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

Abstract

At temperatures between 100 and 110 K, exposing the ͑100͒ face of NaCl to unsaturated SO 2 gas yields a stable adlayer. Infrared spectra of adsorbed SO 2 contain complex resonances near the origins of the molecular symmetric and asymmetric stretching vibrations. On photometric grounds the absolute coverage of the surface is found to be one molecule per exposed Na ϩ Cl Ϫ ion pair. The spectra of this monolayer consist of several sharp lines overlapping one broad feature for each molecular mode. By comparison to vibrational excitons in simpler systems, the coexistence of crystalline and amorphous adlayer structures is strongly indicated. This partial ordering is pressure dependent, and develops spontaneously on a time scale of minutes. The disordered component, in contrast, is never in equilibrium with the gas phase. Computational simulations have detailed the microscopic basis of this behavior. Accurate ab initio models of the SO 2 molecule and NaCl͑100͒ surface were used in a Monte Carlo simulation of the experimental conditions. At both half and full coverage, an amorphous two-dimensional condensate developed. This is minimally consistent with the polarized infrared photometry. Seemingly equilibrated Monte Carlo runs retained some memory of the initial molecular configuration, again consistent with hysteresis observed in the spectroscopic experiments. No structural order developed in the simulations, but the energetic state of affairs was clarified: the potential driving adsorption and condensation is deep in comparison to available thermal energy, but relatively insensitive to molecular orientation.

Heterogeneous oxidation of sulfur dioxide by ozone on the surface of sodium chloride and its mixtures with other components

“…The result shows that the molar ratio of Br − /Cl − is at an upper limit of 0.0015, thus the Br − impurity content of the NaCl sample used in this study is acceptable. Berg et al [1996] found SO 2 adsorbed on NaCl(100) surfaces as monolayer. Thus, in this experiment, SO 2 might react with the adsorptive water in the quasi liquid layer to form species containing S–O groups, resulting in the four IR bands that do not disappear after the sample was evacuated, as shown in Figure 1.…”

Accurate Intermolecular Potentials Obtained from Molecular Wave Functions: Bridging the Gap between Quantum Chemistry and Molecular Simulations

“…The first potentials constructed using IMPT recently have been developed for the adsorption of small molecules on crystal surfaces. H 2 O adsorption on NaCl and MgO surfaces 195 has been studied, as has CO adsorption on NaCl , and SO 2 adsorption on NaCl . The methodology for constructing the water−surface potentials will be briefly discussed, since it differs in part from the derivation of potentials between two isolated molecules.…”

“…In a combined experimental and theoretical study Berg et al . investigated the adsorption of SO 2 on NaCl at cryogenic temperatures. Accurate potentials were used in MC simulations at coverages of 0.5 and 1.0 SO 2 molecule per NaCl unit.…”

“…H 2 O adsorption on NaCl 196 and MgO surfaces 195 has been studied, as has CO adsorption on NaCl 197,198 and SO 2 adsorption on NaCl. 199 The methodology for constructing the water-surface potentials will be briefly discussed, since it differs in part from the derivation of potentials between two isolated molecules. The exchangerepulsion parameters for the cation-water interaction were calculated as the difference between the first-order interaction energy in IMPT and the classical electrostatic energy between the cation and the multicenter multipole expansion for water.…”

“…This is compatible with experimental findings at 35 K that showed a phase transition, from a p(2X1) structure to a p(1X1) structure. 207 In a combined experimental and theoretical study Berg et al 199 investigated the adsorption of SO 2 on NaCl at cryogenic temperatures. Accurate potentials were used in MC simulations at coverages of 0.5 and 1.0 SO 2 molecule per NaCl unit.…”