Picosecond Nonlinear Investigation of Accumulated Damage in Molecular Crystals

Accumulated damage in aromatic molecular crystals at low temperature is studied by a variety of novel picosecond laser techniques. Damage in these materials is qualitatively different than in fused quartz, where dielectric breakdown is the predominant mechanism. In the aromatic crystals, multiphoton ionization creates reactive chemical fragments which are stabilized by the low temperature crystal matrix. These species accumulate until a high concentration is reached. At that point, phonon and photon assisted interactions release enough energy to damage the crystal. In a sense the process can be viewed as a chemical explosion. The picosecond YAG-dual dye laser system used for these experiments is described. With this system we have demonstrated new methods to study damage. Subnanosecond damage is time-resolved with a sampling technique. The mechanism of defect production and crystal destruction is probed using damage detected spectroscopy. Scattering of the optical phonons by the accumulating defects is detected via picosecond time-delayed coherent Raman scattering.