Structure, thermodynamics and kinetics of methane on NaCl(100)

Thesis/Dissertation · Wed Jan 01 04:00:00 EST 1992

OSTI ID:6974014

Quattrocci, L M

Polarized infrared spectra of CH[sub 4] and CH[sub 2]D[sub 2] adsorbed on NaCl(100) are reported. Coverage dependent data indicate that there is only one kind of adsorption site. The adsorbate layer is also seen to grow in the form of constant density islands. In gas phase CH[sub 4], both [nu][sub 3] and [nu][sub 4] are triply degenerate. A partial degeneracy breaking into doublets is observed in both the [nu][sub 3] and [nu][sub 4] infrared active vibrations when CH[sub 4] is adsorbed. Analysis of the [nu][sub 4] doublet reveals one transition dipole component perpendicular to the (100) face of NaCl and the others in the plane of the face. Since [nu][sub 3] transforms under the same representation as [nu][sub 4], the authors expect the transition dipoles of [nu][sub 3] to be aligned similarly to those of [nu][sub 4]. Isotopic dilution experiments of CH[sub 4] and CD[sub 4] show that some of the total [nu][sub 4] doublet separation can be attributed to resonant or dynamic adsorbate-adsorbate interactions. The remaining splitting cannot be specifically assigned to any particular interaction but must involve static adsorbate-adsorbate coupling and surface-adsorbate interactions. Plane group symmetry analysis reveals two possible adsorbate geometries that are compatible with the spectroscopy of CH[sub 4]: one in which two hydrogens rest on the surface (the dipod) and the other in which three hydrogens are against the surface (the tripod). A unique determination of the absorbate geometry is obtained from the statistical photometry of the Ch[sub 2]D[sub 2] data. Since several distinct molecular orientations of CH[sub 2]D[sub 2] can bond to the NaCl surface simultaneously, a theoretical model is developed to account for these geometric permutations in the monolayer. An analysis of the data and the proposed model conclusively establishes a tripod orientation of the monolayer methane molecules over the NaCl(100) surface.

• Research Organization: Indiana Univ. Foundation, Bloomington, IN (United States)
• OSTI ID: 6974014
• Country of Publication: United States
• Language: English

Related Subjects

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY
400201* -- Chemical & Physicochemical Properties
ADSORPTION
ALKALI METAL COMPOUNDS
ALKANES
CHEMICAL BONDS
CHLORIDES
CHLORINE COMPOUNDS
HALIDES
HALOGEN COMPOUNDS
HYDROCARBONS
METHANE
ORGANIC COMPOUNDS
SODIUM CHLORIDES
SODIUM COMPOUNDS
SORPTION
SORPTIVE PROPERTIES
SURFACE PROPERTIES