The paper presents the results of experimental and theoretical studies of the role of avalanche ionization in the process of highly nonlinear generation of electron-hole pairs in the interaction of intense femtosecond laser radiation with dosimetric crystals LiF:Mg,Ti. We assume that the effective formation of defects of hole-interstitial type, which consist of molecular halogen ions placed in one cationic and two anionic vacancies, distinguished by high-temperature peaks in the thermally stimulated luminescence curves, is feasibly connected with avalanche ionization processes occurring in the regions of passage of light filaments. The calculated dependences of the plasma density on time during the laser pulse made with taking into account the avalanche ionization for this type of crystals show that the contribution of avalanche ionization to the electron-hole plasma density is comparable to that of the multiphoton tunneling mechanism.
We present the results of studies of energy accumulation during the non-destructive interaction of extremely intense near infrared laser radiation with model wide band gap dielectric crystals of lithium fluoride, when the intensity of pulses is sufficient for effective highly nonlinear absorption of light and for the excitation of the electron subsystem of matter and the energy of pulses is still not sufficient for significant heating, evaporation, laser breakdown or other destruction to occur. We studied the emission of energy in the form of light sum of thermally stimulated luminescence accumulated under conditions of self-focusing and multiple filamentation of femtosecond laser radiation. It was established that it's the F2 and F3+ color centers and supplementary to them centers of interstitial type which accumulate energy under the action of a single femtosecond laser pulses. When irradiated by series of pulses the F3, F3- and F4 centers additionally appear. F2 centers are the main centers of emission in the process of thermally stimulated luminescence of accumulated energy. The interstitial fluoride ions (I-centers) are the kinetic particles. They split off from the X3- centers in the result of thermal decomposition of latter on the I-centers and molecules X20. I-centers recombine with F3+ centers and form F2 centers in excited state. The latter produce the characteristic emission spectrum emitted in the form of thermally stimulated luminescence.