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Science 30 October 1992:
Vol. 258. no. 5083, pp. 770 - 774
DOI: 10.1126/science.258.5083.770
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Articles

Femtosecond Resolution of Soft Mode Dynamics in Structural Phase Transitions

Thomas P. Dougherty 1, Gary P. Wiederrecht 1, Keith A. Nelson 1, Mark H. Garrett 2, Hans P. Jensen 2, and Cardinal Warde 3

1 Department of Chemistry, Cambridge, MA 02139
2 Center for Materials Science and Engineering, Cambridge, MA 02139
3 Department of Electrical Engineering and Computer Science at the Massachusetts Institute of Technology, Cambridge, MA 02139

The microscopic pathway along which ions or molecules in a crystal move during a structural phase transition can often be described in terms of a collective vibrational mode of the lattice. In many cases, this mode, called a "soft" phonon mode because of its characteristically low frequency near the phase transition temperature, is difficult to characterize through conventional frequency-domain spectroscopies such as light or neutron scattering. A femtosecond time-domain analog of light-scattering spectroscopy called impulsive stimulated Raman scattering (ISRS) has been used to examine the soft modes of two perovskite ferroelectric crystals. The low-frequency lattice dynamics of KNbO3 and BaTiO3 are clarified in a manner that permits critical evaluation of microscopic models for their ferroelectric transitions. The results illustrate the advantages of ISRS over conventional Raman spectroscopy of low-frequency, heavily damped soft modes.

Submitted on June 12, 1992
Accepted on September 29, 1992


THIS ARTICLE HAS BEEN CITED BY OTHER ARTICLES:
Vibrationally coherent photochemistry in the femtosecond primary event of vision.
Q Wang, R. Schoenlein, L. Peteanu, R. Mathies, and C. Shank (1994)
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