Absence of below-threshold ionization and the cumulation effect under conditions of repeated exposure of glasses to laser radiation

Experiments have been carried out to observe below-threshold ionization and to study the cumulation effect under conditions of repeated exposure of glasses to laser radiation. It was found that when the bulk (interior) of a glass was repeatedly exposed to single-frequency laser radiation with hν < E_g/2 (E_g is the ionization potential of the glass matrix), no color center cumulation effect, below-threshold ionization of the matrix, or reduction in the breakdown threshold were observed. The decrease in the breakdown threshold observed on repeated exposure to multifrequency radiation was due to an increase in the probability of formation of high-intensity ultrashort pulses. When glasses were repeatedly exposed to radiation with hν > E_g/2, a color center cumulation effect and high-intensity below-threshold ionization were observed as a result of two-photon excitation of the glass matrix.