Vibrational Properties of Disordered Mono- and Bilayers of Physisorbed Sulfur Hexafluoride on Au(111)

Abstract

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.

Electric Field Changes on Au Nanoparticles on Semiconductor Supports – The Molecular Voltmeter and Other Methods to Observe Adsorbate-Induced Charge-Transfer Effects in Au/TiO2Nanocatalysts

“…54 As with C 3 H 8 , SF 6 -TPD experiments on Au single crystals and separately on metal oxides verify that SF 6 adsorbs only on TiO 2 under the experimental conditions employed here. 55,56 As shown in Figure 5a, when SF 6 is adsorbed on TiO 2 , the CO/Au IR frequency shifts upward toward higher frequencies opposite to the effect observed when C 3 H 8 adsorbs on TiO 2 . Furthermore, when the catalyst is subsequently heated in 5 K increments up to 180 K starting from SF 6 saturation (Figure 5b, black curve) and ending with a clean TiO 2 surface (Figure 5b, red curve), the CO/Au IR frequency exhibits reversed redshifts toward lower frequencies opposite to the blueshift observed for C 3 H 8 desorption.…”

Direct Images of the Virtual Source in a Supersonic Expansion

“…Supersonic molecular beams can be created with neutral atoms at low (thermal) energy, which makes them particularly suited for the investigation of fragile surface structures and insulating materials 7,8 that are difficult to investigate with other methods. They are also useful for surface dynamics experiments because they can probe millielectronvolt as well as microelectronvolt [9][10][11] energy ranges. Recent new developments include measurements with extreme grazing incidence exploring quantum scattering effects, 12 He-spin echo, 13 and scanning helium microscopy.…”

Theoretical Study of Weakly Bound Adsorbates on Au(111): Tests on van der Waals Density Functionals

“…Same as Table 3 ) is a representative polyatomic molecule interacting weakly with metal surfaces, given its inert properties and rotationally averaged symmetry properties comparable to rare gases. 71 Using optPBE-vdW, 12 adsorption geometries of SF 6 on Au(111) were optimized (Figure S8), and the corresponding adsorption energies are compared in Table S5. The molecule locating at the hcp site with three S-F bonds pointing to the surface, denoted as "F3-hcp", is the most stable adsorption structure.…”

“…The octahedral sulfur hexafluoride (SF 6 ) is a representative polyatomic molecule interacting weakly with metal surfaces, given its inert properties and rotationally averaged symmetry properties comparable to rare gases . Using optPBE-vdW, 12 adsorption geometries of SF 6 on Au(111) were optimized (Figure S8), and the corresponding adsorption energies are compared in Table S5.…”