LASER-INDUCED DECOMPOSITION AND ABLATION DYNAMICS STUDIED BY NANOSECOND INTERFEROMETRY - 2 - A REACTIVE NITROCELLULOSE FILM

Laser-induced decomposition and accompanying ablation dynamics of a re active nitrocellulose film doped with a Cu-phthalocyanine derivative a s a light absorber was investigated by applying a nanosecond interfero metric technique. While nitrocellulose does not absorb XeF 351 nm exci mer laser pulse, the film is heated instantaneously via rapid photothe rmal conversion in the doped Cu-phthalocyanine derivative. Below the a blation threshold, the irradiated film expanded transiently with no pe rmanent etching; namely, thermal expansion and contraction processes w ere directly followed in the ns time region. Above the ablation thresh old the expansion of the film was started during the excimer laser pul se, and then explosive decomposition was initiated, continuing in a fe w hundreds ns after the excitation. Generation of shock wave and eject ion of gaseous plume were also observed by nanosecond photographic tec hnique. The shock wave emerged at 100-200 ns after excitation and late r than the typical shock wave formation time reported in general. The slow formation is consistent with the slow initiation of the decomposi tion, suggesting a specific ablation process of the nitrocellulose fil m. Temperature elevation caused by the excimer laser irradiation resul ts in an exothermic decomposition of nitrocellulose, leading to a furt her temperature rise of the film. Consequently self-acceleration of th e reaction is enhanced and an explosive self-sustaining decomposition is induced after reaching the explosive decomposition condition. Ablat ion rate was determined to be 0.63 m/s which is slower than that of de tonation but faster than that of combustion.