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 ($n{\omega }_p$) 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 $1s$ to $2s$ core-level plasmons. The intrinsic contribution diminishes from 21.7% for $1{\omega }_p$ to 4.4% for $2{\omega }_p$, and becomes negligible thereafter (0.5% for $3{\omega }_p$). 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 $1{\omega }_p$. 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.

中文翻译：

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近自由电子金属硬 X 射线光电子能谱中的本征和外在等离子体激元

光电子发射过程产生的固体中的集体等离子体激发是基础研究的一个重要领域。在这项研究中，我们确定了大量（$n$）的多个体等离子体激元（$n{\omega }_p$）在两种著名的近自由电子金属铝（Al）和镁（Mg）的核心能级和价带（VB）的硬X射线光电子能谱中。在早期理论工作的基础上，我们估计了外在、内在和干扰过程对强度的贡献。$1s$到$2s$核心级等离激元。内在贡献从 21.7% 减少$1{\omega }_p$ 至 4.4%$2{\omega }_p$，此后变得可以忽略不计（0.5%$3{\omega }_p$）。外在和内在等离子体激元的贡献在广泛的光电子动能范围内以及两种金属（铝和镁）之间也没有显着变化。干扰贡献从负到零变化为$n$增加。观察到体等离子体激元的不对称线形状，这对于$1{\omega }_p$。在正常发射中检测到表面等离子体激元的特征，并且在掠射发射中表现出显着增加的强度。 Al 和 Mg 的 VB 光谱主要为$s$类状态，与基于密度泛函理论计算的 VB 表现出极好的一致性。 VB 在损失区域表现出四个多重体等离子体峰，除了外在过程之外，这些峰还受到内在过程的影响。在完全氧化的铝表面上，铝金属体等离子体激元的相对强度几乎保持不变，而表面等离子体激元则完全衰减。