EPR and optical ( 4 f 4 photoluminescence and photoluminescence excihdon) speara due to Er centres are studied on bulk &Se crystals, which were gmwn by the high-pressure Bridgman technique and doped with ErF? (and pMly. in addition. with Liz-) during crystal growth. Besides the well known. almost isotropic signal with 8 = 5.94, which has been assignedto isolated E*+ on Zn lattice sires. we observe three strongly anisotropic EPR s p c h a (A, B and B' ) which are due to haositions in non-cubic Er'+ (spectrum A) and @+ (specua B and B') centres. The symmetry axes of these centres have directions close to (111) (A) and parallel to (I 10) @ and B'). The angular dependences of the signals are influenced by twinning effects.In the erystals doped with ErF3 alone. only a single type of centre, which is obviously identical with the type A EPR cenho, manifests itself in photoluminescence. A crystal-field analysis of the companding BPR and optical spectra shows that this antre has a rs-type ground level and is chamtenzed by a clystal-field parameter ratio A 6 ( r 6 ) / A j ( / ) = -0.22 We thhk that this centre is a complex consisling of E?+ on a Zn site and F on a nearest-neighbour interstitial site.The g-factors found for the EPR signals B and B' can be explained on the basis of non-Kramers doublet ground levels of E?+ 4fL2 which result from the splitting of cubic rs triplets due to non-cubic crystal-field components. These two signals are ascribed (0 E?+ on the two zincblende-type interstitial sites, respectively, each forming a complex with some other kind of atom on its next-nearest-neighbour interstitial site.A discussion is given of the result that the EPR spectra (unlike the optical spectra) were detected only in lhe case of Li2C0, codoping.

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In ZnSe crystals grown by different techniques, titanium ions are incorporated as Ti +(d ) andTi +(d') centers on Zn sites. A strong Jahn-Teller (JT) effect acts on the doubly degenerate E ground state of Ti3+. The electron paramagnetic resonance (EPR) at T =3 K indicates a quasistatic JT effect of the ground state and a quasidynamic one of the first excited state for the strain-split vibronic E-A2 manifold. These effects are distinguished by their angular variations and the g values. Ti + causes an isotropic EPR signal. Excitation and sensitization spectra of Ti + and Ti'+ luminescence transitions are explained within a one-electron mode1 connecting internal (d-d) with charge-transfer transitions involving the valence and conduction bands. Both charge states are sensitive to illumination with nearinfrared light. The ions can be mutually converted, as shown by photo-EPR and sensitization experiments. The Ti +/Ti'+ donor level is situated approximately 8500 cm ' below the edge of the conduction band.

In recent years the problem of discrete levels in resonance with a continuum has received a good deal of attention in solid state physics. Preferentially studies have been made of resonant vibrational modes (1) and resonant exciton states (2), whereas the case of impurity-induced discrete electronic states in resonance with an energy band has been investigated less extensively, both theoretically (3 to 5) and experimentally (6, 7).In the present note we report experimental results which indicate that resonant 2+ electronic states are generated by Ti transition metal impurities in this host substance (8, 9), the system CdS:Ti has not been investigated so far.

An E P R investigation is made of the hyperfine interactions with ligand nuclei (SHFS) of the TiZ+ centre in single crystals of hexagonal CdS and CdSe on sites with trigonal point symmetry C3". From the spectra experimental evidence is given for the magnetic nonequivalence of the twelve next-nearest ligands (cadmium) in wurtzite-type crystals. The SHFS-constants for the first three cadmium lattice shells for wurtzite are calculated using MO-theory. From the SHE'S-constants the probability is estimated of finding the delocalized 3d-electrons of the Ti2+ ion in the 5s-orbitals of the cadmium ligands.Es wird die Ligandenhyperfeinwechselwirkang (SHFS) von Ti"+-Zent,ren in Einkristallen von hexagonalem CdS und CdSe auf Platzen niit trigonaler Punktsymmetrie C3" mittels E P R untersucht. &fit den Spektren wird die magrietische Kicht.aquivalenz der zwolf ubernLchsten Nachbarn (Kadmium) in Wurtzit experimentell nachgewiesen. Die SHFS-Konstanten der ersten drei Kadmium-Gitterschalen fur Wurtzit werden mit der MO-Theorie berechnet. Aus den SHFS-Konstanten wird die Aufenthaltswahrscheinlichkeit der delokalisierten 3d-Elektronen des Ti2+-tons in den 5s-Orbitalen der Kadmiumnachbarn bestimmt IntrodlietionHyperfine interactions with ligand nuclei are a powerful mechanism to investigate covalency and charge transfer effects in crystals since a measurement of the hyperfine structure provides a direct measurement of the unpaired spin transfer [l]. Owing to the appreciable portion of covalent binding and the small natural abundance of chalcogen isotopes with nuclear magnetic moments, 11-VI compounds are suitable substances to investigate superhyperfine splitting (SHFS) with the next-nearest neighbours (Cd). The coupling constants of' interaction were found to be nearly isotropic and the observed spectra are described even in the case of the hexagonal crystals of wurtzite type structure with the assumption that the twelve next-nearest ligands belong only t o one lattice shell and having the same SHFS tensor. By reason of the local symmetry of a lattice site in wurtzite, C3", one expects however that the twelve next-nearest ligands belong to three distinct lattice shells and should show differences even in the isotropir SHFS constants.I n this paper experimental evidence is given for the magnetic nonequivalence of the twelve next-nearest ligands (Cd) in the hexagonal 11-VI compounds. At 27 O K in Ti-doped CdS and CdSe single crystals very we11 resolved SHFS lines are observed in the fine structure EPR transitions of the Ti2+ centre due to magnetic interactions of the unpaired 3d electrons with Cdlll and Cd113 nuclei and care-