Alkali halides excitons

Coherent optical phonons can couple with localized excitations such as excitons and defect centers. For example, strong exciton-phonon coupling was demonstrated for lead phtalocyanine (PbPc) [79] and Cul [80] as an intense enhancement of the coherent phonon amplitude at the excitonic resonances. In alkali halides [81-83], nuclear wave-packets localized near F centers were observed as periodic modulations of the luminescence spectra. 

However, in the last two decades it has been shown experimentally [1,7, 8,12-14] and theoretically [15-18] that in many wide-gap insulators including alkali halides the primary mechanism of the Frenkel defect formation is subthreshold, i.e., lattice defects arise from the non-radiative decay of excitons whose formation energy is less than the forbidden gap of solids, typically 10 eV. These excitons are created easily by X-rays and UV light. Under ionic or electron beam irradiations the main portion of the incident particle... 

For the exciton mechanism of defect production in alkali halides the Frenkel pairs of well correlated defects are known to be created [35], the mean distance between defects inside these pairs is much smaller than that between different pairs. The geminate pair distribution function could often be approximated as... 

These processes give rise to the electronic absorption bands of lowest energy observed in the pure undamaged single crystals which occur at 7.68 eV for MgO and 6.8 eV for CaO (142). Defects within the crystal structure are associated with optical absorption bands at reduced energies [for example, the anion vacancy band in the alkali halides (143)] because of the lower Madelung potential. The energy is still absorbed by the processes described in Eqs. (27) and (28), but the exciton is now bound to a defect and is equivalent to an excited electronic state of the defect. 

This was defensible in the inert-gas solids (though we noted that the gap was slightly reduced in those solids), but in the ionic crystal the nonmctallic ion electronic levels are greatly raised and the important excited levels (for exciton levels as well as for lower conduction-band levels) are dominated by the states on metallic ions see Fig. 14-1. Pantelides noted in fact that a critical study of the analysis of experiments in terms of the independent-ion model did not support the model. The model appeared to work for the alkali halides, but this was by fitting 16 experimental numbers with 8 adjustable parameters and the systematic variation made this fitting possible. Little success was had with other compounds. 

The LCGTO-Xa approach described so far has been successfully applied to a large variety of systems, including main group molecules (50,52,53), transition metal compounds, e.g. carbonyl complexes (27,28,55,56) and ferrocene (57), and a number of transition metal dimers (47). Besides these investigations on ground state properties useful information has also been obtained for selected problems involving excited states (52), such as the photolysis of Ni(CO)4 (58,59) and localized excitons in alkali halides (60) and in other ionic crystals ( ). 

Perhaps the most striking feature of a preliminary consideration of the electronic states of lead azide is the diversity of types of excitonic states that it may have. In addition to the charge-transfer excitons, well known in alkali halides, and effective mass excitons, well known in elemental semiconductors, one predicts for PhN intra-cation excitons (describable in terms of excited states of Pb +, and states of the Is 5d %s6p configuration, modified by the complex crystal field of the PbNg structure) and intra-anion excitons (describable in terms of excited states of Nj"). These offer possibilities for the transport of electronic energy. 

The localization of the lowest-energy exciton in ionic solids like the alkali halides is such that neither the Frenkel nor Wannier approximation is valid. An early picture of the excitation by Hilsch and Pohl [58] was that absorption of a photon results in the transfer of an electron from one of the halogen ions to a neighboring alkali ion. This charge-transfer model of the exciton attributed observed double-absorption peaks to the spin-orbit splitting of the halide ion, and successfully predicted exciton energies by the empirical relation... 

This example shows clearly that the emission process is very different from the (simple) absorption process. For all details the reader is referred to the literature [SJ. Finally we draw attention to the fact that the life time of the relaxed self-trapped exciton in the alkali halides is longer 10 s) than expected for an allowed transition (I0 - I0 "s). This is ascribed to the fact that the emitting state contains an amount of spin triplet character. Such a triplet state arises when the spins of the electron and the hole are oriented parallel. The emission transition becomes (partly) forbidden by the spin selection rule (see Chapter 2). 

The emission of the Tl" -doped alkali halides is due to the P - So transition on the Tl ion (see Sect. 3.3.7). It is usually assumed that the afterglow is due to hole trapping in the host lattice (trapped exciton, see Sect. 3.3.1), whereas the electron is trapped by the activator. In Csl Na the emission is due to an exciton bound to a Na ion, in Csl to self-trapped exciton emission. 

A formula was later derived for this charge-transfer exciton by applying an energy cycle for removing a halide electron and placing it on a nearest-neighbor alkali ion [59] ... 

Monovalent Azides. The relation of the cation s first ionization energy to ionic character and band-gap width has already been stressed. The monovalent azides TIN3, AgN, and CuNa have larger values of Ii than the alkali azides and hence less ionic character and smaller band gaps. The conduction band in each is assumed to be primarily formed of neutral cation states. The expected nature of the highest-lying valence band and of possible excitons is discussed below. Comparisons with halide compounds should be qualified by noting that the latter have different structures. 

Other Azides. Preliminary absorption spectra of CUN3 [86], CdNe, and HgNg [121] have been reported by Deb. Thin films were prepared by the solid-solid reaction technique described for PbNg. Mixed crystal systems of metal azide halides were probably formed in the reactions, since the results differed for each combination of alkali azide-metal halide. An earlier CUN3 spectrum was interpreted by Evans and Yoffe [82] to indicate an w = 1 exciton, but here again that appears theoretically unsound (Section C.3.b). 

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