronger than the electronic one, whereas the opposite is true with liquid CHBr₃. The ultrafast response of liquid CHBr₃ also features the occurrence of vibrational quantum beats, involving as many as three normal vibrations of the CHBr₃ molecule. The normal vibrations identified from the beats are the CBr₃ symmetric deformation (υ + ̃₃ = 222 cm^-1) and the CBr₃ symmetric stretch (υ + ̃₂ = 540 cm^-1), in addition to the CBr₃ degenerate deformation (υ + ̃₆ = 154 cm^-1) observed in temporal domain heretofore. Both liquids also exhibit identical ultrafast intermolecular dynamics.