High pressure and low temperature infrared study of solid oxygen: Evidence of a new crystal structure.

Infrared spectroscopy has been applied to the study of solid oxygen at high pressure and low temperature. An absorption line in the vibron fundamental mode region has been observed, and its behavior with pressure measured up to the transition to the epsilon phase. This absorption, not expected according to the symmetry of the delta phase, indicates the existence of a new crystal structure between 2 and 8 GPa at low temperature. The comparison of the infrared and Raman frequency evolution with pressure is done quantitatively on the basis of a simple vibrational coupling model and the intermolecular coupling force constant is obtained. In contrast to solid hydrogen, here the coupling constant is positive and shows a strong dependence on the intermolecular distance which can indicate that significant charge transfer processes are already present in this low-pressure phase.