近自由电子金属硬X射线光电子能谱中的本征与非本征等离激元
Intrinsic and extrinsic plasmons in the hard x-ray photoelectron spectra of nearly free electron metals
AI总结：
本研究在铝、镁两种近自由电子金属的硬X射线光电子能谱中识别多体等离激元，定量分析本征、非本征及干涉过程的贡献，揭示体/表面等离激元的线形、发射角度依赖及氧化表面的演化规律。
摘要：
光发射过程引发的固体中等离激元集体激发是基础研究的重要领域。本研究在两种典型近自由电子金属——铝（Al）和镁（Mg）的芯能级与价带（VBs）硬X射线光电子能谱中，识别出数量可观（$n$）的多体等离激元（$nω_p$）。基于前期理论工作，我们估算了非本征、本征及干涉过程对1$s$至2$s$芯能级等离激元强度的贡献：本征贡献从1$ω_p$的22%降至2$ω_p$的4.4%，之后可忽略不计（3$ω_p$时为0.5%）。在宽范围的光电子动能区间内，以及两种金属（Al和Mg）之间，非本征与本征等离激元的贡献均无显著变化；干涉贡献随$n$增大从负值趋近于零。观测到体等离激元呈非对称线形，在1$ω_p$时最为显著。法向发射中检测到表面等离激元信号，且其强度在掠射发射下显著提升。由类$s$态主导的Al和Mg价带谱，与基于密度泛函理论计算的价带结果高度吻合。价带的损失区呈现4个体等离激元多重峰，除非本征过程外，还受本征过程影响。在完全氧化的铝表面，Al金属体等离激元的相对强度基本保持不变，而表面等离激元则完全衰减。
英文摘要：
Collective plasmon excitations in solids that result from the process of photoemission are an important area of fundamental research. In this study, we identify a significant number ($n$) of multiple bulk plasmons ($nω_p$) in the hard x-ray photoelectron spectra of the core levels and valence bands (VBs) of two well-known, nearly free electron metals, aluminum (Al) and magnesium (Mg). On the basis of earlier theoretical works, we estimate the contributions of extrinsic, intrinsic, and interference processes to the intensities of 1$s$ to 2$s$ core level plasmons. The intrinsic contribution diminishes from 22% for 1$ω_p$, to 4.4% for 2$ω_p$, and becomes negligible thereafter (0.5% for 3$ω_p$). The extrinsic and intrinsic plasmon contributions do not vary significantly across a broad range of photoelectron kinetic energies, and also between the two metals (Al and Mg). The interference contribution varies from negative to zero as $n$ increases. An asymmetric line shape is observed for the bulk plasmons, which is most pronounced for 1$ω_p$. Signature of the surface plasmon is detected in normal emission, and it exhibits a significantly increased intensity in the grazing emission. The VB spectra of Al and Mg, which are dominated by $s$-like states, exhibit excellent agreement with the calculated VB based on density functional theory. The VB exhibits four multiple bulk plasmon peaks in the loss region, which are influenced by an intrinsic process in addition to the extrinsic process. On a completely oxidized aluminum surface, the relative intensity of the Al metal bulk plasmon remains nearly unaltered, while the surface plasmon is completely attenuated.
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