Infrared spectra and structure of thin films of solid oxygen: A metastable phase

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Abstract
The infrared absorption spectra of thin films of solid oxygen prepared by vapor deposition is examined in detail. It is shown that at deposition temperatures between 8 and 11 K the film consists of a mixture of α-O2 and a metastable phase we call m-O2. The spectrum of CO in an O2 matrix is shown to be a good marker for the presence of m-O2 and is informative concerning the structure of this metastable phase. Studies of mixtures of 18O2 and 16O2 indicate that the structure of m-O2 is complex with four nonequivalent sites for O2 molecules and at least eight molecules per unit cell. The isotopic studies suggest the presence of weakly bound dimers as well as tetramers or higher multimers in m-O2. It is shown that the presence of a few percent impurities such as nitrogen or argon allow the formation of and increase the stability of m-O2 at higher temperatures. However, with or without impurities, at deposition temperatures below 8 K less m-O2 is formed, and deposition at 5 K leads to no m-O2 but only poorly crystallized α-O2. It is concluded that m-O2 is formed by epitaxial growth from a 2D structure similar to that observed by others for 1–4 monolayers of O2 on grafoil. The spectrum of annealed α-O2 shows phonon combinations with the fundamental O–O stretch indicative of a phonon density of states from 30–90 cm−1.

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