Impurity induced Raman scattering in alkali halides

Raman scattering has been observed in KBR and KI single crystals doped with hydrogen and deuterium U centers, and NaCl containing silver and copper impurities. An argon ion laser, tandem double monochromator, and photoelectric detection were used for the measurements. Raman scattering from the second hamonics of the high frequency U center localized mode has been measured in KBR:D-, KBR:H-, KI:H-, and KI:D- at 10 K. Lines of A1g, Eg, and T2g symmetry are observed. The peak positions and symmetries are in agreement with the static lattice for the local mode. The anharmonic coupling coefficients needed to calculate the localized oscillator energy levels have been determined from the experiemental data and used to describe the short range repulsive potential in the neighborhood of the U center. The importance of polarization effects in the interaction of the impurity with its surroundings is demonstrated. A weak peak of A1g symmetry has been observed at about 95 cm-1 in KI:H-. It is attributed to the gap mode previously seen in infrared sideband measurements. Raman scattering in NaCl:Ag+ is primarily due to defect activated host crystal phonons. Excellent agreement is obtained between the experimental Raman frequencies and peak positions in the unperturbed, projected densities of states for modes of A1g, Eg and T2g symmetry. In addition, a very strong, broad peak of Eg symmetry appears in a spectrum. It has a frequency of about 86 cm-1 at room temperature and 99.5cm-1 at 7 K. This peak is interpreted as an incipient resonance mode of the neighbors of the silver impurity. In NaCl:Cu+ resonent modes of Eg and A1g symmetry have been observed at 38.6cm-1 and 46.5cm-1 respectively. Their peak positions are temperature independent within experimental error, but their linewidths increase sharply with increasing temperature, indicating that anharmonic effects are important in this system. It appears that anharmonic coupling between the 23.7cm-1 infrared active resonant mode and the Raman active resonances is particularly important. The resonant mode frequencies in NaCl:Ag+ and NaCl:Cu+ demonstrate the inadequacy of existing lattice dynamical models for the interaction of the silver and copper defects with the NaCl lattice. it is suggested that the forces between the nearest and fourth nearest neightbors of the impurity should be considered in a description of these defect systems.U of I Onlydissertatio