Probing local structure of sub and supercritical CO 2 by using two-dimensional Raman correlation spectroscopy

Abstract

Using two-dimensional correlation spectroscopy and curve fitting procedure, we analyzed the hypothesis that each of the two Raman Fermi dyad modes of CO 2 may be resolved in two contributions. The contribution to the highest frequency side of each vibration dyad mode of CO 2 is associated with molecules experiencing a low local density and the contribution to the lowest frequency side is associated with molecules experiencing a high local density. The Raman spectra were recorded along the liquid–gas coexistence curve of CO 2 in the temperature range between 250 K and 303 K, and along the critical isochore between 306 K and 320 K. The two-dimensional synchronous and asynchronous contour maps obtained by processing the dynamic spectra revealed that these two contributions undergo band shift, band width changes and intensity variation. Particularly, it was shown that the temperature affects in an asymmetric way the high and low frequency sides of the Fermi dyad centered at 1385 cm − 1 . The Fermi dyads were also decomposed into two Lorentzian components. The effect of temperature on the position, the width and the intensity of each of these components was quantified. The results show a systematic red shift of the two Fermi dyads along the coexistence curve and almost no dependence along the critical isochore. More interestingly, in the case of the Fermi dyad centered at 1280 cm − 1 , the width of these two contributions increases along the coexistence curve and remains almost constant along the critical isochore. This behavior was traced back to the increase of the heterogeneous distributions of CO 2 molecules and indicates that there are more different environment of individual CO 2 molecules when approaching the critical temperature. Finally, our results demonstrate the complementarities between two-dimensional spectroscopy and curve fitting procedure in order to analyze the Raman Fermi dyads and to characterize the changes in the local structure of sub and supercritical CO 2.