COOPERATIVE 2-PHONON ABSORPTION IN SOLID ALPHA-NITROGEN IN THE 4600-4700CM(-1) REGION

The spectrum of pure alpha-nitrogen was recorded with a Fourier transf orm 1 spectrometer in the near infrared. Two bands at 4657.3 and 4618. 7 cm(-1), previously studied at low resolution by Grundy et al. (Icaru s 105 (1993) 254.), were recorded at 5 K at high resolution and analyz ed. The effect of the temperature, impurities and crystal defects was investigated at lower resolution. The first band, due to a simultaneou s excitation of two vibrons, exhibits a peculiar shape with several si ngularities. The vibron energy and the induced dipole moments have bee n calculated as a function of the crystal wave vector and used in the standard two-phonon theory. By changing somewhat these coefficients fr om their ab initio calculated values, the shape and the intensity are fairly well reproduced. The second band, narrower and much weaker, is due to the simultaneous excitation of the molecules in the N-14 N-15,N -2 couples.