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

The Raman-active vibron modes of solid nitrogen have been investigated by coherent 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 dynamical model for the transitions between the delta, delta (loc), and epsilon phases has been developed. These phase transitions are characterized by different degrees of ordering of the N(2) molecules. The processes can be described by an increase in the orientational order with increasing pressure and a decrease in number in the rotational degrees of freedom at the phase transitions coupled with changes in crystal structure. A structural model for the delta (loc) phase is given, in which the delta-delta (loc)-epsilon transition sequence arises from a group/subgroup relationship and can therefore be considered ferroelastic in nature. Sample annealing was found to have a significant effect on the results. (C) 2001 American Institute of Physics.