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 acid phthalate) crystals has been investigated by optical-pump-X-ray-probe experiments using intense femtosecond laser pulses. A laser-produced plasma X-ray source at 7.12 Å 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 fs up to a few tens of picoseconds in LB films optically excited at laser fluxes from 1.8 to 27 J cm^-2. This loss in the diffracted intensity is understood as a displacement of the diffracting atoms by ∼8 Å from their equilibrium position. The response of the TlAP crystal was significantly slower. The relation to the atomic structure of the materials is discussed.