Two‐Dimensional Phase Transitions and Correlation Field Splitting in Unimolecular Layers of CO Physisorbed on NaCI(100) as Detected by Polarization Fourier‐Transform IR Spectroscopy

Abstract

A phase transition in the monolayer C O physisorbed on NaCl(100) was observed by means of polarization Fourier-transform infrared spectroscopy. Above 35 K the spectra are in concert with molecules oriented perpendicularly to the surface, the unit cell being of p(1 x 1) symmetry. Below 35 K the molecules tilt, and a correlation field splitting appears. In this temperature range the spectra are in accordance with a p(2 x 1) structure with two equivalent molecules per unit cell, which are connected by a glide plane. Submonolayer spectra indicate random distribution of perpendicularly oriented C O molecules above 35 K, while at temperatures below 35 K ordered islands are formed.

Linkage Isomerism of CO Adsorbed on Alkali Halides

“…The obtained results are mostly in agreement with earlier reported data for CO adsorption on alkali halides. − Band positions of adsorbed molecules and their shifts with coverage changes are close to those from the literature.…”

“…From the isotherms of adsorption, the isosteric heat of adsorption was calculated. At half-monolayer coverage, it was found to be 17.1 kJ/mol . For the film, however, the value of 13 ± 3 kJ/mol at 83–99 K was obtained …”

“…At half-monolayer coverage, it was found to be 17.1 kJ/mol. 18 For the film, however, the value of 13 ± 3 kJ/mol at 83−99 K was obtained. 14 The band position of adsorbed CO depends on the surface coverage.…”

“…This assignment was supported by comparison with the spectra of CO adsorbed on single crystals, where an extremely narrow band at 2155 cm −1 was detected 16 with the half-width of 0.08 cm −1 at 6 K. 17 Measurements in polarized light have shown that the CO molecules in the first adsorption layer are perpendicular to the surface, while those in the next layers, which account for the band at 2138 cm −1 , are rather parallel or randomly oriented. 18 From the isotherms of adsorption, the isosteric heat of adsorption was calculated. At half-monolayer coverage, it was found to be 17.1 kJ/mol.…”