A peculiar interference phenomenon observed in femtosecond optical Kerr effect spectroscopy for nearly transparent materials

A sharp negative-going interference signal has been observed in femtosecond optically heterodyne-detected optical Kerr effect spectroscopy using a Ti:sapphire laser for excitation. This signal mainly appears in materials which have OH bonds in their chemical structures and its width is narrower than the duration of the exciting pulse. Careful analysis shows that this phenomenon is connected with the weak light absorption to vibrational overtones and is closely related to the spectral broadening of the exciting pulse through the transient grating created by pump and probe pulses. It is suggested that the lack of this interference effect in water may indicate the presence of a fast relaxation process.