Centers in Sodium Azide

The assignment of the 610 nm absorption was based on the apphcation of continuum models of the F and Fj center [18]. In the continuum model of the F center the electron is bound to the vacancy by a potential of the form e jKr, where K is an effective dielectric constant for the material. The continuum model of the F2 center is based on the analogous assumption that the transitions of the Fj center can be interpreted in terms of transitions of a hydrogen molecular ion immersed in a dielectric medium. The energy levels of the center in this model are the eigenvalues of the Hamiltonian 

Filgure 6. Energy levels and allowed transitions of the F center in C2V symmetry. 

When sodium azide is irradiated with UV light at 298°K, no appreciable optical absorption is detected at wavelengths greater than 500 nm. However, if after this irradiation the crystal is heated to 600°K, a new broad optical absorption with a maximum around 520 nm is produced [26,67]. The band has been attributed to colloidal sodium. The insensitivity of the optical absorption to temperature supports the assignment of the band to colloids. Neither the position of the maxima or the band-width is affected by temperature. The identification is supported by the characteristic electron spin resonance of colloids [26]. The colloidal particle diameter was estimated to be 15 A [26]. 

To summarize, in sodium azide there is experimental and theoretical evidence to associate the 730-nm optical absorption with an F center. The association of colloidal particles with the 520 nm band produced by irradiation and subsequent heating is plausible. An F2 center and a bent triatomic radical have been offered to account for the 610-nm absorption band. The Fj center is less tenable because one would not expect an infrared absorption to be associated 

The bent triatomic radical as a possible model is less than definitive because the assumptions in the analysis are arbitrary. It should be shown that no other molecular radical predicts the shift of the 1721 cm absorption in the isotopically labeled NaNa. Clearly, further theoretical and experimental work is needed before the defect giving rise to the 610 nm band is definitively assigned. Table I contains a summary of irradiation-produced bands in NaNa. 

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The only defects found in the azides which have counterparts in the alkali halides are the F and FJ centers (the F center consists of an electron in an anion vacancy the FJ center is an electron occupying two adjacent anion vacancies). ESR of the F center was observed by Carlson et al. [17] and by King et al. [18] in sodium azide which had been UV-irradiated at 77°K. The observed spectrum consists of 19 hyperfine lines due to the interaction of an electron trapped in an azide vacancy with the nuclear spins (/ = 3/2) of the six nearest-neighbor sodium ions. The ESR signal is correlated with an optical absorption band by thermal and optical bleaching (see below). Bartram et al. [19] have performed calculations of the wave-functions for the F center in sodium azide. Their predictions of the expected hyperfine structure and optical absorptions are in good agreement with experiment. 

Recently, wave functions for an F center in sodium azide have been calculated using the point-ion model, and the electronic structure of the center has been elucidated [19]. In the monochnic phase the point symmetry at the anion site is C2h- There are two allowed transitions of the F center in this symmetry (corresponding to to A and A" to B transition). The calculation predicts that the bands due to these transitions should occur in the area of 730 nm and that the two bands would be very close, perhaps experimentally unresolvable. The calculation also predicts an absorption in the near-infrared due to the A" to B" transition. The energy levels of the F center in C2h symmetry of NaNa are depicted in Figure 6. 

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