arXiv subjects

G. Baldacchini

Publications and source records attributed to G. Baldacchini.

4 recordsLinked to original sources

arXiv2007

Off-center impurity in alkali halides: reorientation, electric polarization and pairing to F center. IV. Reorientational rate

This last Part IV is aimed at deriving relaxation rates (times) of an off-center Li+ impurity. We follow Christov's reaction rate method to define general rate equations in terms of the exact Mathieu eigenvalues, as well as of harmonic-oscillator eigenvalues approximating for the energy spectrum near the bottom of the reorientational wells. To calculate the rate in each particular case, we derive configurational tunneling probabilities by either Mathieu eigenfunctions or by harmonic oscillator eigenfunctions. The electron-transfer probability is calculated by generalizing Landau-Zener's method. Typical examples are considered and compared with experimental relaxation times in KCl:Li+.

physics.chem-ph↗

arXiv2007

Off-center impurity in alkali halides: reorientation, electric polarization and pairing to F center. III. Numerical calculations

We carried out numerical calculations by an extended-Hueckel program in order to check the analytical results reported in the preceding Part I and Part II. We typically consider alkali halide clusters composed of some tens of constituent atoms to calculate electronic energies under static conditions or versus the displacements of particular atoms. Among other things, the off-center displacement of substitutional Li+ impurity in most alkali halides is evidenced. The trigonometric profile of the rotational barriers is also confirmed for KCl.

physics.chem-ph↗

arXiv2007

Off-center impurity in alkali halides: reorientation, electric polarization and pairing to F center. II. In-plane rotation and polarizability

Because of its inherent 2-D character, the eigenvalue equation for the hindered rotation around a normal cation site of an off-center impurity nearest-neighboring an F center is the well-known Mathieu equation. We present an overview of literature data on Mathieu's periodic functions providing exact solutions to the Li+ reorientational problem. We compare them with bottom-well approximating solutions by harmonic-oscillator functions at an effective vibrational frequency renormalized by both first- and third- order electron-mode coupling. We finally discuss the in-plane inversion polarizability of an off-center impurity assumed to form a dipole-dipole coupling with a nearby F center.

physics.chem-ph↗

arXiv2007

Off-center impurity in alkali halides: reorientation, electric polarization and pairing to F center. I. Basic equations

Extending earlier work on a vibronic theory of the FA centers in alkali halides, the reorientation is now considered of an off-center Li+ impurity, either isolated or near an F center. We derive analytically periodic potential energy barriers between metastable reorientational off-center sites, the barriers hindering the impurity rotation around the normal lattice site. Applying to a specific model, electron-vibrational mode coupling constants are calculated up to 3rd order of the expansion of the coupling energy in the T1u mode coordinates. The 3rd order coupling brings about additional renormalization of the reorientation controlling vibrational frequency.

cond-mat.mtrl-sci↗