|Abstract:||The F, V, and Ag+ centers in the alkali halides have been investigated by observing the response of their absorption or luminescence spectra to applied uniaxial stress. The small changes induced in these optical properties have been analysed in the framework of perturbation theory with the results yielding parameters which characterize the center and its interaction with the host lattice.
The method of moments has been used to analyse the stress induced linear dichroism in the F-band absorption in KCl. By measuring the changes in the third moment of the band, the separate contributions to the second moment by lattice vibrations of f Γ1+ , Γ3+ , and Γ5+ symmetry are determined. Extending the work of Schnatter1y, the temperature dependence of these contributions was measured thereby defining an effective vibrational frequency for each symmetry mode.
The stress response for the U-band absorption in KBr was found to be qualitatively identical to that for the F band indicating that the electronic states and their coupling to the lattice are similar in both systems. The broadening of the V band was found to result primarily from cubic vibrational modes. A bump in the dichroism pattern to the high, energy side of the V-band peak position is interpreted as evidence for transitions to higher excited states of the U center.
The stress response of the absorption bands resulting from the vibronically allowed 4d lO → 4d95s transitions of the Ag+ ion was studied in KCl, KBr, and NaCl. Because of the strong anharmonic coupling between the Ag+ ion and its near neighbors, dichroism in the Ag+ absorption bands results from stress induced changes in the vibrational frequencies of the lattice modes responsible for the electronic transitions. Using a simple model in which only the resonance mode of the Ag+ ion is of importance, the final state symmetry assignments for the various bands are determined. Deviations from model predictions are discussed in terms of such factors as covalent bonding and a breakdown in the resonance mode approximation.
The degree of linear polarization in the F-center luminescence induced by uniaxial stress has been investigated in KCl, NaCl, RbCl, and NaF. The amount of polarization was found to be proportional to the magnitude of the applied stress and dependent upon the crystallographic direction along which the stress is applied. The effect was found to be independent of the crystal temperature, the wavelength of the luminescence, the F-center concentration, and the polarization of the exciting light. The results can be successfully interpreted in terms of emission from a p-like relaxed excited state which is very diffuse and largely unaffected by the stress, to a s-like ground state which is still mainly localized at the potential well and into which a small amount of d-like character is mixed by the stress.