LOW-FREQUENCY MODES STUDIED BY ULTRAFAST LASER SPECTROSCOPY

The principle and application of ultrafast laser spectroscopy have bee n presented. The time-domain spectroscopy is particularly suited for i nvestigating low frequency modes in condensed matter, because the freq uency resolution is infinitesimal and the obtained data are directly c omparable with frequency-domain light scattering spectroscopy. The low frequency modes are known to relate with various relaxational and flu ctuational phenomena and also play an essential role in phase transiti on phenomena. In the present report, we demonstrate that the low frequ ency modes are obtainable very accurately for various disordered mater ials such as liquids and polymers. It is also shown that the time beha vior of the relaxational mode shows a rise after impulsive excitation, which clearly contradicts with Debye relaxation model.