Centers In NaCl Research Articles

ESR study of U2centres in BaClF

An X irradiation at 78 K, of a BaClF crystal containing OH- ions leads to the formation of hydrogen atoms in interstitial sites (Hi0 or U2 centres) thermally stable below T approximately 100K. The magnetic properties of the U2 centres are compared with those observed in the case of U2 centres in NaCl crystal. The thermal stability is studied and compared with those of the U2 and U3(Hs0) centres in NaCl.

Excited States of Neutral Silver Center in Alkali Halides Embedded in Continuum

Optical absorption bands associated with the Ag 0 center in NaCl, KCl and KBr are studied. In the wavelength region between two intense bands observed in the visible and ultraviolet region, four weak bands, of which oscillator strengths are less than that of visible band by two order of magnitude, are observed. These four weak bands are ascribed to the transitions from the 2 S ground state to the excited states 2 S , 2 D 3/2 and 2 D 5/2 , the last being split into \(\varGamma_{7}\) and \(\varGamma_{8}\) due to the cubic crystal field. Photoconductivity measurements are also made in KBr and it is shown that the bottom of the conduction band is separated from the 5 s level of the neutral silver by 3.50 eV and situated above the 5 p level. It is concluded that the excited states for four weak bands are embedded in the continuum state consisting of core electrons (4 d 10 ) of Ag 0 and an electron in the conduction band and the asymmetrical shape of these bands is discussed in terms of a resonance effect betwe...

Theory ofMn0Centers in NaCl

Models of various ${\mathrm{Mn}}^{0}$ centers in NaCl, observed by Ikeya and Itoh using EPR, were investigated theoretically. The models for $A, C$, and $E$ centers proposed by these authors were found to be satisfactory but not those for $B$ and $D$ centers. It is concluded that the $B$ center is a ${\mathrm{Mn}}^{0}$ in a divacancy and the $D$ center is a ${\mathrm{Mn}}^{0}$ in a linear trivacancy.

Polarized luminescence and the mössbauer effect of tin impurity centres in NaCl crystals

AbstractAn investigation is made of various types of impurity centres in NaCl;Sn and KCl:Sn crystals using the Mössbauer effect and polarized luminescence. Two types of Sn2+ centres are found in NaCl: Sn. The centres of type I have polarized emission at 2.78 eV due to electronic transitions allowed along the C4 axis of the crystal. A small quadrupole splitting (<0.4 mm/s) of the Mössbauer absorption line at +4.32 mm/s is observed for these centres. The centres of type II show the polarized emission at 2.3 eV due to electronic transitions allowed along the C2 axis of the crystal. For these centres a considerable value of quadrupole splitting Δ = 0.97 mm/s of the Mössbauer lines with the isomer shift + 3.75 mm/s is found. Only the centres of the type I are found in KCl: Sn crystals. It is supposed, that the investigated centres in NaCl:Sn represent two types of associates of Sn2+ ions with cation vacancies vc: associates with C4V symmetry (the type I centres) and associates with C2v symmetry (the type II centres) in the ground electronic state.

Experimental Study of a New Mg Centre in NaCl and KCl

AbstractIn OH‐free NaCl(Mg) and KCl(Mg) electron centres are produced by electrolytical coloration and “decoloration”. It is most probable that the centre is a Mg atom in a vacancy pair. The oscillator strength of its absorption band is determined.

An EPR Study of a Neutral Manganese Center in NaCl

AbstractAn EPR study of the MnO‐E center below room temperature was made in X‐irradiated and F‐bleached NaCl. In addition to the isotropic spectrum with g = 2.0018 and A = 19.5 × 10−4 cm−1 at room temperature, the spectrum at low temperature shows fine structures with an axial field of D = 160 × 10−4 cm−1 and 〈110〉 and 〈111〉 symmetries, which were analyzed from the shift of (M8 = 1/2 ⇋ M8 = −1/2) transitions. The temperature dependence of the hyperfine coupling constant shows anomalies. These results were discussed based on the assumption that the center is an off‐center.

Local and gap modes due to surface or substitutional impurity in diatomic linear lattices

Using the method of ‘extra forces’ we have carried out an exact normal mode analysis for a diatomic linear lattice made up of two masses M 1 and M 2 with the defect atom of mass M at the centre, taking free end boundary conditions and changing the force constant associated with the defect atom. The analysis is limited to the nearest neighbour interaction. The effect of change in surface force constant, in the case of a pure diatomic linear lattice with end atoms heavier, is studied. Localized modes for H − centre in NaCl, NaBr and KCl lattices have been calculated. We have compared our results with the experimental and previous theoretical results.

Zum Problem der Gegenseitigen Abhängigkeit von ε und p0 bei der Thermischen Entfärbung von Farbzentren in NaCl

There was found an approximatelly linear relation between the quantitiese and logp0 of the thermal excitation functionp=p0 exp(-e/kT) in the thermal decoloration of color centres in NaCl and it was shown, that a fictitious dependence can take place. An instruction for the precise measurement ofe andp0 is given.

F-Aggregate Centers in Sodium Chloride. I

Thermal aggregation of electron excess color centers in NaCl, x irradiated at 80°K, was studied by observation of changes in optical absorption in pulse annealing experiments. An initial 80 h x irradiation introduced large concentrations of F, F′ and positive hole centers. As annealing temperatures were raised (80 → 235°K) recombination of thermally released positive holes with electrons in F centers produced α centers. Thermal aggregation of F centers occurred above 240°K when α centers become mobile. M+ centers (F2+) form and disappear and M centers (F2) increase rapidly in number near 265°K as a result of the reactions F + α → F2+ and F2+ + e → F2. R+ centers (F3+) form by the reaction F2 + α → F3+ below 320°K and disappear slowly by the reactions F3+ + e → F3 and F3+ + heat → F2 + α, above 330°K. Optical absorption band maxima at 80°K are: M+, 1.22 eV (1015 nm); M1, 1.74 eV (713 nm); R+, 1.80 eV (690 nm); R1, 2.28 eV (543 nm) and R2, 2.07 eV (600 nm). N1 and N2 band systems were observed but were not studied in any detail. M+ centers also were formed by direct ionization of M centers by 300-nm light. The concentration, however, was low because of a counterbalancing of photoionization and electron capture processes. R+ centers were formed in x-irradiated NaCl by excitation with light absorbed in the R2 band. They could be transformed back to R centers by x-irradiation or F center bleaching.

Studies on non-metals during irradiation — II. The formation and post-irradiation growth and decay of F centers in NaCl at 20°C

Typical NaCl F-center growth curves measured during gamma-ray irradiation at 20°C contain one linear and three saturating components. Following the termination of irradiation at low doses the coloring is constant, but at high doses, the curve contains one increasing saturating exponential, two decreasing exponentials, and one constant component. The resumption of irradiation after an interruption at low doses increases the coloring monotonically. At high doses, the resumption initiates a complex sequence of decreases and increases.

Electron paramagnetic resonance and optical studies of monovalent manganese center in NaCl

EPR and optical absorption spectra of Mn + center in NaCl are studied. EPR spectrum of Mn + at K-band frequency has a hyperfine coupling constant of A=(231±1)×10 −4cm −1 and an isotropic g-value of 1·9992±0·0005. Fine structures caused by second order hyperfine interaction at X-band frequency are analyzed by diagnoalizing the matrix of the spin Hamiltonian which contains the Zeeman and the hyperfine terms. Shf structures are also observed for the magnetic field along <111>. The temperature dependence of the hyperfine coupling constants of Mn 2+, Mn + and Mn o-C is measured and expailed by using theoretical formula based on the phonon-induced hyperfine coupling. It is concluded that the Mn + ion is located at the Na + site. An optical absorption band at 4·47 eV is ascribed to the transitions from the ground state 3 d 54 s( 7 S) to the excited states 3 d 54 p( 7 P) of the Mn + ion. Mn + ion are produced when the Mn o atom traps a positive hole or when the Mn 2+ traps an electron.

Optical and Electron Paramagnetic Resonance Studies of Atomic Manganese Centers in NaCl

EPR and optical absorption spectra of Mn° centers in NaCl are studied. Three kinds of optical absorption spectra A , B and C which are ascribed to atomic manganese centers are found: each of these spectra consists of the allowed transitions from the ground state 3 d 5 4 s 2 to the excited states 3 d 5 4 s 4 p and 3 d 5 4 s 5 p . Each spectrum has a structure: the splitting for the spectrum A is much smaller than those for the spectra B and C . The spectrum A which is formed by the X-irradiation at liquid helium temperature changes above 20°K into spectrum B . It is suggested that the spectrum A and the spectrum B arise from Mn° at a lattice site (Mn°- A center) and in an off-centered position (Mn°- B center), respectively. It is shown that the spectrum C is associated with the Mn° center which produces EPR spectrum with A =15.5×10 -4 cm -1 . It is suggested that the spectrum C is associated with Mn° at the interstitial site (Mn°- C center).

Two-ElectronF′Centers in the Alkaline-Earth Oxides and in the Alkali Halides

The Hartree-Fock-Slater equations for the two-electron orbitals localized about an anion vacancy in MgO, CaO, NaCl, and KCl have been solved numerically in the point-ion-lattice potential. The ionic polarization of the nearest-neighbor ions is treated in a self-consistent manner. It is found that the low-lying ${F}^{\ensuremath{'}}$-center states for MgO and CaO have the following order for increasing values of the energy: $^{1}S(1s, 1s)$, $^{3}P(1s, 2p)$, $^{1}P(1s, 2p)$, and either $^{3}S(1s, 2s)$ or $^{1}S(1s, 2s)$. The states $^{3}S(1s, 2s)$ and $^{1}S(1s, 2s)$ both lie above the other three states, but whether the $^{3}S(1s, 2s)$ state lies above or below the $^{1}S(1s, 2s)$ state depends upon the ionic polarization of the crystal potential. The above ordering, the optical absorption and emission energies between the states $^{1}S(1s, 1s)$ and $^{1}P(1s, 2p)$, and the spin-forbidden emission energy from the state $^{3}P(1s, 2p)$ to the state $^{1}S(1s, 1s)$ agree reasonably with the experimental ordering of the states and with the experimental transition energy values for CaO, respectively. The same physical model gives very different results for the ${F}^{\ensuremath{'}}$ center in NaCl and in KCl. It is found that only the ground state $^{1}S(1s, 1s)$ contains spatially compact (bound) electronic orbitals. The ground-state energies of the ${F}^{\ensuremath{'}}$ center in NaCl and in KCl agree to within 20% of the experimental values. The existence of bound excited states for the ${F}^{\ensuremath{'}}$ center in these monovalent crystals has been investigated. However, definitive statements on such states are not available at present.

Cu+andAg+Centers in Alkali Halides

The displacements of ${\mathrm{Ag}}^{+}$ and ${\mathrm{Cu}}^{+}$ ions in various alkali halides are calculated using a polarizable pointion model. It is found that ${\mathrm{Ag}}^{+}$ in RbCl lies about 0.54 \AA{} along a 111&gt; direction, and that ${\mathrm{Ag}}^{+}$ in NaCl, KCl, and RbBr is on site, in agreement with experiment. ${\mathrm{Cu}}^{+}$ ions are found to be off center in NaCl, KCl, RbCl, and RbBr, also in 111&gt; directions, by 0.2, 1.36, 1.65, and 1.84 \AA{}, respectively. Using a model of a free ${\mathrm{Cu}}^{+}$ ion in a crystalline electric field, the oscillator strength of the ${(3\mathrm{d})}^{10}\ensuremath{\rightarrow}{(3d)}^{9}4s$ transition is calculated using Hartree-Fock-Slater wave functions. It is found that the observed oscillator strengths of the order of 0.025 can be induced by fields of 2\ifmmode\times\else\texttimes\fi{}${10}^{8}$ V/cm. This result suggests displacements in the 111&gt; directions about 50% bigger than those given above for the point-lattice model. It turns out also that for ${\mathrm{Cu}}^{+}$ in NaCl, oscillator-strength measurements alone are insufficient to decide whether the ion is on site.

FCenter in Ionic Crystals. II. Polarizable-Ion Models

The states of the $F$ center are considered on the basis of models which treat the movement of the nearest neighbors to the $F$ center and the $F$ electron in a self-consistent manner. The lattice is first described in terms of a classical ionic-crystal theory. The theory is then extended to treat the nearest-neighbor ions in a quantum-mechanical manner. The one defect electron (the $F$ electron) is treated according to polarizable-ion models. The absorption energy, the emission energy, the lifetime of the first excited state, the zero-phonon transition energies, and the Huang-Rhys factors are evaluated for two models, which differ in the rigor used to compute the polarization of the nearest and next-nearest neighbors. It is found that the model that contains the more rigorous evaluation of the polarization agrees best with the experimental results for CaO and perhaps MgO. In addition, it is found that both these models are least successful for $F$ centers in NaCl and KCl. Not enough data exist to make judgments about the agreement for Ca${\mathrm{F}}_{2}$, Sr${\mathrm{F}}_{2}$, and Ba${\mathrm{F}}_{2}$.

Thermoluminescence of pure and impurity-doped KBr and NaCl crystals

Two thermoluminescence peaks are observed at 90 and 150 °C in highly pure KBr crystals (background divalent cation impurity not greater than 1 p.p.m.). From the growth of F centres with the time of x-irradiation these two peaks have been interpreted in terms of the two types of F centres postulated previously. The presence of Cd impurity in KBr suppresses the formation of both the types of F centres while the presence of Zn impurity enhances the first-stage F centres but suppresses the second-stage F centres. Optical absorption studies of the KBr crystals doped with Cd and Zn are correlated with the thermoluminescence data. Plastic deformation of highly pure KBr crystals considerably increases the area under both the thermoluminescence peaks. No additional glow peak is observed either in plastically deformed or Cd- and Zn-doped KBr crystals. Thermoluminesence experiments performed with highly pure NaCl crystals (background divalent cation impurity not greater than 1 p.p.m.) also show two peaks at 170 and 210 °C. However, in analar purity NaCl crystals (background divalent cation impurity about 20 p.p.m.) or in NaCl crystals containing Ca and Cd, an additional thermoluminescence peak at 80 °C is also observed. The activation energies of both the types of F centres in KBr and NaCl and of the impurity centres in NaCl are evaluated from the thermoluminescence results using the `initial rise method'.

Charged F-aggregate centers in NaCl

Data is presented for NaCl which identifies the absorption and emission bands of the M +, R + and M 1 centers. The transitions for the M + and R + centers differ from those formerly attributed to these defects.

A Calculation of the Wavefunction and Isotropic Hyperfine Structure Constants of the U2‐centre in NaCl and KCl

AbstractThe wavefunction of the U2‐centre is determined by a variational procedure taking into account the charge distribution of the nearest neighbour ions. All wavefunctions concerned are mutually orthogonalized using a combined Schmidt‐Löwdin procedure. The influence of the 2‐centre exchange integral and of a possible distortion of the crystal near the centre is discussed. The isotropic hyperfine and superhyperfine structure constants are calculated.

Lifetime of the first excited state of the M center in NaCl

The lifetime of the first excited state of the M center in NaCl has been determined to be 5 × 10 -8 sec (± 1 × 10 -8 sec). One finds a partial temporary bleaching of the M 1 band, with decay time of 1.5 sec and about 50 sec.

Stark Effect in Color Centers of Alkali Halides

The stark effect of the $F$ center in NaCl, NaBr, KCl, KBr, KI, and RbCl, the $M$ center in KCl, and the ${Z}_{1}$ center in KCl: Sr was measured using an ac electric field. The fractional change of the absorption coefficient, $\frac{\ensuremath{\Delta}\ensuremath{\alpha}}{\ensuremath{\alpha}}$, at the peak of the $F$ band, normalized to a Lorentz local field of 200 kV/cm, was found to be approximately 2\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}5}$ for NaCl and NaBr, 11\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}5}$ for RbCl, and 17\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}5}$ for KCl, KBr, and KI. The value of $\frac{\ensuremath{\Delta}\ensuremath{\alpha}}{\ensuremath{\alpha}}$ at the peak of the $M$ band in KCl was (3.9\ifmmode\pm\else\textpm\fi{}0.4)\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}5}$ for an applied electric field of 97 kV/cm. A measurement of the change in the absorption coefficient, $\ensuremath{\Delta}\ensuremath{\alpha}$, versus photon energy produced a nonsymmetrical curve. This indicates that the effect observed was not the linear Stark effect, and thus provides additional evidence that Seitz's model is incorrect. The absorption coefficient increased at energies lower than the $M$ band, corresponding to a decrease in the absorption coefficient of the $M$ band. The effect is attributed to the mixing of two nondegenerate energy levels of opposite parity. The optical transition to the level of lower energy is normally forbidden. The ${Z}_{1}$ center in KCl: Sr did not exhibit a detectable Stark effect. It was determined, however, that $\frac{\ensuremath{\Delta}\ensuremath{\alpha}}{\ensuremath{\alpha}}l8\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6}$ for an applied electric field of 97 kV/cm. If the ${Z}_{1}$ possesses a permanent dipole moment, the difference between the expectation values of dipole moment in the ground state and excited state is less than 1.0\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}20}$ cgs unit.