A PIR, HAS and LEED Study of the Monolayer CH_4-NaCl(001)

摘要

The adsorbate CH_4-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 v_4 mode of CH_4 into a polarization-dependent doublet absorption was observed in the FTIR spectra. HAS and SPA-LEED measurements reveal a (1x1) superstructure with one CH_4 molecule per NaCl ion pair. Calculations of the potential energy support these experimental results, yielding a perfect dipod arrangement of the CH_4 molecules in the (1x1) structure. Isosteric cooling of the adsorbate results in a two-dimensional phase transition of order > 1. By means of HAS a p (2 x 2) superstructure was found at T < 30 K, whereas SPA-LEED suggests a (2 x 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 (1x1) 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 CH_4 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.