Acoustic Surface Plasmon on Cu(111) as an Excitation in the Mid-Infrared Range

We report on an investigation of the acoustic surface plasmon on Cu(111), an electronic excitation in the infrared range related to the Shockley surface state. As shown here by high-resolution electron energy loss spectroscopy, it contributes together with other low-energetic electronic transitions to a broad excitation feature. Our analysis is similar to that recently reported for the Au(111) surface and clarifies that the group velocity of the acoustic surface plasmon is slower than the Fermi velocity of the surface state. The acoustic surface plasmon thus overlaps with the electron–hole pair continuum and may therefore influence adsorption processes as well.