The structural determination of monolayer CH2D2 adsorbed on NaCl(100) by infrared spectroscopy

Quattrocci, Laura M.; Ewing, George E.

doi:10.1016/0009-2614(92)85774-5

Cited by 7 publications

(4 citation statements)

References 19 publications

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“… 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.…”

Section: Discussionmentioning

confidence: 99%

“…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 .…”

Section: Introductionmentioning

confidence: 99%

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

Davis

Ewing

1999

J. Phys. Chem. B

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Polarized infrared spectra of CH3D adsorbed on NaCl(100) in the temperature range 5−37 K are reported. Analysis of the ν2 spectral region reveals one transition dipole perpendicular to the surface and two oriented at 70 ± 20° (or 110 ± 20°) from the surface normal. These features are assigned to C−D stretching vibrations of molecules oriented in a tripod conformation with the deuterium in the “top” position (D-up) and in a tripod with the deuterium in a “base” position (D-down). The fraction of D-up orientational isomers is shown to decrease with decreasing temperature. An energy splitting of 100 ± 20 J mol-1 between D-up and D-down orientational isomers is consistent with this temperature dependence. A phase transition near 30 K associated with changes in rotatory motions has been observed. Below this transition, a splitting in the D-down region is shown to be statistically consistent with the position of the deuterium atoms on the surface. Some point toward substrate chloride ions and others point toward ‘gaps' between chloride ions.

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Section: Discussionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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

Davis

Ewing

1999

J. Phys. Chem. B

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“…For the tripod arrangement a cogwheeling motion of the adlayer is discussed. Due to recent measurements of the polarization dependency of CHzD z on NaCI(OOI) Quattrocci and Ewing favour the tripod arrangement [14].…”

Section: Discussionmentioning

confidence: 99%

“…The polarization dependency of the v4 absorption was discussed for two possible monolayer structures: the tripod arrangement with three hydrogen atoms directed towards the surface and one up, and the dipod configuration with two hydrogen atoms up and two down. For CHzD z at monolayer coverage on NaCI(OOI) Quattrocci and Ewing favour the tripod orientation [14].…”

Section: Introductionmentioning

confidence: 98%

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

Heidberg

Schönekäs

Weiß

et al. 1995

Ber Bunsenges Phys Chem

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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.

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