Subpicosecond X-ray diffraction study of laser-induced disorder dynamics above the damage threshold of organic solids

Ultrafast disordering in Langmuir-Blodgett (LB) films and TlAP (thallium ac id phthalate) crystals has been investigated by optical-pump-X-ray-probe ex periments using intense femtosecond laser pulses. A laser-produced plasma X -ray source at 7.12 Angstrom wavelength is used to study atomic dynamics by subpicosecond time-resolved X-ray diffraction. It is found that a drop of the X-ray diffracted intensity appears with time constants from below 600 f s up to a few tens of picoseconds in LB films optically excited at laser fl uxes from 1.8 to 27 J cm(-2). This loss in the diffracted intensity is unde rstood as a displacement of the diffracting atoms by similar to 8 Angstrom from their equilibrium position. The response of the TlAP crystal was signi ficantly slower. The relation to the atomic structure of the materials is d iscussed.