Physical aspects of matrix isolation technique: FTIR studies on CO and CO2 in O2 and N2 matrices

Abstract

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.

Elementary excitations in condensed oxygen (α,β,γ, liquid) by high-resolution Raman scattering

“…3 In the meantime the character of the ␣-␤ phase transition was unambiguously assigned to be of first order by Fourier transform infra-red ͑FTIR͒ spectroscopy 6 and by subsequent x-ray studies, 7 but not by Raman scattering.…”

From optical spectra to phase diagrams—the binary mixture N2–CO

“…As a result, we obtained big polycrystalline samples of the low-temperature phases of an excellent optical quality. We measured sample temperature, in the range of 10-80 K, by a calibrated silicon diode, which was directly attached to the sample cell, absolute accuracy of temperature registration was about 0.1 K. This cell with sample chamber (AE 10 mm; three different thicknesses like 1, 5, and 20 mm) equipped with sapphire windows, was mounted on a cold finger of a closed-cycle He cryostat (see for details [7]). Spectra were recorded in this mid IR region by a Fourier spectrometer (Bruker IFS120 HR) both on cooling and warming cycles.…”

“…The amount of CO 2 molecules in the initial gas (N 2 , CO, or mixtures) is about ppm, due to these gas delivering companies. In a series of papers [7,10] we described in detail this probing technique (M/A ratio 10 -6 , M -matrix, A -impurity; typical M/A~10 -3 for standard matrix-isolation technique). In there we explain how we determined the residual amount of CO 2 molecules via optical spectroscopy in combination with other techniques.…”

“…This range DT of phasecoexistence in mixtures should be compared with the thermal hysteresis in pure systems: T ab (warming) -T ba (cooling) is about £ 0.2 K for pure N 2 and £ 0.4 K for pure O 2 . This statement is based on IR spectra of ppm CO in these pure systems [7]. Since we used a calibrated Si-diode to measure sample temperature, the uncertainties in temperature is DT £ 0.1 K in that range.…”

Orientational order in the molecular solidsN2, CO, andO2investigated via combined excitations with FTIR

“…We used the brass cells described in Ref. 10. To investigate the combined modes in the range 2000-6000 cm −1 , we used various windows: diamond windows ͑aperture 3 mm, sample thickness 1.1 mm͒, quartz windows ͑aperture 10 mm, sample thickness 6.7 mm͒, and sapphire windows ͑aperture 10 mm, sample thickness 5.5 or 20.5 mm͒.…”