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

Abstract

The adsorbate CH4‐NaCl(001) at monolayer coverage has been studied by means of helium atom scattering (HAS), electron diffraction (SPA‐LEED) and polarization Fourier‐transform infrared spectroscopy (PIR) in the temperature range between 45 K and 10 K. Above 40 K a splitting of the v4 mode of CH4 into a polarization‐dependent doublet absorption was observed in the FTIR spectra. HAS and SPA‐LEED measurements reveal a (1×1) superstructure with one CH4 molecule per NaCl ion pair. Calculations of the potential energy support these experimental results, yielding a perfect dipod arrangement of the CH4 molecules in the (1×1) structure. Isosteric cooling of the adsorbate results in a two‐dimensional phase transition of order > 1. By means of HAS a p(2×2) superstructure was found at T < 30 K, whereas SPA‐LEED suggests a (2×1) symmetry. Beside of a remarkable sharpening of all IR absorptions two additional features are observed. The phase transition is reversible. Two to three hours after preparation both diffraction methods yield the (1×1) structure even at lowest temperatures. This time behaviour may be indicative of a quantum effect, namely nuclear spin conversion, in the adsorbed methane layer. Knowledge of the interaction between CH4 and NaCl is necessary to predict the gas migration and therewith the safety aspects for a final repository of nuclear waste in permian salt domes.

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 .…”

“…8,12 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. [19][20][21] 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.…”

“…Helium atom scattering (HAS) experiments have been useful in determining the unit mesh structure. 21 Above 30 K a 1 × 1 unit cell was identified while below this temperature a 2 × 2 structure was favored. However, HAS is insensitive to hydrogen position and, consequently, to adsorbate conformation.…”