Vibrational properties of disordered mono- and bilayers of physisorbed sulfur hexafluoride on Au(111)

We have examined the low-energy single-phonon vibrations of disordered mono- and bilayers of sulfur hexafluoride physisorbed on Au(111) with inelastic helium atom scattering. At monolayer coverages, SF ₆ exhibits a dispersionless Einstein mode at 3.6 ± 0.4 meV. We observed two distinct overtones of this vibration as both creation and annihilation events at 7.1 ± 0.7 meV and 10.9 ± 1.4 meV, respectively. The overtones are harmonic multiples of the fundamental Einstein oscillation. Bilayers of SF ₆ exhibit a softer fundamental vibration with an excitation energy of 3.3 ± 0.3 meV. This softening, due to the weaker SF ₆ binding, also results in reduced overtone energies of 6.6 ± 0.7 meV and 9.8 ± 0.6 meV. The disordered bilayer does not exhibit dispersion, indicating that the molecules are still behaving like Einstein oscillators and not beginning to act as bulk crystalline SF ₆. The results have improved our understanding of the adsorbate - substrate and interadsorbate interactions which govern the properties of this model molecular physisorption system.