Raman Studies on Solid CO₂ and N₂ with Excess He at Pressures up to 40 GPa

Raman spectra of CO₂ and the van-der-Waals compound He(N₂)¹¹ were obtained under nearly hydrostatic pressures up to 40 GPa. The pressure induced changes of the librational spectra suggest that solid CO₂ does not transform directly from phase-I (cubic Pa3) to phase-Ill (orthorhombic Cmca). An intermediate phase-IV. which may involve only minor modifications of phase-I, is formed prior to the transition to Cmca structure Phases IV and III coexist over some pressure range. The pressure shifts of all four Raman active librons of phase-Ill were determined over a wide pressure range and are compared to previous theoretical results. Spectra of external and internal modes of the van-der-Waals compound He(N₂)¹¹ are very similar to those of ε-N₂ and imply a close relationship between the structures of He(N₂)¹¹ and ε-N₂. The observed splitting of the main vibron of isotopic species ¹⁴N-¹⁵N indicates that in addition to factor-group interactions site effects are responsible for the observed splittings of the ¹⁴N₂ main vibron of He(N₂)¹¹.