Atkin Joanna M, Sass Paul M, Teichen Paul E, Eaves Joel D, Raschke Markus B
Department of Physics, Department of Chemistry, and JILA, University of Colorado , Boulder, Colorado 80309, United States.
Department of Chemistry, University of North Carolina , Chapel Hill, North Carolina 27599, United States.
J Phys Chem Lett. 2015 Nov 19;6(22):4616-21. doi: 10.1021/acs.jpclett.5b02093. Epub 2015 Nov 9.
Vibrational spectroscopy can provide information about structure, coupling, and dynamics underlying the properties of complex molecular systems. While measurements of spectral line broadening can probe local chemical environments, the spatial averaging in conventional spectroscopies limits insight into underlying heterogeneity, in particular in disordered molecular solids. Here, using femtosecond infrared scattering scanning near-field optical microscopy (IR s-SNOM), we resolve in vibrational free-induction decay (FID) measurements a high degree of spatial heterogeneity in polytetrafluoroethylene (PTFE) as a dense molecular model system. In nanoscopic probe volumes as small as 10(3) vibrational oscillators, we approach the homogeneous response limit, with extended vibrational dephasing times of several picoseconds, that is, up to 10 times the inhomogeneous lifetime, and spatial average converging to the bulk ensemble response. We simulate the dynamics of relaxation with a finite set of local vibrational transitions subject to random modulations in frequency. The combined results suggest that the observed heterogeneity arises due to static and dynamic variations in the local molecular environment. This approach thus provides real-space and real-time visualization of the subensemble dynamics that define the properties of many functional materials.
振动光谱能够提供有关复杂分子系统性质背后的结构、耦合和动力学信息。虽然光谱线展宽的测量可以探测局部化学环境，但传统光谱学中的空间平均限制了对潜在异质性的洞察，尤其是在无序分子固体中。在这里，我们使用飞秒红外散射扫描近场光学显微镜（IR s-SNOM），在振动自由感应衰减（FID）测量中解析了作为致密分子模型系统的聚四氟乙烯（PTFE）中的高度空间异质性。在小至10³个振动振子的纳米级探测体积中，我们接近均匀响应极限，具有几皮秒的延长振动退相时间，即高达非均匀寿命的10倍，并且空间平均收敛到体相系综响应。我们用一组有限的受频率随机调制的局部振动跃迁来模拟弛豫动力学。综合结果表明，观察到的异质性是由于局部分子环境中的静态和动态变化引起的。因此，这种方法提供了定义许多功能材料性质的子系综动力学的实空间和实时可视化。
