飞秒受激拉曼光谱研究金属表面热传导和电子-分子能量传递超快过程

Heat conduction and electron-molecule interactions are fundamental processes related to a variety of scientific interests related to many aspects of natural science and industrial technologies. It is an intellectual challenge to understand nanoscale heat transportation and electron-molecule interaction at interfaces. However, the mechanisms of nano-scale heat transfer, which is still vague, is essentially important for controlling heat or energy transport at atomic scale to ensure the proper function of molecular electronic devices and nano-machines. .We focus our interests on the heat flow and electron-molecule interactions at interfaces of molecules on metal surfaces (model surfaces, but not limited with metals), with sub-picosecond time resolution and angstrom space resolution. In our project, molecules assembled on metal surfaces, then flash-heated by a femtosecond laser pulse to introduce hundreds of degrees of temperature jump. The flash-heating pulse heats up the substrate metal within a picosecond, and initiate heat flow from metal to surface molecule, in the mean time, hot electron created and will interact with the surface molecule as well. As heat flows into molecules, along with hot election interaction, individual vibrations that carry the heat will be excited and populated accordingly. Femtosecond Stimulated Raman Spectroscopy (FSRS) will be adopted to probe surface molecules to follow the heat and energy exchange at molecule-metal interfaces with a detailed vibrational decay pathway of the surface molecules. Although, there is only one layer of molecules on metal surfaces and the Raman cross-sections are very small, FSRS will be still be able to see surface molecules, due to the orders of magnitude higher efficiency than spontaneous Raman. Combination of Ultra fast flash-heating and FSRS can be used to study a large range of material and surfaces, not only limited by metal surfaces, due to the nature of Raman processes. .This research will add a significant amount of knowledge to the data base required for design molecules which enhance or inhibit heat and energy transfer, and help to effectively engineer molecular electronic devices and molecular nano-machine.

热传递和电子分子相互作用是与很多学科和技术前沿领域的重要基础物理化学过程。纳米尺度上的热传递和电子分子相互作用对分子电子学和纳米机械都具有非常重要的意义。但由于缺乏有效的实验手段，这个领域的研究还没有广泛开展。本课题拟采用超快闪光加热和飞秒受激拉曼光谱联用技术，来研究金属表面（以金属为模型，但并不局限于金属）单分子膜体系，纳米尺度上的热传导及其伴随的电子-分子相互作用超快动力学过程。由飞秒激光脉冲闪光加热基底，诱发基底向表面分子的热传递，以及基底中热电子与表面分子的相互作用，由于飞秒受激拉曼比自发拉曼的效率高几个数量级，因此，可以用飞秒受激拉曼光谱实时跟踪表面分子的超快变化过程，在飞秒时间尺度上获取表面分子振动激发和弛豫的详细路径，从而揭示纳米尺度界面上的热传导机理，为分子电子学和纳米机械领域中的分子设计提供理论基础。

研制了宽带受激拉曼光谱系统，主要技术指标为：泵浦光波长可以在可见和近紫外区连续扫描，450-900纳米大范围检测，可以覆盖-4000到4000波数的振动光谱范围（在合适的泵浦波长下），光谱分辨率优于7波数。不同泵浦波长下的共振受激拉曼光谱研究，发现共振谱峰为色散线型，而且受激拉曼损失谱与增益谱中同一振动模式对应的谱峰强度有很大差异并随泵浦波长变化，这些现象与不同泵浦波长下的跃迁几率有关。另外，尝试了用Fano线型对共振受激拉曼谱峰的色散线型进行解析。溶液体系中金属纳米粒子表面分子受激拉曼光谱的测量，发现只有较大共振截面的分子才能检测到共振谱峰。建立了非线性光谱数值解析的算法，可以在不预设物理模型的前提下，对实验测得光谱进行解析还原，获得更精确的谱峰强度、相对相位、振动去相位时间等结构和动力学信息。用飞秒红外脉冲激发金属表面，观测到热电子的生成和弛豫，测得金表面热电子弛豫时间约为1.8皮秒。