The matrix isolation technique is traditionally used to investigate the properties of the matrix-isolated species themselves or to solve some special questions of the theory of defects in solids. We showed here that the optical spectroscopy of real matrix-isolated molecules can be successfully used to investigate the host crystal qualities, too. We demonstrated the capacity of modern FTIR spectroscopy to study the properties of cryocrystals such as phase transitions, solubility boundaries, orientational order parameter, etc., by monitoring the behavior of the IR-active molecules, which are present in matrices under investigation as a natural contamination (40 ppb). Due to the excellent optical quality of our crystal samples, we were able to determine a part of the binary phase diagram CO–O2 (at CO concentrations less than 1 ppm) as well as to investigate the kinetics of phase transitions. Furthermore, we successfully used the spectroscopy of the matrix-isolated molecules to proof that the α-β phase transition of the matrix crystal (O2) is of first order.
We produced mixtures of N2-O2 with different concentrations and performed low-temperature Raman studies at ambient and high pressures. From spectra in vibron and phonon regions, we determined band frequency, bandwidth, and band intensity as a function of temperature, pressure, and concentration. We determined the vibron Raman cross-sections and deduced the true concentrations of mixtures from vibron Raman band intensities. These concentrations were different from those determined from partial gas pressure of the initial gaseous mixtures. From fingerprints in Raman spectra, such as jumps in band frequencies or additional band splitting, we were able to prove phase transitions and propose a preliminary T-x phase diagram. We compared this diagram with two reported in the literature from structural analysis. Comparing all three variants of the T-x phase diagrams we found several discrepancies and inconsistencies, which we associate with different solid sample production techniques. Since we could prove that our samples were in thermodynamic equilibrium, we are convinced that we improved the known phase diagram substantially. From Raman band intensities of the O2 vibrations in different phases of N2 and O2, we were able to determine quantitatively the solubility of O2 in N2. Preliminary Raman studies of 2% and 7% O2 in N2 at high pressure and low temperatures showed that a larger amount of O2 can be dissolved in N2 than at ambient pressure. At the critical pressure (p approximately 15 GPa) we found from Raman spectra that O2 is demixed from 7% O2 in N2 to form epsilon-O2. This was previously called a "new phase" in the literature and not understood up to now. Finally, from band frequencies we determined the environmental shift of oxygen molecules in the mixture which is related to the intermolecular potential U(N2-O2) between different types of molecules.
Solid solutions (N 2) x (O 2) 1-x have been investigated by infrared absorption measurements mainly in the O 2 and N 2 stretching regions, between 60-10 K, completing former similar studies by Raman scattering. We produced thermodynamically stable samples by a careful thermal treatment, followed by cooling/heating cycles over weeks, during which we took spectra. From fingerprints in infrared spectra we deduce phase transition lines, solubility lines and suggest a refined, improved T-x% phase diagram with respect to inconsistencies between those in literature. Spectra of N 2-O 2 mixtures are pretty complex but referring to known spectra of pure systems N 2 or O 2 we were able to assign and interpret broad (~100 cm-1) phonon side bands to fundamentals and electronic transition (O 2) depending on actual temperature and concentration. Narrow features in spectra (<10 cm-1) were attributed to the vibron DOS of N 2 or O 2 , whose bandwidth, band shape and intensity are different and characteristic for each phase. Differences between pure and mixed systems were pointed out. Matrix isolation technique (2 ppm of CO) was used to probe our mixture.
IR and Raman spectra D 6530 Raman Investigation of the N 2 -O 2 Binary System as a Function of Pressure and Temperature. -(MINENKO, M.; KREUTZ*, J.; HUPPRICH, T.; JODL, H.-J.; J.
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