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A quantum-statistical theory of pure and transparent dielectrics optical damage by ultrashort laser pulses is developed. The cornerstone of the suggested model is the idea of direct absorption of light by ions or atoms forming the solid. The direct photon absorption leads, together with thermal :fluctuations of energy, to a diffusion jump of an ion or atom. After such a jump, a defect appears in the lattice. If the diffusion jumps are sufficiently frequent then, by the end of the laser pulse, accumulation of defects leads to amorphyzation of the crystal. Emergence of the amorphyzated region can be considered the initial stage of optical damage. In amorphous media, the photoinduced diffusion jumps result in additional absorption of energy, and the media becomes super-amorphous. Energy dissipation of this state unfolds the next stages of damage, all the way to formation of cracks. In the present paper, a quantum-statistical method is developed for evaluation of mean rate of the diffusion jumps. The value of this rate determines the threshold of a laser wave field causing amorphyzation or optical damage of the crystal. The threshold values are found in the approximation of a constant binding energy of particles in the lattice, and hence are the upper estimates. However even in this approximation the suggested optical damage mechanism may play a significant role at high temperatures of a sample. Keywords: short pulse, dielectric, optical damage, photo-induced diffusion, amorphyzation.
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