Effect of the laser radiation wavelength on the energy threshold of initiation of heavy metal azides

The behavior of silver and lead azides (laminated and filiform crystals, pressed powders) with initiation of their explosive decomposition by a laser pulse is studied. The initiation energy thresholds are measured during irradiation at wavelengths of 1064, 532, 354.7, and 266 nm corresponding to four harmonics of laser radiation. Irradiation is performed under conditions with the sample surface being open or covered with a transparent dielectric (quartz plate) with a pressing force up to 5 $\cdot$ 10$^8$ N/m$^2$. Surface covering does not exert any significant effect on the initiation energy threshold in the domain of transparency of the heavy metal azide matrix (first and second radiation harmonics). In the domain of bandgap absorption (fourth harmonic), the initiation threshold strongly depends on the surface conditions (open/covered). The results are discussed from the viewpoint of the influence of the transparent substrate on gas-dynamic unloading of the heat spot, which is formed in the domain of bandgap absorption owing to laser radiation absorption in a thin subsurface layer.