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Single crystals of anhydrous barium azide have been irradiated at room temperature with 2537 Å ultraviolet light. Upon cooling to 77°K, an ESR spectrum is observed, the angular variation of which is expressed by the spin-Hamiltonian H = βH·g·S + I·A·S with gx≈gy = 1.997 ± 0.001, gz = 1.979 ± 0.0001, Ax = 3.6 ± 0.2 G, Ay = 20.0 ± 0.2 G (see text), Az = 4.1 ± 0.2 G. The spectrum is interpreted in terms of the theoretical predictions of Gelerinter and Silsbee for an N2− molecular ion in a field of rhombic symmetry. The ESR data are used to speculate on the orientation of the azide ions in the Ba(N3)2 lattice, the structure of which is as yet unknown. The optical absorption spectrum of thin single crystals in the range 200-300 mμ has been measured at room temperature and is correlated with the wavelength threshold for production of the paramagnetic center. It is postulated that the initial step in production of the center is the creation of an excited azide ion by the ultraviolet light.

Earlier work based on specular reflectance measurements on single crystals of the cyclic polynitramine known as RDX indicated the presence of an absorption band near 340 nm. Since a similar band was not observed in dilute solution, the absorption was attributed to the formation of a charge-transfer self-complex in the crystalline state. This work compares measurements of the transmittance of thin single crystals and new specular reflectance measurements to absorption spectra of saturated solutions of the polynitramines known as RDX and HMX. The results are found to support the charge-transfer self-complex hypothesis.

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