Infrared spectra of carbon monoxide adsorbed on evaporated films of calcium halides

Abstract

Infrared spectra of CO adsorbed at 77 K on evaporated films of CaF₂, CaCl₂, CaBr₂ and CaI₂ have been investigated. The system CO/CaCl₂ has been studied in the far-infrared region.

Two absorption bands have been found in each system. With CaF₂, the main band at 2156 cm^–1 has been assigned to molecules adsorbed on cations in the (111) face of the adsorbent. Heats of adsorption and spectral shifts have been calculated for this site. Fairly good agreement with experimental values has been found. These results and the reversible behaviour on desorption indicated that the adsorption is physical.

In the other three systems, the frequencies of the main band depended strongly on coverage and varied from 2185–2190 cm^–1 to ca. 2170 cm^–1 with the amounts adsorbed. The desorption was reversible, but needed a much higher temperature (ca. 190 K). No additional evidence for charge transfer or complex formation has been found and therefore physical, although relatively strong, adsorption has been assumed.

Assignments for the second band in each case have been suggested.