Self-trapped Excitons In Alkali Halides Research Articles

Hole burning on the time-resolved optically detected ESR spectra of self-trapped excitons in alkali halides

The homogeneous and inhomogeneous line widths of the ESR spectra of self-trapped excitons (STEs) in KBr have been investigated using the methods of time-resolved optically detected magnetic resonance (ODMR) and hole burning. The line shape of each partially resolved hyperfine line is the Gaussian function whose width is between 13 and 15 mT (FWHM). The homogeneous line width is estimated to be ⩽0.2 mT from the microwave power dependence of the ODMR signal intensity. In contrast, from the hole-burning experiment, we acquired knowledge of the broad hole whose shape is the same as that of the single hyperfine line. We suppose that an initially narrow burned hole becomes broad within a shorter period less than millisecond by so-called spectral diffusion.

Self-trapped excitons in pure and Na- and Tl-doped caesium halides and the recombination luminescence

On the basis of a method used earlier to predict the off-centre relaxed triplet self-trapped excitons in alkali halides, similar systems in caesium halides are studied. This work confirms the expected off-centre relaxation along the [100] cubic axes with increasing magnitude in the order CsI, CsBr and CsCl. The calculated emission energies are in reasonable agreement with observed values. The well-known 2.95 eV emission band of CsI:Na has been studied as a tunnelling recombination between a close pair consisting of a VK centre and a Na atom. For a number of close-pair geometries the emission energies are close to 3 eV. The strong emission bands of CsI:Tl at 2.25 eV and 2.55 eV have been interpreted as arising from tunnelling recombination of close pairs each consisting of a Tl0 and a VK centre. The calculated emission energies and polarizations are discussed in conjunction with the experimental data.

Configuration of the self-trapped exciton in alkali halides

Vinti sum-rule analysis of the optical absorption of self-trapped excitons (STEs) in alkali halides reveals two STE classes: (1) Large excitons with electron orbits encompassing many ions. The STE radii correlate roughly with the dielectric constant as in the Wannier model. (2) Small excitons with orbits less than the lattice spacing. The radii are of the same size as F centers and show the same linear dependence on lattice spacing. This absorption-based classification mirrors the luminescence-based scheme of Kan'no et al. The classes correspond to on-center and off-center STEs, respectively.

Parity-broken and -unbroken self-trapped excitons in alkali halides

Abstract

Three Band Theory for Three Types Self-Trapped Excitons in Alkali Halides

Self-trapped excitons (STE) in alkali halides are theoretically studied by the following three band model. A hole moves about p -orbitals centered at anions. While an electron moves over vacancy orbitals centered at regular anion sites as well as s -orbitals of cations. Thus, the electron, initially in a s -orbital, can fall into the vacancy orbital, when an anion is removed from its lattice site. Only the iodine series is studied, and we have found three types of minima, A, B and C in the adiabatic potential energy surface of the STE. The type A is a one-center STE with even parity, while B is a deformed V K + e , and C is a charge-separated STE. The type A is always the highest state, independently of the alkali atom. While, relative stability between the B and the C depends on the size of alkali ion. When it is small the B is the lowest, and the C is absent. As the size increases, the C appears as well as the B and becomes the lowest.

Metastable one-halide self-trapped excitons in alkali halides

Fast (τ < 2 ns) short-wavelength emissions have been detected overlapping the continuous intraband luminescence on exciting KBr, RbBr, KCl and RbCl crystals by an electron beam. These emissions are attributed to the radiative decay of metastable one-halide self-trapped excitons. The weak 5.8 eV emission can effectively be excited in a KBr crystal in the region of the formation of excitons (6.73-7.3 eV).

Correlation between the Spin State and Structure of Self-Trapped Excitons in Alkali Halides

In order to clarify the origin of the preferential relaxation of singlet self-trapped excitons (STEs) into the on-center configuration in alkali halides, we have studied variation of exchange splitting energy along the relaxation path from the on-center to the off-center configurations using a simple phenomenological model. Estimated exchange energy is several tens of meV both at the on-center and off-center configurations, and takes its maximum in between. It provides a larger potential barrier intervening between the on- and off-center minima for the singlet STE than that for the triplet STE. This will inhibit the singlet STE to be relaxed into the off-center configuration.

Stable geometries of the self-trapped exciton in alkali halide crystals

According to recent works, the self-trapped excitons (STE) in alkali halides are grouped in three different types. In some (e.g. RbI), a population transfer is observed among coexisting types as temperature is raised. In others (NaBr and NaI) extra type emission bands have been observed under dilational strain. We studied the adiabatic potential energy surface (APES) of the STE under the effect of expanded or compressed lattices, and of rotation of the Vk core in several directions, based on both extended-ion and ab initio Hartree-Fock methods. A critical study of the zero field splitting parameter D of the spin Hamiltonian is made in assessing the effect of rotation. It is shown that: (a) the potential barrier separating the adjacent local minima on the APES becomes smaller as the lattice is dilated; (b) the APES associated with rotation of the halogen molecule-ion axis from [110] toward [001], upto about 30°, is flat; Rotations in other directions are stiff, however; (c) the observed anomaly of the D parameter in NaCl is compatible with the rotation described in (b) above. The geometries of the three types are discussed on the basis of this study.

Resonance-Raman-scattering spectroscopy of the self-trapped excitons in alkali halides.

We have studied resonance Raman scattering tuned to the ${\mathrm{\ensuremath{\sigma}}}_{\mathit{g}}$\ensuremath{\rightarrow}${\mathrm{\ensuremath{\sigma}}}_{\mathit{u}}$ hole transition of the self-trapped excitons (STE's) in alkali halides. The Raman spectrum is characterized by a sharp line of 137 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ in NaBr, 361 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ in NaCl, and 139 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ in RbI. Each line has been assigned to the stretching vibration of the halogen molecular ion (${\mathit{X}}_{2}^{\mathrm{\ensuremath{-}}}$) composing the STE. Comparison of the Raman frequencies with those of ${\mathit{V}}_{\mathit{K}}$ and H centers obtained by similar measurements indicates that the frequency of the STE in NaBr coincides with that of the ${\mathit{V}}_{\mathit{K}}$ center, while those in NaCl and in RbI are almost identical to the frequencies of the H centers. These results provide evidence that the ${\mathit{X}}_{2}^{\mathrm{\ensuremath{-}}}$ of the STE in NaBr occupies two adjacent halogen sites and the ${\mathit{X}}_{2}^{\mathrm{\ensuremath{-}}}$ of the STE in NaCl and in RbI occupies a single halogen site.

Model of self-trapped excitons in alkali halides.

We have carried out an ab initio many-electron variational calculation of the adiabatic potential-energy surface (APES) for the lowest triplet state of the self-trapped exciton (STE) in LiCl, NaCl, and KCl. Calculations of the H center in these crystals show that the 〈111〉 orientation is favored, in agreement with experimental results for NaCl but not for KCl, in which it is oriented along a 〈110〉 direction (no measurement exists for LiCl), and hence most detailed calculations for STE's are carried out for NaCl. It is found that the APES minimum for each crystal occurs when the ${\mathrm{Cl}}_{2}^{\mathrm{\ensuremath{-}}}$ molecular ion is displaced along its molecular axis from its symmetrical position (${\mathit{D}}_{2\mathit{h}}$) nearly halfway to the nearest halogen lattice point. The calculated transition energies for the optical absorption and luminescence at this configuration agree with the experimental values for the triplet STE, although the calculated stretching vibration frequency of the ${\mathrm{Cl}}_{2}^{\mathrm{\ensuremath{-}}}$ molecular ion in NaCl is much smaller than that for the H center, contradictory to recent resonant Raman studies. Other minima are found at the nearest F-H pair configuration, in which the ${\mathrm{Cl}}_{2}^{\mathrm{\ensuremath{-}}}$ molecular ion is reoriented by 90\ifmmode^\circ\else\textdegree\fi{} from the initial orientation and next-nearest F-H pair. Extremely small luminescence energy at these configurations excludes the possibility that they are the candidates for the luminescent state of the STE. It is found that, after the displacement of the ${\mathrm{Cl}}_{2}^{\mathrm{\ensuremath{-}}}$ molecular ion beyond the first minimum of the APES towards the nearest F-H pair configuration, the total energy is lowered by reorientation, inducing an anomaly on the APES. The results of a recent experimental investigation, including existence of several types of relaxed configuration of the STE in alkali halides, the stretching vibration frequency, and the femtosecond oscillation on APES, are discussed on the basis of the results of the calculation.

Resonance Raman scattering of the self-trapped exciton in alkali halides.

We have studied resonance Raman scattering of the self-trapped exciton (STE) in NaCl tuned to the ${\mathrm{\ensuremath{\sigma}}}_{\mathrm{g}}$\ensuremath{\rightarrow}${\mathrm{\ensuremath{\sigma}}}_{\mathrm{u}}$ hole transition and to the 1s\ensuremath{\rightarrow}2p electron transition. The former spectrum is characterized by a sharp line at 366 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$, which coincides with the stretching-vibration frequency of the H center, while the latter is characterized by broad bands, similar to that of the F center, situated below the LO phonon frequency. The result provides evidence that the STE is essentially an interacting F-H pair.

Mollwo-Ivey relation for the electron transition energy of the self-trapped excitons in alkali halides.

We investigate the dependence on the interionic distance R of the transition energy \ensuremath{\Delta}E of the electron from 1s to 2p states of the self-trapped exciton (STE) for alkali halides. It is found that the \ensuremath{\Delta}E-vs-R plots, except for NaBr and NaI, fall in two distinctive straight lines, showing that the STE in alkali halides can be divided into two groups. This grouping is found to be in accordance with the grouping in terms of the Stokes shift presented by Kan'no, Tanaka, and Hayashi. The relaxed configuration of excitons in alkali halides has been discussed on the basis of these results.

A new model for the self-trapped exciton in alkali halides

The authors report a quantum mechanical study of the structural and spectroscopic properties of the self-trapped exciton in LiCl. Their method interfaces the quantum cluster calculations on a Li10Cl4 cluster with a Mott-Littleton treatment of the relaxation of the surrounding lattice. Calculations on the Vk centre give structures that are in very good agreement with Mott-Littleton calculations, and yield optical absorption energies that compare well with experimental values. Their study of the triplet ground state of the self-trapped exciton reveals a minimum energy structure of C2v symmetry caused by a small of f-centre displacement (of approximately=0.07 AA) of the Cl2- (Vk) ions which comprise the hole component of the exciton. The electron is in a largely delocalized state around the hole with both components localized more on the Cl ion that is displaced towards the perfect lattice site. The magnitude of the off-centre displacement is much less than in the earlier studies of Song et al. (1990). A key feature in the success of these calculations was the self-consistent polarization of the surrounding lattice to the quantum cluster.

A study of the structure of the self-trapped exciton in alkali halides by ab initio methods

Hartree-Fock calculations have been performed for various possible geometries of the self-trapped exciton (STE) in several alkali halide crystals. The computations treat a four- to twelve-ion cluster embedded within an ionic lattice representation that may or may not respond self-consistently to the charge distribution in the cluster. Both the triplet and open-shell singlet excited states of the exciton are considered. The energy of each state decreases as the molecular component of the exciton moves from the on-centre D2h symmetry to an off-centre C2v symmetry site. This distortion is accompanied by a separation of the electron and hole components of the exciton leading to a nearest neighbour F-H pair. These findings are in general accord with earlier analyses and one-electron calculations presented by K.S. Song and co-workers (1989).

High-pressure spectroscopic studies of free and self-trapped excitons in alkali halides at low temperatures

Abstract A brief survey of our studies of free and self-trapped excitons (FE and STE) in alkali halide crystals under hydrostatic pressure up to 12.5 kbar at 4.2–140 K is presented. Main attention is paid to the following effects observed: (1) the strong coupling of three energy levels of FE in CsI revealing itself as an exciton analog of pressure-scanned Fermi resonance; (2) emergence of a new emission band of STE in CsI under pressure; (3) a large pressure shift of the thermal quenching curve for STE emission in NaCl.

Optically-detected magnetic resonance of intrinsic and impurity centers in ionic crystals

The methods of optical detection of electron spin resonance (ODESR) and electron nuclear double resonance (ODENDOR) which can be applied to defects with strong electron-phonon coupling having broad absorption and emission bands are briefly reviewed, in particular the methods using the magnetic circular dichroism of the absorption (MCDA) and the magnetic circular dichroism of the emission (MCPE). Results of ODESR and ODENDOR measurements obtained for intrinsic defects such as F centers or self-trapped excitons in alkali halides and impurity centers such as transition metal ions, heavy metal ions, and rare earth ions in various ionic crystals are briefly described. It was attempted to include all work in this field since the beginning of ODESR spectroscopy in the early sixties.

A theoretical study of the pi -luminescence from self-trapped excitons in alkali halide crystals

It is shown that the equilibrium axial shift of the Vk core of the self-trapped exciton (STE) in alkali halides is correlated with the lifetime and Stokes shift of the pi -emission band and the binding energy of the STE. A configuration coordinate diagram for the STE in KC is constructed which is consistent with existing data and accounts for the large Stokes shift and large binding energy of the STE. A detailed comparison is made between the configuration coordinate diagrams based on the off-centre (F-H pair) model and the on-centre (Vk+e) model of the STE. Using a previously developed extended-ion program package coupled with a CNDO code, the pi -emission band peak energies and the binding energies of the STE are calculated for 12 alkali halides. The results are in general agreement with experiment.

Modelling of the Self-Trapped Excition π Luminescence in Alkali Halides

On the basis of recent works which show that the self-trapped exciton in alkali halides and other ionic halides are essentially a nearest neighbour pair of F – H centers, we show that the emission band peak energy and lifetime of the π band in pure and heavier halogen doped alkali halides are correlated to the off-center shift of V k -core. It is suggested that parameters of the π emission bands can be predicted on the basis of this correlation.

Energy competition of self-trapped excitons in alkali halides

We report the results of the calculation on the stability of self-trapped excitons localized on lattice sites with different geometrical structures in twelve alkali halides; AH, A = Li, Na, K, Rb, and H = Cl, Br, I. Present model leads us to the conclusions that excitons and holes in lithium halides, NaBr and NaI are most stable in energy when they are self-trapped as the three-center type and that those in potassium halides, KCl and NaCl are most stable when they are self-trapped as the two-center type. The energies of the two types of excitons are almost same in KBr and KI. This result shows us an important possibility that the Ex emission comes from the recombination of the three-center type self-trapped exciton.

Stabilities of the self-trapped holes and excitons in alkali halides

We study theoretically the stability of self-trapped hole and self-trapped exciton in alkali halides, AH, A=Li, Na, K, Rb, and H=Cl, Br, I. From a good correspondence between the emissions obtained experimentally and the present numerical results, it is found that a probable origin of Ex emission is three-center like state, while that of π and σ emissions is two-center state.