Elementary excitations of ${\mathrm{C}}_{60}$ from the far infrared to the far vacuum ultraviolet studied by high-resolution electron-energy-loss spectroscopy

We report on measurements by high-resolution electron-energy-loss spectroscopy (HREELS) of the excitation spectrum of thin films of ${\mathrm{C}}_{60}$ (containing less than 10% of ${\mathrm{C}}_{70}$) deposited onto Si(100) in ultrahigh vacuum. Most of our spectra have been taken in the specular direction of the reflected beam. By varying the primary-electron energy from 2 to 150 eV, the frequency regions extending from the far ir, through the visible-uv, to the far vuv have been investigated on one single specimen in the same spectrometer with a resolution varying from 10 meV in the ir to 60 meV in the vuv. Since no strict selection rules apply to electron inelastic scattering, the present data contain information on many of the elementary excitations of the ${\mathrm{C}}_{60}$ fullerite film. Several of the spectral features are comparable to the observations of photon, photoelectron, and neutron spectroscopies which address separate parts of the spectrum. Important additional features are revealed by HREELS, including a transition at 1.55 eV which appears to be the lowest one-electron excitation observed in ${\mathrm{C}}_{60}$ and two collective excitations at 6.3 eV (π plasmons) and 28 eV (σ plasmons). When possible, the band intensities are discussed with the help of the dielectric theory of EELS. The present data further advance our knowledge of the rich spectrum of elementary vibrational and electronic excitations of the ${\mathrm{C}}_{60}$ fullerite.