Emission In KI Research Articles

Effect of uniaxial stress on dynamics of electronic excitations in alkali halides

The effect of uniaxial stress supplied at various temperatures along the 〈100〉 crystal axis on the different stages of electronic excitation relaxation has been studied for several alkali halides. An increase in the intrinsic/extrinsic emission intensity ratio has been found and explained by the stress-induced increase of the self-trapping efficiency of electronic excitations and the corresponding decrease of their migration length, as well as by the stress-induced reduction of the efficiency of the non-radiative decay of electronic excitations. The increase of energy barriers between various self-trapped exciton configurations has been detected. The origin of the E x emission in KI and RbI crystals as well as of the fast ultraviolet emissions of localized excitons in CsI is discussed.

Decay of fluorescence emission of KI: Cu+

Lifetime and quantum yield measurements concerning the fluorescence of Cu+ impurity centre in KI were performed in the range 19–300 K. The emission mechanism was shown to involve two excited states in thermal equilibrium, one of them being metastable and lying close under the emitting level,i.e. according to the Pedrini model for Cu+ fluorescence in alkali halides.

Identity of Self-Trapped Exciton Configurations for the π Emission of NaI and the σ Emission of KI

The interrelation between the intrinsic emission bands of NaI and KI has been examined by observing changes of luminescence spectra and their decay profiles in mixed Na 1- x K x I crystals at 10 K. As x is increased from 0 to 1, the π emission band of NaI changes continuously into the σ emission band of KI. Furthermore, the decay component with a long lifetime (∼100 ns) diminishes, while the component with a short lifetime (∼1.0 ns) intensifies. An emission band changing into the π band of KI appears above x =0.95. It is concluded that the self-trapped exciton (STE) responsible for the π band of NaI corresponds to that for the σ band, rather than the π band, of KI. Lattice configurations of initial states for these emission bands are briefly discussed in connection with the recent off-center model of the STE.

Luminescence of KI:Br and RbI:Br

Emission spectra and decay times of luminescence in KI and RbI containing small amounts of Br - ions are investigated. The Br - ions induce a reduction of Ex emissionin KI, but an enhancement of it in RbI. The Ex emission in KI seems to be of extrinsic origin. The luminescence center associated with the Ex emission in RbI:Br is deduced to be an STE of RbI perturbed by nearby Br - ions. The states responsible for the Ex and π emission bands in RbI are discussed in relation to the model of on- and off-center STEs.

Relaxation Processes in the Triplet States of Self-Trapped Excitons in KI and KBr

Temperature dependence of lifetimes τ f (fast component) and τ s (slow component) of the π emission in KI and KBr has been measured precisely to investigate the decay kinetics of zero-field-split triplet sub-levels A u and B u of the self-trapped excitons. Spin-lattice relaxation rates W ab and W ba between the A u and the B u levels are determined directly from the observed values of τ s and τ f at various temperatures. In KI, the upward transition rate W ab was found to decrease while the downward one W ba approaches to a constant value in lowering temperature. On the other hand, both the upward and the downward ones in KBr decreases in lowering temperature. It is concluded that the one-phonon direct process is effective in KI, whereas a multi-phonon process is effective in KBr.

Time-resolved study of hot luminescence in KI:Se −2

Transient behavior of weak emission in KI:Se − 2, assigned to “hot luminescence”, has been studied at about 80 K by means of pico-second spectroscopy. It has been found that the decay time of the emission is shorter than 50 ps and the component with the short decay time spreads not only in the higher energy region of of the 0-0 line of the ordinary luminescence but also in the lower energy region of the line.

Fine structure in the emission spectrum of doped s2ions: KI:Sn2+

The broad emission of KI:Sn2+ has been recorded at 2 K between 2.15 and 2.25 eV in a standard photon-counting experiment under CW A-band excitation at 3.46 eV. The energy dependence of the photon-counting statistics were measured. Deviations from Poisson statistics exhibit several peaks on the high-energy side of the emission band with a spacing of 7.4(2) meV which is not present in the usual emission spectrum and can be explained as vibrational fine structure within the molecular approximation.

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.

Optical properties of FAand (F2)a centres in doped KI

Abstract Crystals of additively coloured KI doped with lithium or sodium exhibit, after irradiation with F-light, new absorption and emission bands due to associated and perturbed centres. The defects responsible for these bands have been identified using excitation spectra and polarization measurements. Fa(I) and (F2)a centres are found to be present in both KI:Li+ and KI:Na+ crystals. A weak emission in KI: Li+ is ascribed to FB(II) centres. The possibility of using these systems as laser active media is discussed.

Luminescence Quenching Due to the Dynamical Nonradiative Transition of F Center in KI Crystal

It has been observed that the quantum efficiency of the F center emission in KI near liquid helium temperature drops drastically at the high energy region beyond the F absorption band. This drop suggests that, when the F center is excited to a higher vibrational level than the crossing point of the excited state and the ground state, it has a large probability to make dynamical nonradiative transition to the ground state above the crossing point.

Symmetry andgvalue of the relaxed excited state of KI:TI+AXemission

We report the effect of a magnetic field applied along the $〈111〉$ and $〈100〉$ directions on the luminescent decay time of the ${A}_{X}$ emission of KI:T${\mathrm{I}}^{+}$. From the directional dependence we show that the ${A}_{X}$ emission arises from a trigonal Jahn-Teller (JT) well. The field dependence of the decay time also provides an estimate of the $g$ value of the emitting state, which we find as $g\ensuremath{\sim}1$. This is shown to be consistent with the emitting state being a JT minimum on the $^{3}T_{1u}$ adiabatic potential-energy surface in the presence of strong spin-orbit interaction.

Stimulated luminescence of KI single crystals

Luminescence is observed in KI with red and/or infrared stimulation after previous uv preexposure. The results indicate the onset of band-to-band transitions and provide evidence for the thermal dissociation of self-trapped excitons. A commonly observed emission in KI at 3.0 eV is observed through stimulation. A model for this emission is presented.

Non-radiative processes for complex potential surfaces: III. Magnetic circular polarization in the A T emission of KI:Tl +

To account for negative magnetic circular polarization in the A T emission of KI:Tl +, we have modified the potential model of Part II by inclusion of minima on the upper branch of the potential and assuming circular symmetry in the q ϵ, q ζ- space. The WKB approximation enters in this analysis for the evaluation of non-radiative transition rate in going from upper to lower minimum.

Determination of excited state symmetry and g-value from the magnetic field dependence of the radiative decay time: KI: In + and KI: Sn 2+

An expression for the decay time as a function of an applied magnetic field is derived for the particular cases of A-band emission in KI: In + and KI: Sn 2+. Application of this analysis to the measured field dependence gives the value of g ⊥ for the emitting level as well as the symmetry of the relaxed excited state.

Polariton effects in the resonant emission of KI and RbI

We have observed the narrow resonant emission in KI and RbI crystals under electron irradiation. Its properties as a function of the penetration depth of electrons in the crystal, as well as estimated lifetime and half-width, can be interpreted in the point of view of the polariton theory.

Magnetic field effect on the radiative decay time of A TΠ, A TΣ and the new red emission in KI:Sn 2+

We show that the magnetic field effect on the decay times of the three emission bands of KI = Sn ++ (excited at ≈ 370 nm) can be used to determine both the symmetry of the relaxed excited state and its g-value.

Edge Emission of Excitons in KI and RbI

The intensity of the edge emission in KI and RbI has been measured from 5 K to 100 K. The temperature dependence is interpreted in terms of a simple exciton decay model. Thermal quenching of the emission can be ascribed to the increase in the self-trapping probability of excitons. The edge emission is assumed to originate from a metastable free exciton state.

Effect of hyperfine — Structure interaction in the emission of KI:Tl

It is suggested that the hyperfine interaction plays an important role in determining the radiative lifetime or the slow component of the A X emission in KI:Tl. A theoretical estimate of such decay time shows very good agreement with experiment. Previous arguments, discarding the hyperfine interaction mechanism, are discussed.

Temperature dependence of the decay modes of the 420 nm emission in KI:Tl +

The time behaviour of the A X luminescence emitted from KI:Tl + crystals, after pulsed excitation in the A absorption band, has been studied in the temperature range 20–295°K. The obtained results show that the decay mode consists of three different slopes, each one characterized by a different value of the lifetime. On the basis of the available theoretical models it is possible to conclude that at least two radiative levels contribute to the A X emission.

Quadratic Jahn-Teller effect in the emission of KI:T1-type phosphors

The $^{3}T_{1\ensuremath{\mu}}$ and $^{3}A_{1\ensuremath{\mu}}$ adiabatic potential energy surfaces of ${\mathrm{T}1}^{+}$-type impurities in alkali halides have been computed taking into account the spin-orbit interaction and the linear and quadratic Jahn-Teller interactions with ${E}_{g}$ vibrational modes. Effects of ${T}_{2g}$ modes are assumed to be negligible. The inclusion of the quadratic effect leads to tetragonal nondegenerate $T$ minima with underlying trap levels, besides $X$ minima and their trap level already predicted for the linear Jahn-Teller effect. This level scheme is required in order to explain many experimental features concerning ${A}_{T}$ and ${A}_{X}$ emissions.