Cooperative two-phonon absorption in solid α-nitrogen in the 4600–4700 cm−1 region Original Research Article

The spectrum of pure α-nitrogen was recorded with a Fourier transform spectrometer in the near infrared. Two bands at 4657.3 and 4618.7 cm−1, previously studied at low resolution by Grundy et al. (Icarus 105 (1993) 254.), were recorded at 5 K at high resolution and analyzed. The effect of the temperature, impurities and crystal defects was investigated at lower resolution. The first band, due to a simultaneous excitation of two vibrons, exhibits a peculiar shape with several singularities. The vibron energy and the induced dipole moments have been calculated as a function of the crystal wave vector and used in the standard two-phonon theory. By changing somewhat these coefficients from 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 14N15N,N2 couples.