Centers In KI Research Articles

Luminescence efficiency ofFcenters in KI studied by lifetime measurements

The luminescence efficiency of isolated $F$ centers in KI was studied as a function of the excitation energy by measuring the lifetime for ordinary luminescence. The emission intensity measured under a pulse excitation was found to obey nonexponential decay. The luminescence efficiency decreases to below unity when the excitation energy exceeds 1.96 eV, which corresponds to the slope position at the higher energy side of an $F$ absorption band. The threshold value for the luminescence efficiency is lower than that previously obtained from cw measurements but close to that predicted theoretically by Leung and Song. We have traced the origin of this difference by performing experiments based on the cw method and found out the reason. It is shown that there is a relationship between quantities obtained by the cw method and those obtained by our lifetime measurement. Our experimental data are thoroughly analyzed using a model for the radiationless process of Leung and Song, which is based on a $1s\text{\ensuremath{-}}2p$ model. A $1s\text{\ensuremath{-}}2p\text{\ensuremath{-}}2s$ model is also discussed.

Persistent nuclear wave packet oscillation coexistent with incoherent vibrational population at excited F centers in KI

We investigated nuclear wave packet dynamics in the excited state of KI F centers at 10 K using time-resolved luminescence spectroscopy. Observed transient spectrum is divided into oscillatory and non-oscillatory components. The former lasts over 11 ps without appreciable damping and is attributed to the oscillation of the wave packet consisting mainly of the A(1g) mode around the center. The non-oscillatory part rises quickly after photo-excitation exhibiting a cooling of incoherent vibrational population. This behavior suggests the fast energy dissipation due to the dephasing of the bulk phonon modes.

Luminescence of excitons and Eu2+vc– centres in KI:Eu2+ crystals

AbstractLuminescence characteristics of KI:Eu2+ crystals are studied at 11.5–300 K under excitation in the 3.0–6.3 eV energy range. Besides the known 2.88 eV emission of Eu2+vc– centers, the new emission bands peaking at ∼3.75 eV, ∼3.3 eV, 3.06 eV, and 2.38 eV are found. The bands, located in the 3.1–4.3 eV energy range and strongly distorted due to their overlap with the Eu2+vc– absorption bands, are ascribed to the self‐trapped exciton emission. The 3.06 eV and 2.88 eV bands are ascribed to the electronic transitions from two close levels of the splitted T2g excited state of an Eu2+ ion. The 2.38 eV emission is ascribed to the radiative decay of an exciton localized near an Eu2+ ion. It is assumed that the appearance of the self‐trapped exciton emission under excitation in the Eu2+‐related absorption bands is connected with the radiative decay of excitons, which are formed at the recombination of electrons and holes, produced in the crystal as a result of the photostimulated charge transfer from the Eu2+‐perturbed I– states. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Luminescence of KI: Pb Crystals

AbstractSpectral, polarization, and kinetic characteristics of triplet luminescence of Pb2+vc− centres in KI:Pb are studied in the temperature range from 4.2 to 250 K. It is suggested that thermally stimulated T → X transitions as well as the transitions between T or X minima of various orientations can explain the temperature dependences of the intensities and the polarization degrees of luminescence (near 70, 30, and 130 K, respectively). It is concluded that in spite of a strong spin‐orbit coupling in the KI:Pb crystal, the Jahn‐Teller effect can be considered as the main interaction in the relaxed excited state (RES) of Pb2+vc− centres. A considerable influence of spin‐orbit interaction on the structure of RES was also detected.

Photostimulated desorption studies on potassium iodide

Abstract In this work we present the results of photostimulated desorption measurements obtained on KI under excitation in the fundamental absorption region. Yield curves of both potassium and iodine desorbed atoms are reported at several sample temperatures. We discuss the experimental results in terms of the optical constants and of the production efficiency of F and H centers in KI.

Thermally activated non-radiative decay process in KI: Se -2

The relaxation process of the optically excited state of the Se- 2 center in KI has been investigated by the measurements of the luminescence intensity and its decay time as a function of temperature . It has been found that the decay time of the luminescence is 85 ns at 2 K. It starts to decrease around 50 K and becomes about 2 ns at 100 K. The decrease of the luminescence intensity is similar to that of the decay time for the change in temperature. The temperature dependence of the luminescence intensity and its decay time is interpreted in terms of thermally activated non-radiative decay with activation energy Δ E=53 meV.

Selective two-step photoionization of F-aggregate centers using 106-μm radiation

A two-step photoionization of several potentially laser-active F-aggregate centers is observed in additively colored crystals by using high-intensity 1.06-microm pulses from a Q-switched Nd:YAG laser. The centers produced include the lithium (F(2)(+))(A) center in KI, which has lased with tunability from 2.4 to 4.0 microm, and N(1) (+) centers in KBr, which show promise of lasing. Steady-state concentrations of ionized (F(2)(+))(A) and N(1) (+) centers are found to be considerably larger than those produced by using blue or near-ultraviolet excitation.

Luminescence polarization of dimer centers in KI:Tl+ crystals: Analysis of experimental data and structure of dimers.

Luminescence polarization of dimer centers in KI:Tl+ crystals: Analysis of experimental data and structure of dimers.

FA centers in additively colored KI:Li crystals.

Optical properties are reported for ${F}_{A}$ centers produced in KI:Li additively colored under nitrogen-free conditions. Contrary to previous studies for KI:Li, these centers exhibit two distinct absorption bands centered at 0.673 and 0.733 \ensuremath{\mu}m. Furthermore, their optical properties are consistent with type-I ${F}_{A}$ centers. The elimination of nitrogen at each stage of crystal preparation was essential to the optimum production of these centers in KI:Li.

StimulatedC'-emission of Ag?-centers in KI, RbBr, and CsBr

Stimulated emission around 335 nm in theC' emission band of Ag− centers in KI, RbBr, and CsBr has been observed at low temperatures and the gain coefficient in the band maximum has been determined. Laser action has been demonstrated using a simple broad band cavity.

Theoretical study and assignment of absorption bands of dimer centers (Tl+)2 in KI.

The absorption bands of (${\mathrm{Tl}}^{+}$${)}_{2}$ dimer centers in KI are investigated theoretically by means of an extension of the study which Honma performed on the electronic states of the paired-${\mathrm{Ag}}^{\mathrm{\ensuremath{-}}}$-ion center. We have considered the mixing among the lower and upper triplet states and between triplet and singlet states owing to the value of the spin-orbit coupling constant (\ensuremath{\zeta}=0.51 eV), which is comparable to the minimum energy separation (\ensuremath{\Delta}${E}_{\mathrm{min}\mathrm{\ensuremath{\simeq}}0.45}$ eV) between the states concerned.

Picosecond configuration relaxation in the1 T 1u -state of Ag?-centers in KI

The relaxation of the optically excited Ag−-center in KI is investigated by measuring the risetime of the spontaneous emission from the relaxed excited state. The energy and number of phonons created in the relaxation process are determined.

Decay Kinetics and Polarization of the Ax Emission of KI:In Crystal

AbstractThe decay kinetics and polarization of the AX emission are investigated for In centres in KI:In at 0.35 to 80 K. The parameters of the relaxed AX state are determined. Good agreement is obtained between the experimental results and theory, which suggests a strong interaction of impurity optical electrons with non‐totally symmetric vibrations of the crystal lattice.

On “the Anomalous Effect” of the Magnetic Circular Polarization of the F Center Luminescence

When the pumping light is modulated between right and left circular polarization, the magnetic circular polarization of F center luminescence observed after passing through a linear polarizer has been called “the anomalous effect”. We measured this effect for the F center in KI as a function of the magnetic field H and the photon energy. These experimental data are explained by deriving a form which includes the parameter ( e I ) for the unbalancing of circular polarization in pumping light and the spin-lattice relaxation rate of the relaxed excited state ( T 1 * ) -1 . The H -dependence of ( T 1 * ) -1 is presented.

Geometrical structure of(Tl+)2luminescent center in alkali halides: Presence ofD2handD4hdimer centers

Absorption, emission, and excitation spectra of the ${({\mathrm{Tl}}^{+})}_{2}$ center in KI, KBr, and KCl have been investigated and compared with the excitation spectrum of the ${({\mathrm{Au}}^{\ensuremath{-}})}_{2}$ center with ${D}_{2h}$ symmetry in KCl. Dichroism produced in the ${\mathrm{Tl}}_{2}^{+}$ absorption band after bleaching with polarized light has also been investigated in $\ensuremath{\gamma}$-ray-irradiated KCl:${\mathrm{Tl}}^{+}$. From these results and from the magnetic-circular-dichroism measurements, it is shown that the ${({\mathrm{Tl}}^{+})}_{2}$ absorption bands observed thus far arise from a ${D}_{2h}$-symmetry ${({\mathrm{Tl}}^{+})}_{2}$ center. An additional absorption band called ${A}^{\ensuremath{'}}$, which has been found at a position close to the peak position of the $A$ band, produces an emission band which is different from the previously observed ${({\mathrm{Tl}}^{+})}_{2}$ emission bands. From a systematic investigation of the separation between the ${A}^{\ensuremath{'}}$ and $A$ bands in various alkali halides, it is shown that the ${A}^{\ensuremath{'}}$ band arises from a ${D}_{4h}$-symmetry ${({\mathrm{Tl}}^{+})}_{2}$ center.

Polarized At Emission of In+ Centres in KI:In

AbstractSpectral, polarization, and kinetic characteristics are investigated at 4.2 to 300 K for the blue luminescence of KI:In crystals with various In+ concentrations. It is established that the At emission band of In+ centres is elementary and its decay consists of only one component. The At emission is polarized only at excitation in the B and A absorption bands, the polarization degree is positive and grows as the energy of exciting photons decreases. The doublet structure of the At emission band as well as the second slower decay component which shows in KI:In with large In+ concentration a large negative polarization on excitation in the C band at 80 K, are caused by the presence, besides of In+, of some anisotropic indium centres in the crystal. Their emission is shifted by about 0.04 eV to lower energies with respect to the At emission of In+ centres.

Decay and polarization of luminescence pulses in the 440 nm emission band of KI:In

Model suggestions for the impurity luminescence center in KI:In crystals from the literature are found to agree with experimental results obtained by excitation of KI:In at 80 K with polarized light pulses in the A, B and C impurity absorption bands. By excitation in the C absorption band the population of the metastable M state goes not only via A state, but directly from the C state too.

Continuous tuning of a color-center laser between 2 and 4 μm

Efficient laser action is reported for the (F(2)(+))(A) center in KI with tunability now extending from 2.38 to 3.99 microm. For the F(2)(+) and (F(2)(+))(A) centers coexisting in KI, the combined lasing range is 1.98-3.85 microm. The 1.73-microm line of a pulsed Er:YLF laser pumped both centers simultaneously.

Luminescence of color centers in KI:Li +

New luminescence bands have been observed in KI:Li + excited in the F band region. As suggested by the dependence of the bands upon the Lithium content of the samples, these bands are due to perturbed F and M centers (F A, F B and M A centers). The excitation spectrum, the wavelength and the polarization of the emission indicate that the 1.75 μm band is due to the M A center and the 2.08 μm band to the F A (I) center. The band at 2.88 μm is tentatively assigned to the F B (II) centers.

Broadly tunable laser action beyond 3 μm from (F_2^+)A centers in lithium-doped KI

Laser action has been achieved by using lithium (F(2)(+))(A) centers in KI. The laser is continuously tunable from 2.59 to 3.65 microm; the 1.73-microm line from a pulsed Er:YLF laser is used as pump source.