Interaction induced Raman spectroscopy of the dipole forbidden pgr;umode in neat CO2and solutions with N2as supercritical fluids

摘要

The interaction induced Raman spectra (IIRS) of 1:0, 1:1, and 1:3 CO2:N2solutions have been studied at 50thinsp;deg;C and 75thinsp;deg;C at densities up to 2.5 times the triple point density (90ndash;3010 bars).M0,M2, andM4evaluations have been made upon the pgr;ungr;2Raman 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 ngr;2and is identified with a CO2oscillating on a large cluster of CO2; this perturbs the isolated ngr;2frequency; 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 75thinsp;deg;C at near solid densities of the fluid. The nature of the ngr;2signal shows that there is a significant difference between the neat and N2ndash;CO2solutions. In particular, depolarization measurements upon ngr;2in the 1:0 and 1:3 solutions at 50thinsp;deg;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 30ndash;60 femtosecond regime, we believe that such fast times can be related to the behavior of individual CO2moieties. Last, this IIRS (lsquo;lsquo;earsrsquo;rsquo;) 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 quadrupolendash;inducedhyphen;dipole (QID) allowed ngr;2signal.