Coherent anti-Stokes Raman scattering spectroscopy of solid nitrogen to 22GPa

The Raman-active vibron modes of solid nitrogen have been investigated by c oherent anti-Stokes Raman scattering (CARS) spectroscopy to 22 GPa at room temperature. Frequencies and linewidths were measured with an accuracy of 0 .1 to 0.2 cm(-1). From the pressure dependence of the linewidths a dynamica l model for the transitions between the delta, delta (loc), and epsilon pha ses has been developed. These phase transitions are characterized by differ ent degrees of ordering of the N-2 molecules. The processes can be describe d by an increase in the orientational order with increasing pressure and a decrease in number in the rotational degrees of freedom at the phase transi tions coupled with changes in crystal structure. A structural model for the delta (loc) phase is given, in which the delta-delta (loc)-epsilon transit ion sequence arises from a group/subgroup relationship and can therefore be considered ferroelastic in nature. Sample annealing was found to have a si gnificant effect on the results. (C) 2001 American Institute of Physics.