Spectroscopic study of the epsilon phase of solid oxygen

The infrared spectrum of the epsilon phase of solid oxygen has been studied between room temperature and 20 K as a function of pressure up to 63 GPa. Besides the strong absorption in the fundamental O2 vibron mode and the broad doublet in the overtone region, another peak is detected in the far infrared region. The analysis of the overtone bands allows the determination of the density of states of the O2 vibron region which consists of two separated energy regions, including one the infrared and the other the Raman bands observed in the 1500-1650 cm-1 range. This result, consistent with the analysis of the other Raman and infrared bands at lower frequency, is interpreted on the basis of a crystal composed by a molecular unit formed by four oxygen atoms. This hypothesis explains the strong infrared absorption which is in contrast with the model of a crystal composed by diatomic oxygen molecules. Very thin crystalline slabs (about 0.4 um) allowed to measure the intensity of the strong infrared absorption at 1500-1550 cm-1. The measurement of the Raman spectrum as a function of the incident power and of the laser excitation frequency shows how the intensity and the frequency of the Raman lines are affected by the experimental conditions. Finally, a simple chain model provides indirect proof of our assignment of the low-frequency infrared mode and allows to rule out an association in polymeric units formed by more than four atoms even at pressures close to the insulator-metal transition.