Researchers at Johannes Kepler University in Linz have been investigating the physical properties of Cu(110), a surface obtained by cutting a single copper crystal in a certain direction, for years. Their most recent study, featured in Physical Assessment Letters, provides the first evidence of so-called resonant Raman scattering from the surface of the metal. This phenomenon involves the inelastic scattering of phonons by matter.
“We have done a lot of research on Cu(110) and are particularly interested in the surface state transition at 2.1 eV. Since the surface state electrons are confined to the first few layers of the crystal, the Cu(110 ) surface condition is a sensitive measure of the surface condition, we use this high sensitivity to study various physical processes on the surface, such as reconstruction of the surface after adsorption or molecular growth,” Mariella Denk, one of the researchers who conducted the study, told Phys.org.
“During discussions with the group of Prof. Dr. Norbert Esser in Berlin, which mainly deals with Raman scattering of semiconductors, but also has experience in studying metal surfaceswe came up with the idea of just trying to see if Raman scattering from surface phonons can be seen on Cu(110).”
In a series of initial experiments, Denk and her colleagues observed a very high intensity Raman scattering of phonons on the surface of a Cu(110) sample. They then decided to further investigate this surprising observation to determine the underlying mechanisms.
In their experiments, the researchers used a technique called Raman spectroscopy. This is a non-destructive method of performing chemical analysis, which focuses the light from a laser onto the surface of a sample, covering a spot approximately 100 m in size. The light emitted from this spot is collected with the help of a lens and enters a monochromator (ie an optical instrument that measures the spectrum of light).
“Elastic scattered radiation at the wavelength corresponding to the laser line (Rayleigh scattering) is filtered out, while the rest of the light is scattered on a detector,” explains Denk. “The laser light interacts with vibrations, phonons or other excitations in the system, changing the energy of the laser photons. The difference in the energies of the incident and scattered light provides information about the excited vibrational modes.”
The surface phonons of Cu(110) – as well as their dispersion – have been intensively studied with complementary techniques and are well understood. However, Denk and her colleagues were the first to show that Raman scattering of surface phonons on Cu(110) can be observed and that the high intensity obtained in the experiments is due to scattering in resonance with the electronic transition of the surface state of Cu(110) at a 2.1 eV. They did this by collecting polarization and excitation energy dependent Raman measurements on their sample using 10 laser lines, within a photon energy range of 1.8 to 3 eV.
“Our study provides the first evidence for Raman scattering by surface phonons on a metal surface,” explains Denk. “The Raman experiments, along with electronic band structure and lattice dynamics calculations, paint a coherent picture of the interaction between surface phonons and surface-localized electronic states.”
The findings collected by this team of researchers could greatly improve the current understanding of Cu(110) and other metal surfaces. In the future, they could pave the way for further theoretical work focusing on: electron-phonon coupling occur on metal surfaces.
“We now plan to conduct further experiments to test whether the method can be used for high resolution surface vibrational spectroscopy, in particular whether optical transitions on surfaces and interfaces can be used to enhance Raman scattering of vibrations from adsorbed species,” Denk said.
M. Denk et al, Surface Resonant Raman Scattering of Cu(110), Physical Assessment Letters (2022). DOI: 10.1103/PhysRevLett.128.216101
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