Modes in Alkali Azides

Color centers can be produced in the alkali metal azide by ultraviolet light and ionizing radiation at low temperatures. The phenomenon has been of interest for some time since the defects produced are involved in the process of photochemical decomposition (cf. Chapter 7). In earlier studies [54a, b, c] purely speculative identifications of optical absorption bands with F, V, and aggregate F centers were made by analogy with the alkali halides. The most prominent visible absorption band in each case was attributed to the F center—a defect involving an electron trapped at an azide (N3) vacancy. In the case of NaNa, spin resonance [55] and recent point ion calculations [56] clearly point to the existence of a F center. However, in the case of KN3, spin-resonance studies [54a] point to the existence of molecular centers of type N2 (on low-temperature irradiation) and NJ (on room-temperature irradiation). Infrared absorptions [57] and Raman scattering [58] have been observed in the irradiated alkali azides, which can be correlated with modes associated with these defects. 

In NaNa irradiated at 77°K, an infrared peak at 1721 cm appears which bleaches out at ambient temperatures. Experiments with Na NNN and NaN NN, performed by Bryant [59], show isotopic shifts and splittings which 

In KN3, RbN3, and CSN3 irradiated at 77°K, infrared peaks at 1637,1623, and 1613 cm , respectively, have been observed. More recently polarized infrared measurements in KN3 have indicated that the peak at 1637 cm is polarized parallel to the (001) layer plane of the azide ions [60b]. Furthermore, splittings have been observed of the infrared peak on warming up the crystals, and this has been interpreted to correspond to the relaxation of a center to that of C2V symmetry [60a]. 

In conclusion, infrared absorption studies on irradiated alkah azides indicate the presence of a triatomic defect of D3J symmetry at 77°K which could be either a trapped N3 excitation or a N3 center. Raman scattering experiments with 6328 A excitation show a resonance-enhanced spectrum for a crystal of KN3 irradiated at 77°K, with a fundamental peak at 307 cm . This peak can be related to a local mode associated with the NJ defect. 

Until the early 1960s no direct observation of the fundamental external vibrations of metal azides had been made. Some tentative analyses of infrared-active, intemal-external-mode combination bands had been proposed however, the Raman studies of Bryant and coworkers [42,44,61] provide the first direct observations and assignments of lattice modes in these compounds. 

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