Laser absorption characteristics and energydeposition patterns of CL-20 explosives

Abstract

In order to explore the energy absorption mechanism of femtosecond laser in explosives, the electronic structures, linear and nonlinear optical parameters of five crystal forms of CL-20 molecules (α, β, ε, ξ, and γ) were systematically calculated under laser irradiation based on density functional theory and vibrational frequency analysis in this study. The corresponding ultraviolet–visible (UV) and infrared (IR) absorption spectrum characteristics and laser energy deposition patterns were revealed. The results indicated that single-photon energy level transitions and molecular resonance absorption were not the primary photon absorption mechanisms of CL-20 explosives under laser irradiation (with wavelength ranging from 300 to 1200 nm). The reflection coefficient R and linear absorption coefficient α of explosives were related to the parameters such as crystal form and laser wavelength. Under lasers irradiation with wavelength range from 200 to 250 nm, the only significant single-photon absorption was exhibited in five crystal forms of CL-20 molecules (α, β, ε, ξ, and γ). The corresponding linear absorption coefficients were ranked in descending order as γ-CL-20, α-CL-20, ξ-CL-20, β-CL-20, ε-CL-20, respectively, which was ranked in the opposite order with predicted ablation thresholds. However, under femtosecond lasers irradiation with wavelength range from 400 to 500 nm and from 650 to 750 nm, the obvious two-photon, three-photon absorption and weaker single-photon absorption were exhibited in five crystal forms of CL-20 explosives, respectively. In addition, the multiphoton nonlinear absorption effect of explosives varied significantly with laser power density, while total absorption coefficient A(I) and multiphoton absorption capacity were positively correlated with laser power density.