Dynamic measurement of the HMX beta-delta phase transition by second harmonic generation

We demonstrate the use of optical second harmonic generation (SHG) to ident ify the beta-delta phase transition in polycrystalline HMX. This phase tran sition has been implicated in the thermal decomposition of HMX for over 30 years. We measure an order of magnitude difference in SHG cross section fro m the beta and delta phases and use this intensity difference to observe th e phase transition during CO2 laser irradiation. This is the first known il l situ measurement of a phase transition in such an energetic material duri ng a dynamic experiment, and the first example of SHG from this class of or ganic nitramine molecular crystals. [S0031-9007(98)08215-5].