The analysis of the far infrared spectrum of thin high quality alpha Y-N-2 Single crystals as a function of temperature was carried out by high resolution Fourier-transform infrared spectroscopy. The bandwidths at T=8 K of the two translational modes, correlated to the two-phonon density of states, are considerably smaller than the Raman active modes, This result can be interpreted on the basis of a lower anharmonicity of the translations with respect to the librational modes. The alpha-N-2 is the first molecular crystal where such difference is found. The evolution of the T-u((1)) linewidth was studied until the alpha-beta phase transition. The relaxation of this mode is essentially dominated by fourth-order decay processes. The study of the frequency shifts of both infrared phonons allowed the contributions relative to different orders of the anharmonic potential to be separated. Finally a careful analysis of the relative intensities of the two phonons was carried out in the alpha phase. (C) 1996 American Institute of Physics.