Centers In KCl Research Articles

A study of the FH(CN-) centre in KCl and CsCl: the potential energy surface and EV energy transfer

Adiabatic potential energy surfaces (APESS) of the FH(CN-) centre in KCl and CsCl are calculated for the excited and ground states. It is shown that in CsCl the APES of the excited state intersects that of the ground state on the path of de-excitation leading to the quenching of emission. In KCl, the opposite situation is obtained in agreement with experiment. The calculation for CsCl shows a rapid variation of the dipole moment of the CN- molecule near the crossing region. The electronic-to-vibrational energy transfer (EV transfer) between the F centre and CN- molecule is discussed in terms of the non-radiative transitions between the two intersecting APESS. The dipole-dipole coupling between the F centre and CN- molecule (Dexter-Farster theory) is also considered to analyse certain aspects of the EV transfer.

A Contradiction between Pulsed and Steady-State Studies in the Recombination Kinetics of Close Frenkel Defects in KBr and KCl Crystals

Theoretical study of the kinetics of the correlated annealing of pairs of close (geminate) F-H centers in KCl and KBr crystals controlled by their diffusion and elastic attraction shows that the multi-step (kink) decay in defect concentrations observed more than once in thermostimulated experiments takes place only for very close F-H center pairs which are no further than fourth nearest neighbors. On the other hand, it is demonstrated (both theoretically and experimentally) that such F-H center pairs should be destroyed by the tunneling recombination already at time ≤10 -4 s, i.e. much before beginning of the thermostimulated experiments. Possible explanations of this contradiction are suggested.

Spectroscopic properties of (F2+)AH centres in oxygen- and sulphur-doped alkali halides

In search of potential laser-active colour centres, we have created and spectroscopically studied for the first time (F2+)AH centres in KCl:Li+:O2- and RbCl:Na+:O2-. We compare them with the 'classical' (F2+)AH centres in KCl:Na+:O2- and KBr:Na+:O2-. We describe preparation procedures, and absorption and emission spectra at 100 and 5 K. We study the stability of the centres in the dark at room temperature, an important property for laser applications. Preliminary data concerning (F2+)AH centres in KBr:Na+:S2- are also reported.

Picosecond spectroscopy of the resonant secondary radiation of F centers in KCl

Resonant secondary radiation of F centers in KCl is studied, using time-resolved picosecond spectroscopy. A novel mechanism is proposed, viz., that the optically excited F electron is de-excited as the lattice relaxes in a 2p adiabatic potential.

Self-consistent analysis of absorption and emission data forF A centres in alkali halides

A theory which explains self-consistently the emission and the polarization rotation of transmitted light has been developed forF A centres in crystals with cubic symmetry. Azimuthal dependences ofF A luminescence in alkali halides are very informative for determining the electron-phonon interaction as well as the parameters related to the structural and optical quality for coulour centre lasers crystals. The theoretical results are applied to the interpretation of the luminescence properties ofF A (Li) centres in KCl, RbCl and KF.

Paraelectric behavior of OH − -dipole centers in KCl crystals

OH - centers in KcL are shown to be oriented with their dipole axes in the six degenerate (100) directions of the crystal. High electric fields cause redistributions among these six orientations and can lead at low temperatures to a complete alignment of the diluted dipole system. This was studied by the electric dichroism of the OH - -absorption in dependence on field, temperature and crystal orientation. A theoretical model can quantitatively explain all measurements and provides values for the dipole moment and optical anisotropy of the center.

Optical Absorption of the Fd Center in KCl: Ca2+

AbstractThe optical absorption of the Fd center in KCl: Ca2+ is studied by using point‐ion and ion‐size correction models. For the calculations the Gourary and Adrian wave function types I, II, III and Gaussian functions are used. The principal axes are obtained for several positions of the impurity‐vacancy dipoles, and the corresponding ground and excited states of the Fd center are calculated. Good agreement with the experimental values is found for the Gourary and Adrian wave functions of type I.

Theoretical simulation of VK-centre migration in KCl. II. Phenomenological theory

For pt.I, see ibid, vol.4, p.7417 (1992). An adequate theoretical basis is developed and presented for a nonstationary technique for the experimental detection of reactions in insulating crystals controlled by tunnelling recombination of mobile defects (hole centres) with electron centres. The effect of finite hop lengths of point defects in a crystalline lattice is carefully analysed. This method is based on measuring the inertial increase or decrease in the recombination luminescence intensity after a stepwise change of temperature (defect mobility). Theoretical analysis of the transient process observed experimentally for VK centres in KCl:Tl crystals argues for their motion through 60 degrees rotations with the frequency factor St=4*1013 s-1, very close to that obtained earlier in the optical dichroism studies.

On the off-centre angle and the refractive index difference due to the birefringence of FA (II) centres in KCl:Li crystals

Based on the dichroism of the FA (II) KCl:Li centres the off-centre angle of the Li+ ion was measured to be 13.5+or-1 degrees at 26 K. Furthermore, the birefringence of the centres was verified and the difference between the refractive indexes relating to the two preferred directions was directly derived from the absorption spectra measured with polarized wave.

Spin coupling and memory effects in the optical cycle of FH centers involving electron-vibrational energy transfer

Spin-coupling and spin memory effects of FH(OH−) and FH(CN-) centers have been studied in various hosts at 2 K. The spin-memory loss ϵ, i.e. the probability of spin-flip during a complete electronic excitation and relaxation cycle, has been determined. The main result is a strong increase of the spin-memory loss ϵ for FH(CN-) in CsCl and CsBr compared to F centers, while ϵ remains nearly unchanged (only a slight decrease) for FH(OH-) centers in KCl and KBr. First attempts will be made to explain these spin-memory loss results in correlation to the electron relaxation and electron-vibrational energy transfer processes occuring in the same optical cycle.

Teaching the optical memories

Some simple experimental ideas are introduced to demonstrate optical memory. The optical memory is based on the use of the linear dichroism of FA color centers in KCl:Li crystal.

EPR of Triclinic Fe Centers in KCl and RbCl

A new paramagnetic Fe center with triclinic g-tensor symmetry is reported. This type of defect is detected in both KCl and RbCl single crystals doped with iron, after they were quenched, irradiated at 77 K, and annealed at 160 K. The g-tensors exhibit a very large g-tensor anisotropy with g∥ ≈ 10 and g⟂ ≈ 5 in an axial approximation. Several models for this type of paramagnetic defect are discussed. On rapporte la découverte d'un nouveau centre paramagnétique Fe, dont le tenseur g possède une symmétrie tricline. Ce type de défaut fut détecté tant dans le KCl que dans le RbCl monocristallin. Les échantillons furent assujettis à un écrouissage, à une irradiation à 77 K et ensuite à une conservation à 160 K durant une durée variable. Les tenseurs g sont fortement anisotropes: g∥ ≈ 10 et g⟂ ≈ 5 dans l'approximation axiale. Divers types de modèles pour ces défauts paramagnétiques sont traités.

Optical properties of O2 − centres in KCl∶NaCl mixed crystals

The optical properties of O2 − centres in KCl∶NaCl mixed crystals were investigated. The timeresolved spectra and the fluorescence decay time are shown. From the measurements of the radiative decay time, the oscillator strength of the absorption,f ab=0.045, the oscillator strength of the fluorescence,f fluo=0.019, and the activation energy ΔE=0.22 eV for the O2 − centre in KCl∶NaCl were determined.

Determination of the refractive index difference caused by the birefringence ofFA(II) centers in KCl:Li

Abstract A method is described for determining the refractive index difference caused by the birefringence of oriented FA (II) centers in KCl:Li crystals. It is shown that the portion induced by the birefringence can be separated from an absorption spectrum measured through a polarizer-analyzer system. From this portion the refractive index difference can then be calculated with ease.

Vibrational fluorescence of CN−centers in KCl by electronic excitation of neighbouring Tl+centers

Abstract Infrared emission from higher vibrational states of CN− in KCl has been achieved by optically exciting nearby Tl+ ions with an excimer laser. The electronic vibrational energy transfer, already known from FH(CN−) in alkali halides, leads to population of vibrational states up to at least the tenth level.

Formation and optical properties of laser-active N2 centers in KCl.

Recent experimental results on the formation and optical properties of the laser-active ${\mathit{N}}_{2}$ center in additively colored pure KCl are reported. The ${\mathit{N}}_{2}$ center in KCl is most efficiently formed through a photoaggregation process in heavily colored crystals exposed to light on the long-wavelength shoulder of the F band. The observed room-temperature stability of the ${\mathit{N}}_{2}$ center indicates that it is an electrically neutral defect. Absorption and emission bands peak at 1.02 and 1.25 \ensuremath{\mu}m, respectively. The ${\mathit{N}}_{2}$-band absorbance grows proportionally with the ${\mathit{R}}_{1}$- and ${\mathit{R}}_{2}$-band absorbances. Polarized ${\mathit{N}}_{2}$-center fluorescence data is consistent with two degenerate orthogonal dipole moments lying on {111} planes of the crystal. Excited-state absorption spectra show higher transitions coincident in wavelength with the ${\mathit{R}}_{1}$ and ${\mathit{R}}_{2}$ absorption bands of the trigonal ${\mathit{F}}_{3}$ color center in KCl. Similar experiments on the ${\mathit{R}}_{1}$ band correlate well with the ${\mathit{N}}_{2}$-center data. Results are consistent with the identification of the laser-active part of the ${\mathit{N}}_{2}$ band as a transition of the trigonal ${\mathit{F}}_{3}$ color center.

Optical properties and synchronously pumped mode-locked 1.73–2.10 μm tunable laser operation of [formula omitted] centers in KCl:Na +:O -2 crystals

Using synchronous pumping with ∽100 ps wide 1.32 μm Nd 3+:YAG laser pulses, we realize mode-locked KCl:Na +:O - 2 (F + 2) AH color center laser operation with nearly transform limited ∽5–10 ps wide pulses throughout a 1.73–2.10 μm tuning range. Pulses shorten continuously with increasing pump power levels. At maximum available pump power (∽2.3 W), average output power up to ∽380 mW is obtained in the peak of the tuning curve at 1.83 μm. Pulse shape and width measurements show that stable mode-locked operation is relatively insensitive towards cavity length detuning.

The FA(II) centres of KCl:Li in a reoriented centre distribution

The magnetic circular dichroism (MCD) spectra of FA(II) centres in KCl:Li crystals have been measured at a magnetic field of 11.5 kG at 17 K and room temperature. When a crystal is irradiated with linearly polarized FAl light, a non-zero circular dichroism (CD) signal is detected at zero field. The MCD spectra are compared with the zero-field CD spectra, which were obtained in systems containing both a random distribution of centres and an aligned distribution. The meaning of the MCD signal obtained in the reoriented anisotropic system using a conventional CD polarimeter is discussed.

Energy transfer and picosecond relaxation of F centers in RbCl, KCl, and KBr.

Possible decay mechanisms of excited F centers are investigated in both additively colored and x-irradiated RbCl, KCl, and KBr, containing typically more than 3\ifmmode\times\else\texttimes\fi{}${10}^{16}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$ F centers and sizable amounts of dimer (${\mathit{F}}_{2}$) and higher F aggregates. Time-resolved ground-state recovery measurements under F-band excitation at low temperature and optical-absorption data show a picosecond deexcitation process, occurring for a substantial number of F centers and possessing shorter decay times for higher relative concentrations of ${\mathit{F}}_{2}$ and N centers. This establishes energy transfer from excited F to ${\mathit{F}}_{2}$ centers, a controversial topic throughout the literature, and towards N centers in KCl to be both competitive relaxation channels. Transient absorption measurements in heavily colored KCl by Gadonas et al., which show increased transparency in the ${\mathit{F}}_{2}$-band region 500 ps after pulsed F excitation, fully support the above conclusion. A similar behavior was also observed in RbCl and KBr. This additional decay channel could adversely affect laser operation in several color-center lasers.

The property of FH(NCO-) center in KCl

The property of the newly discovered F-aggregate center, FH(NCO-) center, was investigated. Several new peaks located in ultraviolet region and infrared region are found, which are considered to be the new electronic state of F-center and the new vibration states of NCO- of the coupling F-NCO- pairs. The quite novel behavior of these peaks under thermal treatment is given.