Excess Electron Color Centers in the Alkali Halides

The F center is the prototype of the excess electron color centers. Although many of the properties of this center are well understood, some interesting questions still remain concerning the nature of its relaxed excited state. The influence of uniaxial stress upon the luminescences has yielded new evidence for the diffuse nature of this state. A contrast will be made of the influence of uniaxial stress upon the absorption and the luminescence. The association of F centers forms higher order complex centers. Studies of the polarization of the optical absorption and the luminescence bands, the stress induced splittings of the zero phonon lines, the magneto-optic effect associated with absorption, and the electron spin resonance have been particularly valuable in establishing the symmetry and microscopic configuration of a number of these centers. A brief review will be given of the properties of the neutral complex, the singly ionized complex, and the complex centers that have trapped an extra electron. Some recent results will be presented of the excitation spectra of the luminescence of a number of theabsorption bands associated with the M center and of the magneto-optic effect of the R center. R6sum6. Le centre F est le prototype des centres a exces d'electron. Bien que beaucoup des propriktbs de ce centre soient bien comprises, quelques questions intbessantes restent poskes au sujet de l'ttat excite relax&. L'influence de contraintes uniaxiales sur la luminescence a apporte de nouvelles evidences sur la nature diffuse de cet etat. I1 est utile de comparer l'influence de contraintes uniaxiales sur l'absorption et sur la luminescence. L'association de centres Fforme des centres plus complexes. Les etudes de la polarisation optique et de la luminescence, le dedoublement des raies a zero phonon sous l'influence des contraintes uniaxiales, l'effet magneto-optique associe a l'absorption et la resonance paramagnetique Blectronique ont permis d'etablir la symetrie et la configuration microscopique de beaucoup de ces centres complexes. Une breve revue des proprietes des centres neutres est donnee ainsi que celles des etats une fois ionises et celles des centres complexes qui ont pikg6 un Uectron supplkmentaire. Quelques resultats recents seront prCsentCs concernant le spectre d'excitation de la luminescence de nombre de bandes d'absorption associCes au centre M, ainsi que l'effet magnkto-optique du centre R. The F center is the prototype of the excess electron color centers. Although many of the properties of this center are well understood, some interesting questions still remain concerning the nature of its related excited state. The influence of uniaxial stress upon the luminescence has yielded new evidence for the diffuse nature of this state. A contrast will be made of the influence of uniaxial stress upon the absorption and the luminescence. The association of F centers forms higher order complex centers. Studies of the polarization of the optical absorption and the luminescence bands, the stress induced splittings of the zero phonon lines, the magneto-optic effect associated with absorption, and the electron spin resonance have been particularly valuable in establishing the symmetry and micros(*) A portion of this work was supported by a grant from the National Science Foundation. copic configuration of a number of these centers. A brief review will be given of the properties of the neutral complex, the singly ionized complex, and the complex centers that have trapped an extra electron. Some recent results will be presented of the excitation spectra of the luminescence of a number of the absorption bands associated with the M center and of the magneto-optic effet of the R center. The F Center.The excess electron color centers in the alkali halides constitue a family of defects that have received a substantial amount of attention [l]. Starting with the simplest of these centers, the Fcenter, and progressing to larger and larger complexes containing two, three, four and perhaps more, one moves from the M to the R and perhaps to the N centers. In nearly all cases, the understanding of the properties of the higher complex centers has been determined by the detailed understanding of the F center itself. Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1967402 EXCESS ELECTRON COLOR CENT 'ERS IN THE ALKALIHALIDES C 4 11 Since the model of the F center is so simple, it has been traditional to describe the system by a single dimension configuration coordinate model, such as that shown in figure 1. Here is represented the total energy of the system for the ground state and the excited state of the system with optical absorption represented by the arrow A-B and emission represented by the arrow C-D. According to the Franck-Condon principle, these arrows are drawn vertically to indicate that the transition takes place in a time short compared to any lattice motion. The minimum of the ground state A is displaced from the corresponding minimum in the excited state C, since the electronic configuration of the system in the ground and excited states is different. The Stokes shift is substantial in these centers with the luminescence occurring at approximately one-half of the photon energy of the absorption. The different couplings of the electronic system to the lattice in the two states result in the change of curvature of the total energy curves with configuration coordinate.