Centers In KCl Research Articles

Electron-spin-resonance and optical study of the Bi0(6p3) center in KCl.

An electron-spin-resonance study of a tetragonal ${\mathrm{Bi}}^{0}$(6${\mathit{p}}^{3}$) center in KCl:${\mathrm{Bi}}^{3+}$ crystals is presented. The defect is produced either after x irradiation above 220 K, or after a warmup above this temperature consecutive to low-temperature irradiation, demonstrating that the release and/or capture of anion or cation vacanices is involved in the defect production. The g values, the quadrupole parameter, and the dipolar contribution to the magnetic hyperfine parameters have been calculated in a crystal-field model, yielding good quantitative agreement with the experimental electron-spin-resonance results and permitting the determination of the signs of the hyperfine and the quadrupole parameters. To this end, Coulomb, spin-orbit, and crystal-field interactions were diagonalized between the states of the 6${\mathit{p}}^{3}$ configuration. The hf analysis points to a small delocalization of the valence electrons and to a crystal field, either without or with only a small odd component. Preliminary optical-absorption measurements are presented, and absorption bands at 629 and 725 nm are tentatively correlated with weakly allowed transitions between the calculated energy levels arising from the ground manifold of the ${\mathrm{Bi}}^{0}$ center.

Magnetic circular dichroism in the absorption of theF A centre in KCl doped with Li and Na

The spin-orbit structure ofF A centres in KCl:Li and KCl:Na have been studied by means of the magnetic circular dichroism. Due to theirC 4V symmetry theF A centres have two different spin-orbit parameters, Δ‖ and Δ⊥, which only in the KCl:Li case follow the relation: Δ‖ < ΔF < Δ⊥ as expected from the theory. For a close comparison we have also reported our measurement of the spin-orbit coupling ofF centre in KCl. The spin-orbit parameters of theF andF A centres have been determined using the method of moment.

Tl+Tl0(1)-like dimer centers in KCl and RbCl doped with Tl+, In+, and Ga+: A resonant Raman study.

The Raman spectra of x-ray-irradiated KCl and RbCl, doped with ${\mathrm{Ga}}^{+}$ and ${\mathrm{In}}^{+}$ impurities, demonstrate the presence of ${\mathrm{Ga}}^{+}$${\mathrm{Ga}}^{0}$(1) and ${\mathrm{In}}^{+}$${\mathrm{In}}^{0}$(1) dimer centers through a completely polarized defect-induced broadband spectrum, similar to the one reported for ${\mathrm{Tl}}^{+}$${\mathrm{Tl}}^{0}$(1) in these alkali halides under resonant excitation of its third optical transition (OT3) [W. Joosen et al., Phys. Rev. B 32, 6748 (1985)]. The dynamical modes of this broadband spectrum are shown to possess ${\mathit{C}}_{4\mathit{v}}$:${\mathit{A}}_{1}$ character, consistent with a high degree of \ensuremath{\sigma} polarization of the OT3 of the dimer center. The broadband spectrum is characteristic for the host crystal and shows the same polarization properties and spectral composition as the monomer ${\mathit{M}}^{0}$(1) center (M=Tl,In,Ga). This observation supports the model, derived from optically detected magnetic resonance experiments, of a Tl dimer, in which the unpaired electron is moving between two perturbed ${\mathrm{Tl}}^{0}$(1)-like configurations. The absence of low-frequency vibrational modes in the dimer spectra is probably related to this nonadiabatic electron motion. Despite the resonant enhancement and the different defect structure, the ${\mathit{M}}^{+}$${\mathit{M}}^{0}$(1), broadband spectrum in KCl and RbCl exhibits common features with the ${\mathrm{Tl}}^{+}$-induced first-order Raman spectrum [R. T. Harley et al., Phys. Rev. B 3, 1365 (1971], indicating a larger electron density in the anion vacancy for the third excited state than for the ground state.

Experimental evidence for Stark effect on F A(II) centers in KCl:Li relaxed excited state

The effect of an electric field on the emission from F A(II) centres in KCl:Li has been detected in a colour centre laser. A clear evidence has been obtained by comparing the visible absorption and infrared emission signals in two different experimental configurations, either by applying a sinusoidal field to the crystal or by modulating the pump beam intensity. We found that the Stark effect on the emission depends on the polarization angle of the pump light.

33S splittings of some sulphur centres in KCl and NaCl

A new sulphur centre has been detected in KCl by means of EPR. The defect is shown to be the same as the B centre detected already by Matthys and co-workers in NaCl. By enriching the sulphur centres with 33S it is shown to be an S2- ion in a divacancy site, in contrast to the S2- ion detected two decades ago and located in a single anion vacancy by Vannotti and co-workers. In KCl both varieties of S2- may coexist. Further experimental evidence is also presented to identify the earlier reported A centre as an interstitial S- ion.

Optical Detection of the Hyperfine Interaction of the F Center in KCl in the Absence of Resonant Oscillating Fields

By adopting Mezger and Jaccard's method, we have optically determined the isotropic and anisotropic hyperfine (hf) parameters for the K nuclei at the fifth shell in the ground state of the F center in KCl at 4.2 K. We have found three additional facts: (1) the hf signal intensity is decreased with an increase of the frequency of the modulation field superimposed on the static magnetic field; (2) the spin-lattice relaxation time in the relaxed excited state is an important factor for the occurrence of the hf signal; (3) the hf signal intensity is independent of the excitation power, P , in the low magnetic field range. However, it is reduced with an increase in P of the high-power pulse laser.

Tunnelling processes in colour centres

The drop, at low temperature, of the relaxation time in the excited state of KI: Ag− system is interpreted by a vibrational relaxation using a classical description analogously to what done for theFA(II) centre in KCl: Li. In the present case, however, an important role is played by a tunnelling mechanism accounted for by the instanton-bounce method.

The Indirect Determination of the Cross Section of Stimulated Emission of the F Center in KCl

The intensity of the F center emission in KCl is reduced by simultaneous irradiation of 1.06 µm pulse from an Nd:YAG laser, whose second harmonic pulse is used for the F excitation. The reduction is related to the stimulated emission in the relaxed excited state of the F center. The cross section of the stimulated emission is estimated to be 4.0×10 -17 cm 2 at 80K.

EPR of a New O Centre in KCl

AbstractEPR results are given of a new O2− paramagnetic defect, formed following room temperature X‐ray irradiation in KCl, heavily doped in the melt with KNO2. The EPR spectra indicate that the O2− molecular ion has two equivalent equilibrium positions between which it jumps. Above 10 K the EPR spectrum of this molecular ion is orthorhombic, with principal axes [110], [001], and [110]. Below this transition temperature the principal axes are tilted by 1° with respect to the [110] and [110] directions in the (110) plane. A possible model for the perturbing impurity responsible for this tilt is proposed.

Absorption lineshape ofF A centres in alkali halides

The lineshapes of the absorption bands ofFA centres in alkali halides have been studied for the first time. The new method used for this investigation is based on the determination of the overlap between theFA1 andFA2 bands from luminescence measurements. The experimental results have been compared with calculated values deduced from the theoreticalFA bands of different shapes. For bothFA (I) centres in KCl: Na+ andFA (II) centres in KCl: Li+ and RbCl: Li+ the absorption lineshape at low temperature is much closer to a sum of two Lorentzian curves than that of two Gaussian or Poissonian bands. This result shows an unexpected difference with theF centres, whose absorption lineshape is known to be Poissonian at the same temperatures.

Off-center position of the Li+ ion in the FA center in KCl

The photostimulated reorientation of FA centers in KCl: Li+ is affected by the off-axis geometry of the dipoles and by the overlap between the FA1 and FA2 absorption bands at each pumping wavelength. A detailed model of such a process, so far successfully compared to experimental results of optical absorption and emission in stationary equilibrium conditions, has been applied to studies of transient effects. The results further confirm the value of the off-axis angle of FA (Li) centers in KCl.

Influence of a vVacancy on Luminescence of Pb+ Centres in Alkali Halides

AbstractLuminescence of Pb+ centres in X‐irradiated Pb2+ doped KCl, KBr, and KI crystals is first observed at 1.29, 1.21, and 1.14 eV, respectively. It is excited in the bands of KCl:Pb2+ at 4.93, 4.45, 3.70, 2.03 eV, of KBr:Pb2+ at 5.20, 4.55, 4.20, 3.20, and at 5.10, 4.30, 3.91, 3.52, 3.02, eV for KI:Pb2+. The effect is studied of the presence of the cation vacancy on both, the excitation and emission spectra of the Pb+ centre. Temperature, kinetic, and polarization characteristics of Pb+ and Pb+vc−‐centres in KCl:Pb2+ are investigated.

Hot luminescence in F center under one- and two-photon excitations

The spectra of hot luminescence from F center in KCl excited by one and two photons are theoretically investigated on the basis of a vibronic model. The depolarization spectra as well as the total emission intensity are calculated, and compared with experiments in good agreement. It is also shown that the non-Markov effect plays very important roles in the vibrational relaxation.

Spatial coherence in large-sample superflourescence of O -2 centers in KCl

Superflourescence resulting from O - 2 centers at temperatures below 30 K contained in an excitation volume with Fresnel numbers F > 1 exhibits characteristic interference patterns indicating self-organization of spatially coherent states. Experimental studies of this phenomenon are presented.

Hot luminescence and non-radiative transition of F centers in KCl and NaCl crystals

The hot luminescence from F centers in KCl has been measured at LNT and LHeT. At both temperatures the intensity shows exponential dependence on wave number shift up to 6000 cm -1 from the excitation energy, while the degree of polarization depends only weakly on the wave number shift in the same region. The possibility of the non-radiative transitions due to level crossing of ground and excited states has been studied in F centers in KCl and NaCl, by using photocalorimetric spectroscopy (PCS). The results show that the ratio of the non-radiative and radiative transition probabilities depends on photon energy of excitation and that level crossing is dominant for the non-radiative process in NaCl:F.

Large-sample effects in superfluorescence of O -2 centers in KCl

Superfluorescence is the spontaneous collective emission of coherent light from an ensemble of two-level systems which are initially all in the excited state. In this contribution we attempt to distinguish this phenomenon from standard laser activity as well as to indicate common features of both processes. We report on experiments with O - 2 centers in KCl and emphasize a number of novel experimental observations such as multicolor superfluorescence, transverse mode structure and superfluorescence-brightened laser activity.

Dynamical relaxation processes of optically excited F centers studied by the resonant secondary emission

The resonant secondary emission (RSE) and its linear polarization for F centers in KCl, KBr, KI and RbCl have been measured over the whole Stokes region at 80 K. The spectral shape of F-absorption and RSE including F-luminescence inn KCl and RbCl is described on the basis of a classical damping A 1g oscillator in an anharmonic adiabatic potential. A relaxation time of 2.4 × 10 -3 s is estimated for RbCl. Immediately after the resonant excitation, the linear coupling with the T 2g mode disappears because of a quadratic electron-lattice interaction which is induced by orbital expansion of the F electron. A fractional probability of the dynamical transition at the 2s-2p level crossing is estimated as 0.6. An anomaly of the F-luminescence band found at the lower energy tail can be related to the turning point of a classical damping oscillator.

Detailed model of the photostimulated realignment ofF FA centres in KCl:Li+

The photostimulated reorientation of theFA centres in KCl:Li+ is described by a simple model that takes into account the off-axis angle of the centres and the overlapping of the tails of the two absorption bands. With this model we have interpreted the dependence of the luminescence intensity on the polarization of the exciting beam. The fitting of the experimental data gives an off-axis angle of approximately 5° and an overlapping of the bands larger than that predicted for Gaussian line shapes.

Dynamics and electronic properties of the Tl+-perturbed Tl0(1) center in KCl, KBr, and RbCl as probed by resonant Raman scattering.

Inelastic light scattering of the ${\mathrm{Tl}}^{+}$-perturbed ${\mathrm{Tl}}^{0}$(1) center [denoted as ${\mathrm{Tl}}^{+}$${\mathrm{Tl}}^{0}$(1)] in KCl, KBr, and RbCl is resonantly excited in its two visible optical transitions. The absorption spectrum of ${\mathrm{Tl}}^{+}$${\mathrm{Tl}}^{0}$(1) shows some similarities with that of the laser-active ${\mathrm{Tl}}^{0}$(1). The Raman spectra obtained under excitation of the third optical band of ${\mathrm{Tl}}^{+}$${\mathrm{Tl}}^{0}$(1) contrast with those of ${\mathrm{Tl}}^{0}$(1) under ``F-like'' excitation by the spectral composition of the defect-induced first-order spectrum, and by the absence of a low-frequency ${\mathrm{Tl}}^{0}$(1)-like mode at about 30 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$. A behavior-type (BT) analysis of the polarized Raman intensities establishes an apparent ${C}_{4v}$ symmetry. The selective resonant enhancement of one diagonal component of the Raman tensor under excitation of the third optical transition of ${\mathrm{Tl}}^{+}$${\mathrm{Tl}}^{0}$(1) obscures the low defect symmetry. The latter shows up in the polarized Raman spectra obtained under excitation of the fourth optical band. These spectra do exhibit a low-frequency mode at about 25 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$, which is associated with the motion of one or both of the thallium nuclei. The analysis of this mode by means of the extended BT method for resonant Raman scattering establishes that it belongs to the totally symmetric representation of either the ${C}_{1h}$[010] or the ${C}_{2v}$[110] point group. The former defect symmetry corresponds to a weakly perturbed ${\mathrm{Tl}}^{0}$(1) center, while the latter is compatible with electron tunneling between two equivalent thallium nuclei.

Polarization Correlation of Resonant Secondary Emission from the F Center in KCl: Picosecond Time-Resolved Spectroscopy

Transient behavior of secondary emission from the F center in KCl is investigated in the picosecond time region at 77 K. Time response of resonant Raman scattering (RRS), hot luminescence (HL) and ordinary luminescence (OL) components is observed up to the Raman shift of 5600 cm -1 . The degree of linear polarization due to the RRS and HL components separated from the CL component is determined as a function of Raman shift. This result exhibits the large polarization correlation up to the onset region of OL and is in contrast to the large depolarization observed by the stationary excitation. Therefore, it is found that the depolarization in the onset region of OL observed by the stationary excitation is due to fate inclusion of the unpolarized CL component in the degree of polarization of HL, but not due to the 2p-2s nonradiative transition.