Spectroscopic study of the epsilon phase of solid oxygen - art. no. 104110

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 O-2 vibron mode and the br oad doubler in the overtone region, another peak is detected in the far inf rared region. The analysis of the overtone bands allows the determination o f the density of states of the O-2 vibron region which consists of two sepa rated energy regions, including one the infrared and the other the Raman ba nds observed in the 1500-1650 cm(-1) range. This result, consistent with th e analysis of the other Raman and infrared bands at lower frequency, is int erpreted on the basis of a crystal composed by a molecular unit formed by f our oxygen atoms. This hypothesis explains the strong infrared absorption w hich is in contrast with the model of a crystal composed by diatomic oxygen molecules. Very thin crystalline slabs (less than or equal to0.4 mum) allo wed 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 inciden t power and of the laser excitation frequency shows how the intensity and t he frequency of the Raman lines are affected by the experimental conditions . Finally, a simple chain model provides indirect proof of our assignment o f the low-frequency infrared mode and allows to rule out an association in polymeric units formed by mon than four atoms even at pressures close to th e insulator-metal transition.