Intrinsic and extrinsic plasmons in the hard x-ray photoelectron spectra of nearly free electron metals

TL;DR

This study investigates collective plasmon excitations in nearly free electron metals Al and Mg using hard x-ray photoelectron spectra, revealing multiple bulk plasmons, their contributions, and surface plasmon signatures with implications for understanding electron dynamics.

Contribution

It provides the first detailed analysis of multiple bulk plasmons and their intrinsic, extrinsic, and interference contributions in nearly free electron metals using experimental spectra.

Findings

Multiple bulk plasmons identified in Al and Mg spectra.

Intrinsic plasmon contribution decreases with higher order.

Surface plasmon signature observed in normal and grazing emission.

Abstract

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 1$s$ to 2$s$ core level plasmons. The intrinsic contribution diminishes from 22% 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.