Infrared spectroscopy of monolayer CH4 on NaCl(100)

Abstract

Polarized infrared spectra of CH4 adsorbed on NaCl(100) in the temperature range 4–47 K 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. A partial degeneracy breaking into doublets is observed in both the ν3 and ν4 infrared-active vibrations. Analysis of the ν4 doublet reveals one transition dipole component perpendicular to the (100) face of NaCl and the others in the plane of the face. Isotopic dilution experiments of CH4 in CD4 show that some of the ν4 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. A structural analysis consistent with the spectroscopic data and the symmetry of slab groups is developed. Two possible adsorbate geometries are considered. A unique determination of the adsorbate geometry will have to await other surface measurement techniques.

A PIR, HAS and LEED Study of the Monolayer CH4‐NaCl(001)

“…Since for a single molecule in each unit cell no coupling except for that of the unit cells is expected, the local orientation of the adsorbed molecules can be directly deduced from the PIR spectra. As already pointed out by Quattrocci and Ewing [13] the doublet absorption in the p-polarized v4 spectra results from a site symmetry splitting. The two signals are attributed to a twofold degenerate and a nondegenerate species.…”

“…The remarkable narrowing and frequency shift of all absorptions has been modelled successfully by means of a vibrational dephasing mechanism, where any structural change is neglectable [13].…”

An amorphous monolayer: Infrared spectroscopic and theoretical studies of SO2 on NaCl (100)

“…The corresponding resonance would appear in E P spectra only, not in E s . [1][2][3][4][5] As we see in Fig. 3, all features appearing in E P polarization for the 1 region also have absorption in E s polarization.…”

Orientational Isomers and Monolayer Structure of CH3D Physisorbed on NaCl(100)

“… In this equation, N is the number of surfaces available for adsorption ( N = 4 in our experiments), S is the surface density of Na + Cl - pairs ( S = 6.4 × 10 14 molecules/cm 2 ), and σ g is the integrated absorption cross section of the adsorbate. Here, as elsewhere, − the adsorbed CH 4 molecules are taken to be over Na + sites, so S may be read as the molecular density of the monolayer. The trigonometric terms in θ account of the tilt of the NaCl(100) surfaces (and the adsorbed monolayer) with respect to the interrogating light.…”

“…Various studies have preferred a tripod conformation (three hydrogens nearest to the surface) or a dipod conformation (two hydrogens nearest to the surface) for physisorbed methane. It has been proposed that, in general, methane will physisorb in a tripod conformation when the methane−substrate interactions dominate methane-methane interactions, while a dipod conformation is favored in the opposite case. , Current data suggest that methane dipods are favored on an MgO(100) substrate , while tripod methane is preferred on the basal plane of graphite. , Polarized Fourier transform infrared (FTIR) spectroscopic studies of CH 4 physisorbed on NaCl(100) support a symmetric adsorption conformation (i.e., tripod or dipod) but no conclusive preference for either. − On the basis of theoretical arguments, these studies have placed CH 4 on top of Na + sites. Helium atom scattering (HAS) experiments have been useful in determining the unit mesh structure .…”