Exciton In KCl Research Articles

Luminescence properties of KCl:Ag − crystals excited near the fundamental absorption edge

Luminescence properties of KCl single crystals doped with Ag − centers have been investigated under various excitation energies around the fundamental absorption edge at low temperatures. Under the excitation at 6.89 eV, which is lower than the intrinsic exciton energy by 0.87 eV, the A ′ luminescence band due to the intraionic transition in the Ag − ion is dominantly observed at 2.91 eV. On the other hand, the excitation at 6.66 eV induces a broad luminescence band at 2.60 eV in addition to the A ′ luminescence band. From the comparison with the localized excitons in KCl:I crystals, the 2.60 eV luminescence band is attributed to the two-center type localized exciton related with the Ag − ion. The adiabatic potential energy surfaces of the excited states in the Ag − center and the localized exciton in KCl:Ag − are discussed.

Infrared transient absorption and electronic state of localized self-trapped excitons in KCl:I.

The infrared transient absorption and electronic state of the on-center-type self-trapped exciton (STE) localized at an iodine dimer in KCl:I were investigated. The polarization properties and vibronic structure of the absorption band indicate that the electronic state of the on-center-type STE is rather extended so that the splitting of the 2p-like sublevels is very small.

Time-Resolved Luminescence Study of Relaxed Excitons in KBr:I and KCl:Br

Luminescence from relaxed excitons associated with guest halogen dimers was investigated for KBr:I and KCl:Br through time-resolved measurements of emission and excitation spectra using synchrotron radiation pulses from UVSOR under single-bunch operation. A weak slow decay component was found in addition to a main fast one in the 4.89 eV band in KCl:Br, in contrast to the 4.35 eV band of KBr:I which consists of a weak fast decay component and a main slow one. The fast and the slow decay components are assigned to transition from the singlet and the triplet relaxed excitons in the on-center configuration. It is suggested that the adiabatic potential energy surface for the triplet state, as well as that for the singlet state, must have a local minimum at the on-center configuration but that the triplet exciton in KCl:Br relaxes preferentially into the off-center configuration.

Relaxation of localized excitons in KCl:I and RbCl:I

Luminescence from KCl:I and RbCl:I has been investigated. It is found that two different types of relaxed exciton states coexist with a potential barrier between them. The one is (ICl - + e)-state of two- center type. The other is one-center type, where an electron-hole pair is located mainly on the site of central iodine ion.

Luminescence of Localized Excitons in KCl:I at Low Temperatures

Temperature dependence of the intensities of 5.88 eV ( Near-Edge ) and 2.64 eV ( Blue-Green ) emission bands in KCl:I has been investigated in crystals containing various amount of iodine impurities under excitation into each of three absorption bands due to isolated I - ions. Localized excitons created on the I - monomer sites have two relaxed states, NE-state ( one-center type ) and BG-state ( two-center type ). It is confirmed that there is a thermal activation process from the NE-state to the BG-state. Fraction of the relaxation rate into the BG-states, γ BG , is found to increase with increasing iodine contents. This means that the relaxation mechanism of the localized exciton is influenced markedly if another iodine ion is present in the vicinity of the localized exciton.

Evolution of Frenkel pairs from excited states of self-trapped excitons in alkali halides

Abstract We find that the transition to the first and second electron excited states from the lowest self-trapped excitons in KCl and RbCl are π- and σ-polarized, respectively. For KCl the orientation of the H centers evolved from the self-trapped exciton by exciting to these states are shown to be in parallel to the orientation of the self-trapped excitons.

Non-Radiative De-Excitation of Highly Excited Self-Trapped Excitons in Alkali Halides: Mechanism of theFandHCentre Production

We calculate the rates of non-radiative transitions among the excited states of the self-trapped exciton in KCl. The transition matrix elements are calculated using the same semi-empirical molecular orbital method (CNDO) as in our earlier calculations of the energy surfaces and radiolysis mechanism. The results are consistent with current experiments. One important result is that the non-radiative decay of the lowest electron-excited state of the self-trapped exciton proceeds more rapidly to the lowest hole-excited state than to the ground triplet state. Transfer of excitation energy from the electron to the hole (and hence the production of F and H centres) can proceed rapidly.

F-Center Formation at Highly Excited Triplet States of Self-Trapped Excitons in KCl

Measurements have been made of the creation of the F centers by the optical excitation of the self-trapped excitons with a light pulse from a tunable dye laser. It is found that the ratio of the F -center concentration created by the light pulse to the triplet exciton concentration at the incidence of the light pulse depends on the integrated photon flux \(\varPhi\) as \(a\{1-\exp(-b\varPhi)\}\), where a and b are constants. Kinetic models are developed for treating the non-radiative processes of the highly excited self-trapped exciton, and the efficiency of the F -center creation from the highly excited triplet exciton was determined using the experimentally obtained parameters a and b . It is found that the efficiency is almost constant between 1.77 and 2.08 eV above the lowest triplet exciton and becomes higher above 2.58 eV. The dependence of the efficiency on energy is discussed in terms of existing models of the defect formation.

A semiempirical calculation of the axial shift of the self-trapped exciton in KCl

A method of calculating electric-field gradients is given for the off-centred self-trapped exciton in KCl, first observed by Block, Wasiela and Merle d'Aubigne (1978). The method is based on the model for defect formation in alkali halides described previously and closely follows the treatment by Gourary and Adrian (1960) for F-centres. Using this expressions obtained for the electric-field gradients and the observed values of the quadrupole interaction parameters for the two central chlorine ions, the magnitude of the axial displacement is calculated to be 0.22 Bohr radii.

Off-centre configuration of the self-trapped exciton in KCl

Based on the results of ENDOR measurements it was proposed that the self-trapped exciton in KCl is displaced from the centrosymmetric position in the lattice along the (110) direction. Using the same model presented earlier for defect formation in alkali halides, it is shown that axial shift of the self-trapped exciton is very likely to occur. The magnitude of the axial displacement is estimated from the experimental values of the quadrupole interaction parameters.

ENDOR of the self-trapped exciton in KCl

ENDOR of the self-trapped exciton in KCl is studied using the optical detection technique. The contact interaction with the neighbouring nuclei is very similar to that of the F centre in its ground state which indicates that the radial extent of electronic wavefunctions are similar in the two cases. The quadrupole resonance of the two central halogen nuclei was also detected, observation of two different quadrupole splittings indicates that the self-trapped exciton is slightly displaced from the centrosymmetrical position between two lattice sites.

Electronic structure of the self-trapped exciton in the potassium halides

We present pseudopotential calculations of the energy levels and wave-functions of the electron associated with the self-trapped exciton in KCl, KBr and KI. Accurate values are obtained for the separation of the singlet and triplet luminescent bands, for the electronic transitions, and for the spin resonance linewidths. Lattice distortion is shown to be particularly important in KCl. Additional low-energy electronic transitions, at 0.7 eV in KCl and 0.94 eV in KBr, are predicted, but have not been seen experimentally.

Luminescence of the self-trapped exciton in KCl

Recent data on the luminescence of the self-trapped exciton in KCl are analysed, and a theoretical description is given of the temperature-dependence of the intensity, polarisation and lifetime of the emission. We conclude that emission is seen from both the lowest triplet ψT and the corresponding singlet ψT, and that recombination from both these states can occur radiatively or non-radiatively. In the processes in which an electron is captured by a VK centre and decays to these lowest singlet and triplet states we conclude (i) that when capture is initially into a triplet state the lowest state ψT is reached in almost all cases, (ii) that ψS is only populated via ψT, i.e. that when initial capture is into a singlet state recombination is almost certain to occur before ψS is reached, and (iii) during the decay processes following capture, some reorientation of the self-trapped hole can occur.

Electronic Structure of Excitons in KCl

The electronic structure of exciton in KCl is analyzed using the valence band structure and wave function of Howland and the effective mass and the wave function of the conduction band calculated by the authors. The effective mass formalism gives 1.1 and 0.28 eV for the binding energies of 1 s - and 2 s -excitons, respectively. The effect of valence band warping is found to make the J =2 states to interact with the J =1 states and thus to reduce the spin-orbit splitting. This effect is small in the case of \(\varGamma_{1}\)-exciton in KCl because of small effective mass of the conduction electron. Short range electron-hole exchange interaction estimated from the band wave functions gives a reasonable ratio of intensities of higher and lower energy components for both n =1 and n =2 halogen doublets. Also calculated spin-orbit splittings for n =1 and n =2 states are in good agreement with experiment. The total oscillator strength of this doublet is found to be 0.15.

Absorption Band Shape and Urbach's Rule of Localized Excitons

An empirical description of the shape of optical absorption bands due to localized excitons in ionic crystals is proposed. The new formula links a nearly Gaussian description of the band near its maximum with an exponential description of the band on the low-energy tail (Urbach's rule). The applicability of the proposed shape formula over a range of 4\textonehalf{} orders of magnitude of absorption constants is demonstrated for localized excitons in KCl. The theoretical background which led to the proposed formula suggests that the existence of an exponential tail (Urbach's rule) is indicative of a predominantly quadratic interaction of certain lattice modes with the exciton formation.

Electronic Structure of the Exciton I. Localized Exciton in KCl

The present situation of the exciton problem is discussed briefly. The localized excitons of KCl in both optical and X-ray absorption have been worked by using Wannier-Slater's equation for the perturbed lattice, which may be considered to essentially correspond to the quantum-analogue of von Hippel's semi-classical model of the exciton. The perturbing potential fields in Wannier-Slater's equation are evaluated numerically by the use of Hartree's data and then the best-fit analytical functions are constructed with fair accuracy. The solutions of the equation are worked by the variational method. The result for the optical exciton is observed to be in fair agreement with the experiment. As for the X-ray exciton, the quantitative comparison with our results can hardly be carried out at present, since the position of the bottom of conduction band can not be measured accurately on the X-ray absorption spectra. On the basis of our results for X-ray exciton, however, some discussions on the characteristic struc...