Centers in the Tetragonal Azides

The thermal annealing of the 565 and 680 nm bands casts doubt on the correlation of the 575 nm absorption to the 1637 cm infrared absorption in KN3. The correlation rests on the similarity of the growth and annealing be- 

It is also possible that the 565 nm band is related to the NJ radical [69]. The 565 nm band thermally anneals in the temperature region in which the ESR of the N2 anneals. However, the kinetics of the decay of the ESR of the NJ remains uncertain and may be neither first nor second order. The ESR of the 

Low-temperature UV-irradiation of KN3 also produces an absorption at 360 nm. This band thermaUy anneals at a lower temperature than the 565 nm absorption and does not have annealing properties similar to any of the known paramagnetic defects. The 360 nm band displays some vibrational structure at low temperature, suggesting a molecular defect is involved. Like the 565 nm absorption, the 360 nm band is dichroic, with maximum absorption occurring when the incident light has its E vector perpendicular to the c axis of the tetragonal unit cell. There is insufficient evidence to assign a defect model for this absorption. 

Since it appears that some optical absorptions in KN3 are associated with the radicals N4 and possibly NJ, it is of interest to consider the electronic structure of the radicals in order to assign the specific transitions responsible for the optical absorptions. Molecular orbital calculations of the NJ in NaN3 indicated that the ordering of the states in the NJ radical is la, la, 2og, lou, Itt, htg [23]. The transition Itt to litg is allowed and could account for the 565 nm absorption. This transition should be dichroic, with maximum absorption occurring when the E vector of the incident light is parallel to the internuclear axis of the radical. Since the ESR studies of the N2 indicate the radical is parallel to [110], the litu to Ing transition is consistent with the observed dichroic properties of the 565 nm band [69]. 

Radiation-induced optical absorption bands have also been observed in nonalkali azides. Absorption spectra in Pb(N3)2 and TIN3 appear to be associated 

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