Energetic material response to ultrafast indirect laser heating

The initial evolution of thermal energy transfer into a solid explosive is studied using an indirect femtosecond laser heating technique on a picosecond timescale in order to elucidate the role of temperature in the shock-induced initiation of explosives. The indirect laser heating method is presented; time-resolved visible transient absorption (TA) spectroscopy was used to monitor the energetic material response following heat transfer from the laser-heated gold (Au) layer to the sample. Reported here are visible TA data in the spectral region from 500 to 750 nm for indirect laser-heated thin films of cyclotrimethylene trinitramine (RDX), oxidized polyethylene (OPE), and RDX with 1%, 2.5%, 5%, or 10% OPE prior to decomposition. TA was observed for RDX and RDX with OPE; however, no TA was observed for pure OPE. Compared to pure RDX, the TA intensity of RDX with OPE decreases as the OPE content increases and the time required to observe the TA signal from RDX increases. Our results suggest that the thermal energy produced by a femtosecond laser pulse with an energy of 15  mJ cm^-2 is sufficient to induce changes in the electronic structure of RDX, resulting in promotion of the RDX molecules into an excited state. We also determined that the heat transfer rate in RDX depends on its homogeneity and degree of purity.