Formation time of F centers in alkali halides

It is argued that in most alkali halides an appreciable fraction of the self-trapped excitons may undergo non-radiative transitions from vibrationally excited states of B_3u to the A_1g state during the relaxation of the self-trapped excitons created by ionizing radiation. Numerical calculations show that the non-radiative transition probabilities, from vibrational levels of B_3u near or above the crossing point of the A_1g and B_3u potential curves to the A_1g state, are consistent with the observed formation times of F centers in various alkali halides. The exceptional case of KI is also discussed.     

On the temperature dependence of the F-coloring efficiency in KCl

A simple model is presented to account for the major features of the temperature dependence of the primary F-coloring efficiency in KCl. It assumes that irradiation creates Frenkel pairs in three basic configurations followed by some correlated diffusion, leading either to annihilation of the pair or formation of a “surviving” primary pair. Steady irradiation as well as fast pulse data are analyzed in terms of the model.     

Electron-lattice coupling of F centers: Particular case of NaI: F

Absorption and magnetic circular dichroism studies of the F centers in NaI reveal a strong electron-lattice coupling with the vibration modes of Γ₁⁺ symmetry in comparison with those of Γ₃⁺ and Γ₅⁺ symmetry. On the other hand, the F electron exhibits in its first excited state a large spin-orbit coupling, in agreement with the value previously calculated. These results are confirmed by the Raman spectra which show a predominant intensity for the Γ₁⁺ modes, particularly for the resonant mode at 113 cm^?1.First and second-order Raman spectra have been calculated using a “quasi-resonant” theory in which the vibrations of the ions up to the fifth shell have been taken into consideration. We have been able to determine from these calculations absolute values for the contributions of the different types of vibration modes to the broadening of the F-absorption band.     

Spin-selective electron tunneling from excited FA centers to F+ centers in CaO

From pulsed phosphorescence microwave double resonance experiments it is established that electron tunneling between F_A and F⁺ centers in CaO occurs from F_A centers in the excited ³A₁ state. Only the radiative sub-levels within this ³A₁ state act as precursor states in the electron transfer process. The tunneling rate at 1.2 K is determined to be 1.5±0.2s^-1.     

Spin polarization in the relaxed excited state of the F center in case of saturated optical pumping

The dependence of the F center MCP on the polarization of pumping light is explained in terms of the spin polarization in the RES (relaxed excited state) which is derived by examining the spin-mixing processes; the spin-mixing parameter in the RES and that associated with the relaxation from the URES (unrelaxed excited state) to RES are evaluated. A new ESR method in the RES is proposed.     

Effect of V K centers on positron annihilation in KCl and Ag-doped KCl

Positron lifetime spectra have been measured at 77 K for KCl and Ag-doped KCl before and after x-irradiation at 77 K and after annealing at room temperature. Radiation at 77 K reduces the intensity of the intermediate lifetime (τ₂) component. Radiation-induced defects were monitored optically and by ESR. The experiment shows that the changes observed in the positron decay are associated with the presence ofV _K centers. Work support in part by the Research Corporation. Paper C1 presented at 3rd Internat'l Conf. Positron Annihilation, Otaniemi, Finland (August 1973).     

On the luminescence quenching of F centres in alkali halides

Non-radiative transitions from the excited state of F center to the ground state occur very efficiently around and above, over a rather large interval of energy, the crossing point of the two adiabatic potential energy curves. Taking this effect into account the criterion for luminescence quenching originally proposed by Dexter et al. is re-examined. The luminescence quantum efficiency is estimated for those alkali halides in which the luminescence is weak, and the results are in general accord with experiment. The border-line case of NaCl is also explained.     

Bi-directional holographic information storage based on the optical reorientation of FA centers in KCl:Na

The dichroism of F_A centers in KCl is used to store erasable, high-resolution and high-contrast Bragg-angle holograms. The photochromic system has a bi-directional photo-response, high-writing sensitivity, practically non-destructive low-temperature read-out, and permits good recyclability. The holograms have nearly unlimited lifetime at freezer temperatures.     

Fourier transform emission spectroscopy of the FΔ-XΦ system of TiF

The emission spectra of TiF have been reinvestigated in the 4200-15 000 cm⁻¹ region using the Fourier transform spectrometer associated with the National Solar Observatory at Kitt Peak. TiF was formed in a microwave discharge lamp operated with 2.5 Torr of He and a trace of TiF₄ vapor, and the spectra were recorded at a resolution of 0.02 cm⁻¹. The TiF bands observed in the 12 000-14 000 cm⁻¹ region have been assigned to a new transition, F⁴Δ-X⁴Φ. Each band consists of four sub-bands assigned as, ⁴Δ_1/2-⁴Φ_3/2, ⁴Δ_3/2-⁴Φ_5/2, ⁴Δ_5/2-⁴Φ_7/2, and ⁴Δ_7/2-⁴Φ_9/2. A rotational analysis of the 0-1, 0-0, and 1-0 bands has been obtained and spectroscopic constants have been extracted.     

ESR of Vk centers in CsI

V_k centers (self trapped hole I^?₂) have been obtained in CsI doped with Tl⁺ or Na⁺ by X-irradiation at low temperature and studied by ESR. Spin-Hamiltonian parameters are calculated and compared with those obtained for I^?₂ in the other alkali iodides, as well as the energy of the forbidden transition ²Σ⁺_u→²Π_u and the s and p components of the fundamental state.     

Utilization of balanced detection with optical hard-limiters in T/W OCDMA system

In this paper, we utilize optical hard-limiters to further enhance the system performance in balanced detection T/W optical CDMA system. In T/W optical CDMA systems, it had been investigated that the balanced detection scheme is not superior to the OOK scheme because of the adverse impact of beat noise. The employment of optical hard-limiter is an effective way to alleviate the deteriorating effect of beat noise. The carrier-hopping prime code is utilized as signature sequences whose out of phase autocorrelation is zero and the cross-correlation is at most one. Due to the orthogonality of signature sequence, the correlator of shifted version of carrier-hopping prime code is utilized in the receiver. The numerical results reveal that our proposed scheme can greatly reduce the error floor; also it is superior to that using OOK scheme with optical hard-limiter. Most important of all, our proposed scheme has fixed and optimum thresholds which do not vary with system parameters; and this will lead to dynamic estimation of optimal thresholds unnecessary in the receiver.     

Effect ofV K centers on positron annihilation in KCl and Ag-doped KCl

Positron lifetime spectra have been measured at 77 K for KCl and Ag-doped KCl before and after x-irradiation at 77 K and after annealing at room temperature. Radiation at 77 K reduces the intensity of the intermediate lifetime (τ₂) component. Radiation-induced defects were monitored optically and by ESR. The experiment shows that the changes observed in the positron decay are associated with the presence ofV _K centers.     

Impossibility of perturbative restoration of F type spontaneously broken supersymmetry

We prove that higher order corrections cannot restore supersymmetry if it is spontaneously broken at the tree level by an F type mechanism even if not all terms consistent with the symmetries have been included.     

A study of the optical absorption and the hyperfine constant of the F-centre in alkali halides by means of the multiple scattering Xα method

The F-centre in several alkali halides has been investigated by means of the multiple scattering Xα method. The calculated values of the first absorption peak and the hyperfine splitting constant are compared to experimental results and other theoretical work. It is found that the results agree quite well with experimental values.     

Optical properties of silver colloidal centers in KCl crystals

The Ag colloidal centers in additively colored KCl: Ag crystals have been studied by optical and electron microscope methods. The transmission electron microscope studies reveal that the colloidal particles are spherical in shape and have a size distribution which can be described by the skewed zeroth order logarithmic distribution (ZOLD) function. The band shape of the experimental absorption spectra of colloidal particles agrees well with that calculated on the basis of Mie theory, if the optical constants are suitably modified and if the particle size distribution is taken into account. If the particle size is so small that the size distribution cannot be measured by electron microscope the optical absorption bands can be used to determine the size distribution parameters provided the distribution is well described by ZOLD, which in turn is used to calculate theoretical extinction bands.     

Ab initio calculation of many-particle effects in the optical spectra of semiconductors

We present the first investigation of an optical spectrum including electron-hole pair interaction which starts from an ab initio calculation of Wannier functions and band structure. The resulting spectrum of TlCl agrees well with experiment, demonstrating the importance of both, Coulomb and self-consistently screened exchange interaction. Finally we present a Clausius-Mosotti-like formula with one fitting parameter which quantitatively takes into account the interaction effects.