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, SF6 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 SF6 exhibit a softer fundamental vibration with an excitation energy of 3.3 +/- 0.3 meV. This softening, due to the weaker SF6 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 SF6. The results have improved our understanding of the adsorbate-substrate and interadsorbate interactions which govern the properties of this model molecular physisorption system.