Cole-Filipiak Neil C, Knepper Robert, Wood Mitchell, Ramasesha Krupa
Combustion Research Facility, Sandia National Laboratories, Livermore, California 94550, United States.
Sandia National Laboratories, Albuquerque, New Mexico 87185, United States.
J Phys Chem A. 2021 Sep 16;125(36):7788-7802. doi: 10.1021/acs.jpca.1c04800. Epub 2021 Aug 31.
The coupling of inter- and intramolecular vibrations plays a critical role in initiating chemistry during the shock-to-detonation transition in energetic materials. Herein, we report on the subpicosecond to subnanosecond vibrational energy transfer (VET) dynamics of the solid energetic material 1,3,5-trinitroperhydro-1,3,5-triazine (RDX) by using broadband, ultrafast infrared transient absorption spectroscopy. Experiments reveal VET occurring on three distinct time scales: subpicosecond, 5 ps, and 200 ps. The ultrafast appearance of signal at all probed modes in the mid-infrared suggests strong anharmonic coupling of all vibrations in the solid, whereas the long-lived evolution demonstrates that VET is incomplete, and thus thermal equilibrium is not attained, even on the 100 ps time scale. Density functional theory and classical molecular dynamics simulations provide valuable insights into the experimental observations, revealing compression-insensitive time scales for the initial VET dynamics of high-frequency vibrations and drastically extended relaxation times for low-frequency phonon modes under lattice compression. Mode selectivity of the longest dynamics suggests coupling of the N-N and axial NO stretching modes with the long-lived, excited phonon bath.
分子间和分子内振动的耦合在含能材料从冲击到爆轰转变过程中引发化学反应方面起着关键作用。在此，我们通过使用宽带超快红外瞬态吸收光谱，报道了固体含能材料1,3,5-三硝基全氢-1,3,5-三嗪（RDX）从亚皮秒到亚纳秒的振动能量转移（VET）动力学。实验揭示了VET发生在三个不同的时间尺度上：亚皮秒、5皮秒和200皮秒。中红外区所有探测模式下信号的超快出现表明固体中所有振动存在强烈的非谐耦合，而长时间的演化表明VET是不完全的，因此即使在100皮秒的时间尺度上也未达到热平衡。密度泛函理论和经典分子动力学模拟为实验观测提供了有价值的见解，揭示了高频振动初始VET动力学的压缩不敏感时间尺度以及晶格压缩下低频声子模式显著延长的弛豫时间。最长动力学的模式选择性表明N-N和轴向NO伸缩模式与长寿命的激发声子浴耦合。
