Laser-induced decomposition and ablation dynamics studied by nanosecond interferometry. 2. A reactive nitrocellulose film

Laser-induced decomposition and accompanying ablation dynamics of a reactive nitrocellulose film doped with a Cu-phthalocyanine derivative as a light absorber was investigated by applying a nanosecond interferometric technique. While nitrocellulose does not absorb XeF 351 nm excimer laser pulse, the film is heated instantaneously via rapid photothermal conversion in the doped Cu-phthalocyanine derivative. Below the ablation threshold, the irradiated film expanded transiently with no permanent etching; namely, thermal expansion and contraction processes were directly followed in the ns time region. Above the ablation threshold the expansion of the film was started during the excimer laser pulse, and then explosive decomposition was initiated, continuing in a few hundreds ns after the excitation. Generation of shock wave and ejection of gaseous plume were also observed by nanosecond photographic technique. The shock wave emerged at 100-200 ns after excitation and later than the typical shock wave formation time reported in general. The slow formation is consistent with the slow initiation of the decomposition, suggesting a specific ablation process of the nitrocellulose film. Temperature elevation caused by the excimer laser irradiation results in an exothermic decomposition of nitrocellulose, leading to a further temperature rise of the film. Consequently self-acceleration of the reaction is enhanced and an explosive self-sustaining decomposition is induced after reaching the explosive decomposition condition. Ablation rate was determined to be 0.63 m/s which is slower than that of detonation but faster than that of combustion.