Interaction induced Raman spectroscopy of the dipole forbidden πu mode in neat CO2 and solutions with N2 as supercritical fluids

The interaction induced Raman spectra (IIRS) of 1:0, 1:1, and 1:3 CO2 :N2 solutions have been studied at 50 °C and 75 °C at densities up to 2.5 times the triple point density (90–3010 bars). M0, M2 , and M4 evaluations have been made upon the πu ν2 Raman dipole forbidden signal. In addition, the absolute Raman scattering cross sections have been evaluated for the 1:0 and 1:1 solutions. A broad low intensity high frequency shoulder was seen in the 1:0 data of ν2 and is identified with a CO2 oscillating on a large cluster of CO2 ; this perturbs the isolated ν2 frequency; the shoulder is not found in the 1:1 or 1:3 solutions. Significant many body cancellation has been observed in the 1:0 data at 50 and 75 °C at near solid densities of the fluid. The nature of the ν2 signal shows that there is a significant difference between the neat and N2 –CO2 solutions. In particular, depolarization measurements upon ν2 in the 1:0 and 1:3 solutions at 50 °C and 1255 bars of pressure showed very different behavior. This would seem to indicate quite a different local field environment, i.e., local structures. All Raman intensity correlation functions show times in the 30–60 femtosecond regime, we believe that such fast times can be related to the behavior of individual CO2 moieties. Last, this IIRS (‘‘ears’’) study is consistent with the earlier findings of Amos, Buckingham, and Williams that angle averaged pair entities are the principal intermolecular scattering species and must be present to give rise to the quadrupole–induced-dipole (QID) allowed ν2 signal.