钠纳米团簇的光吸收和体等离激元:理论分析

指出了体等离激元是一种重要的元激发,在固体里它不能与光波耦合所以无法用光波分波来研究,但是在团簇里由于边界的存在,情况不太一样.此外,光与物质之间的相互作用是非常根本的,但简单金属团簇在紫外光区的光吸收从未被实验研究过,尽管这方面的理论计算非常多.采用光倒空技术对N a20和N a92团簇进行了光吸收实验,覆盖了近紫外部分以及可见光范围(2.0~5.5 eV);实验还研究了其它一些团簇在近紫外范围内的吸收截面.在可见光部分N a20的实验数据与现有的数据非常吻合.除了表面等离激元在可见光区域很强的吸收峰外,还观察到紫外区域明显的光吸收.对吸收截面光谱进行洛伦兹拟合揭示了中心略高于4eV宽峰的存在,这个宽峰的频率与权重均与理论预测非常接近,被确定为纳米团簇"体等离激元"共振.这些吸收光谱是金属纳米团簇由光子激发的"体等离激元"集体电子态的首次实验观测与证实,这种实验现象只存在于有限体系.

Photoabsorption of Nan Clusters and Volume Plasmon: Theory Review

The volume plamson is an important elementary excitation,it could not couple to lightwaves in the bulk and thus could not be investigated by optical spectroscopy,however the situation is different for clusters due to the presence of the boundary.What is more,it is fundamental to study the interaction between light and matter,but the photoabsorption of simple metal clusters in the UV region has remained unexplored experimentally.We have carried out experiments on the photoabsorption of Na20and Na92 covering the near UV part as well as visible range (2.0～5.5 eV)employing the phorodepletion technique.The absorption cross sections for clusters of some other sizes in the near UV range have also been studied.The experimental data for Na20 in the visible part agree with existing data very well.In addition to the strong surface plasmon absorption peaks in the visible,we observed appreciable photoabsorption in the UV region.These cross section spectra have been fitted by Lorentzian line shapes and revealed broad peaks slightly above 4 eV.The frequency and weight of these peaks match the theoretical predictions and are identified as the nanocluster"volume plasmon"resonance.The photoabsorption spectra provide the first experimental observation of optically excited"volume plasmon"collective electronic states in metal nanocluster particles,a phenomenon unique to finite systems.