Dynamic Measurement of the HMXβ-δPhase Transition by Second Harmonic Generation

We demonstrate the use of optical second harmonic generation (SHG) to identify the $\beta -\delta$ phase transition in polycrystalline HMX. This phase transition has been implicated in the thermal decomposition of HMX for over 30 years. We measure an order of magnitude difference in SHG cross section from the $\beta$ and $\delta$ phases and use this intensity difference to observe the phase transition during ${\mathrm{CO}}_2$ laser irradiation. This is the first known in situ measurement of a phase transition in such an energetic material during a dynamic experiment, and the first example of SHG from this class of organic nitramine molecular crystals.