Observation of the acoustic surface plasmon by inelastic neon scattering

Carraro1, G. Bracco1, L. Vattuone1, M. Smerieri2 and M. Rocca1

1.Dipartimento di Fisica dell’Università di Genova and IMEM-CNR, Genova, Italy

2.IMEM-CNR Unità Operativa di Genova, Italy

The Acoustic Surface Plasmon (ASP), arising from the counter-phase motion of electrons in the Shockley Surface State and in bulk, has been reported so far only by High Resolution Electron Energy Loss Spectroscopy (HREELS). The experiments, performed on Be(0001) [1] Cu(111) [2,3], Au(111) [4,5] and Au(788) [6], confirmed the linear dispersion and evidenced that, in the short wavelength limit, the slope of the ASP dispersion on noble metal surfaces can be markedly smaller than the initially predicted Fermi velocity of the 2D electron gas. HREELS is, however, unable to explore the long wavelength limit because the ASP loss overlaps with the Drude tail. Alternative methods have thus to be seeked to explore this very important region extending down to terahertz frequencies which is most promising for applications.

We show here that the ASP can be efficiently excited on Cu(111) by hyperthermal (0.24 eV) Ne beams, obtained by seeding in He. The Time of Flight (ToF) spectra recorded at different polar angles show evidence of energy losses larger than the highest phonon frequency (30 meV) and extending up to 150 meV at small transferred momentum, thus fully compatible with the ASP dispersion on Cu(111) extrapolated from the HREELS data. From the energy width of the observed ASP losses we infer a lower limit to the ASP mean free path of about 25 nm, an information relevant to assess the feasibility of ASP based devices operating in the Terahertz region.

Surprisingly, the intensity of the ASP losses in the Ne ToF spectra is comparable to, and in some cases even larger than, those due to phonon excitations. Our data demonstrate, therefore, that the ASP represents a relevant, and so far not considered, channel for energy dissipation in molecule surface scattering and in adsorption processes.

[1] Diaconescu, B. et al, Low energy Acoustic plasmons at metal surfaces, Nature 448, 57- 59 (2007)

[2] Pohl, K et al, Acoustic surface plasmon on Cu(111) Europhysics Letters 90, 57006 (2010)

[3] Pischel, J., Welsch, E., Skibbe, O.,  Pucci, A., Acoustic Surface Plasmon on Cu(111) as an Excitation in the MidInfrared Range JPCC 117, 26964−26968 (2013)

[4] Park, S.J. & Palmer, R.E. Acoustic Plasmon on the Au(111) Surface PRL 105, 016801 (2010)

[5] Vattuone, L. et al., Correlated Motion of Electrons on the Au(111) Surface: Anomalous Acoustic Surface-Plasmon Dispersion and Single-Particle Excitations PRL 110, 127405 (2013)

[6] Smerieri, M. et al, Anisotropic Dispersion and Partial Localization of Acoustic Surface Plasmons on an Atomically Stepped Surface: Au(788) PRL 113, 186804 (2014)