Time-resolved Spectroscopy of Anharmonic Systems

麻省理工学院的基思·纳尔逊（Keith Nelson）得到了实验物理化学项目的支持，他继续利用时间和空间分辨飞秒光脉冲整形技术研究大振幅晶格振动。最初，他将专注于经历结构相变的晶体，特别是不同铁电相之间的转换。此外，他将研究半导体材料，其带隙可能受到光诱导的大振幅相干晶格振动的影响，甚至可能闭合。尼尔森还计划研究分子晶体的行为，其中光脉冲诱导沿反应坐标运动以影响晶体化学反应。他将研究大块钙钛矿和薄膜钙钛矿，以研究结构相变和声子-极化子腔；二乙炔、苝和芘的分子晶体研究；Ti2O3和GeTe作为半导体。Nelson教授已经证明，使用高能脉冲可以在分子晶体中诱导显著的核位移。此外，如果波长足够长，对晶体的损伤就会降到最低。在这些研究中，Nelson将研究实现大核位移的替代策略以及分子特性对最佳策略的影响类型。Nelson教授将把他在实验室开发的利用飞秒光脉冲的技术应用于各种各样的材料，如铁电晶体和小间隙半导体，以确定是否有可能影响化学变化。除了这项工作可能为晶格光谱和分子过程的光学控制领域提供直接的进步之外，可以预期，计划中的研究将对光开关和信息传输原型设备的设计者具有技术兴趣和实用性。

Keith Nelson of the Massachusetts Institute of Technology is supported by the Experimental Physical Chemistry Program in his continuing investigations of large amplitude lattice vibrations using time- and space-resolved femtosecond optical pulse-shaping techniques. Initially, he will focus on crystals that undergo structural phase transitions, in particular the switching among different ferroelectric phases. In addition, he will look at semiconducting materials whose bandgaps may be influenced and perhaps closed by optically induced large-amplitude coherent lattice vibrations. Nelson also plans to examine the behavior of molecular crystals in which the optical pulses induce motion along reaction coordinates to affect crystalline chemical reactions. He will investigate both bulk and thin film perovskites to study structural phase transitions and phonon-polariton cavities; diacetylene, perylene and pyrene to study molecular crystals; and Ti2O3 and GeTe as semiconductors. Prof. Nelson has demonstrated that significant nuclear displacements can be induced in molecular crystals using high energy pulses. In addition, if the wavelength is long enough, damage to the crystal is minimized. In these studies Nelson will investigate alternative strategies for achieving large nuclear displacements and what types of effects the molecular properties have on the optimum strategy. Prof. Nelson will apply techniques developed in his laboratory that exploit femtosecond optical pulses to a wide variety of materials such as ferroelectric crystals and small gap semiconductors, in order to determine if it is possible to effect a chemical change. In addition to the direct advances this work will likely offer to the fields of lattice spectroscopy and optical control of molecular processes, it can be anticipated that the planned research will be of technological interest and utility to designers of prototype devices for optical switching and information transmittal.