利用超快二维电子光谱实现物质亚表面的层析分辨

Layer-Resolved Characterization of Subsurface of Materials using Ultrafast Two-Dimensional Electronic Spectroscopy

物质的表面区域除了最外层表面之外,通常还包含亚表面.亚表面的分子及电子结构与最外表面存在差异,且能够对表面能量及电荷转移过程产生决定性的影响.因此,具有层析分辨能力的新方法的开发对于表面科学技术发展具有重要意义.尽管光谱学方法可以用于解析分子结构,但是它们的空间分辨率往往远大于分子层间距,因此无法在空间上区分不同分子层.同时,谱线展宽效应使得在频域实现层析分辨也极具挑战,从而阻碍了层间结构演化和能量转移动力学过程的探测、本文讨论如何利用超快二维电子光谱来实现吸收频率和能量转移动力学的层析分辨.通过二维光谱数值模拟,研究了两种层状体系,表面:亚表面:体相,以及附着分子层:表面:亚表面:体相.模拟中预设了由层间耦合所导致的体相到表面的定向能量转移速率.由于激发能量从亚表面到表面的转移时间短于从体相到表面所需时间,因此在某个特定时间窗口内,来自于亚表面的非对角峰信号占据绝对主导,能够被有效提取.模拟结果表明通过分析非对角峰差谱,最外表面、亚表面以及体相的不同激发频率能够得到解析.相对于三层体系,四层体系的非对角峰远离对角峰的干扰,可以更好地实现层析分辨.

Surface of matter normally contains sublayers with molecular or electronic structure different from the outmost surface and the bulk,which may play a critical role in surface energy and/or charge transfer processes.Therefore,the development of layer-resolved characterization methods is of great importance for surface science and techniques.Although optical spectroscopy methods are very sensitive to structure,their spatial resolution is often much larger than the inter-layer distance of the sublayers,resulting in the inability to achieve laminar resolution.In this work,we discuss the possibilities of utilizing two-dimensional(2D)electronic spectroscopy to distinguish spectral information and energy transfer between different layers,which cannot otherwise be obtained from linear spectroscopy methods owing to lineshape broadening.By theoretical 2D spectral simulations,we investigated two layered systems by numerical simulations,material surface:subsurface:bulk and molecule:surface:subsurface:bulk.The directional energy transfer rates from the bulk to the surface layer owing to the surface-bulk coupling was preset.Due to the fact that the energy transfer between the subsurface and the outermost surface layer is faster than that between the bulk and the outermost surface layer,there will be special time periods during which only the former takes place and gives rise to the distinguishable dynamics of off-diagonal peaks.Our simulation results showed that by analyzing the difference between the projections of the off-diagonal peak,the excitation energy of the surface,subsurface and bulk layers can be determined.Furthermore,for the four-layer system,the off-diagonal peaks can be kept away from the interference of diagonal peaks,providing a better chance for realizing laminar resolution compared with the three-layer system.