Center In CaO Research Articles

Investigations of the spin Hamiltonian parameters and defect structure for the rhombic Gd3+ center in calcium oxide

The spin Hamiltonian (SH) parameters (zero-field splittings , and ) and the defect structure for the rhombic Gd3+ center in CaO are theoretically investigated by the superposition model. Meanwhile, the g-factor is also quantitatively studied from the approximation formula on the basis of an admixture of the ground 8S7/2 and the excited 6L7/2 (L = P, D, F, G) states via the spin–orbit coupling interactions. The defect structure of this center can be described by the impurity Gd3+ occupying the host octahedral Ca2+ site, associated with one nearest neighbouring cation vacancy V Ca in the [1 1 0] direction. On the basis of these studies, the impurity Gd3+ is found to shift towards the V Ca along the [1 1 0] axis by about 0.582 Å, whereas the oxygen ligands closest to the V Ca suffer a displacement towards the latter by about 0.211 Å. The calculated SH parameters based on the earlier analyses show reasonable agreement with the observed values.

Optical absorption and luminescence energies of F centers in CaO from ab initio embedded cluster calculations

We calculated the optical absorption and luminescence energies of electrons trapped at oxygen vacancies in CaO using a consistent embedded cluster method which accounts for the long-range polarization effects and partial covalence of CaO. Optical absorption and luminescence energies of neutral (F center) and positively charged (F+ center) vacancies are calculated by means of time dependent density functional theory using the B3LYP exchange-correlation density functional. Our results demonstrate that using large basis sets to describe a diffuse nature of excited states, and properly accounting for long-range polarization induced by charged and excited defect states, is crucial for accurate predictions of optical excitation and luminescence energies of these defects.

Studies on the Local Structure of the Tetragonal Er3+ Center in CaO

Abstract The local structure of the tetragonal Er3+ center in CaO is theoretically studied by using the perturbation formulas of the g factors for a 4f11 ion in tetragonal symmetry. In these formulas, the contributions to the g factors from the second-order perturbation terms and the admixtures of various states are taken into account. Based on the investigations, this center is suggested to be the impurity Er3+ substituting the host Ca2+ site, associated with a Ca2+ vacancy VCa in the [100] (C4) axis due to charge compensation. By studying the g factors of the tetragonal center, impurity Er3+ is expected to undergo an off-center displacement ΔZ(≈ 0.2 Å ) towards the VCa along the C4 axis because of the electrostatic attraction. The calculated g factors based on the displacement ΔZ show reasonable agreement with the observed values.

Long lasting phosphorescence properties of Tb-activated reduced calcium aluminate glasses

A long lasting green phosphorescence was found in -doped calcium aluminate glasses prepared under a strongly reducing atmosphere. An electron-trapped centre giving an EPR signal at g = 1.999, an analogue of the centre in CaO, was induced by illumination with ultraviolet (uv) radiation and decayed slowly after the illumination was stopped. A partial oxidation of ions to ions by uv illumination was observed. A mechanism of the long lasting phosphorescence was suggested to consist of recombination of an electron released thermally from the -like centre with the photo-oxidized .

Symmetry assignments of Raman scattering by F+ and F centres in calcium oxide

Data on the Raman scattering of F and F+ centres in CaO are recorded. Symmetry assignments of the Raman effect of F+ centres are in agreement with the F+ emission spectrum measurements of Hughes, Pells and Sender. Furthermore, polarization results show that the frequency separation between T2g and Eg modes is about 17 cm-1. The vibrational modes of F centres are also discussed.

Zeeman measurements of Pr 3+ centres in CaO and CaF 2

Three Pr 3+ centres in two different crystal hosts have been studied by site selective laser spectroscopy and Zeeman spectroscopy with attention directed at the 3P 0, 3P 1 and 1I 6 crystal field level assignments. The 3H 4→ 3P 0 excitation line is readily identified in each case. For a tetragonal and a trigonal centre in CaF 2 there is, in each case, a further weak excitation line higher in energy which exhibits a large Zeeman splitting. The associated excited levels are attributed to 1I 6 states. For the Pr 3+ centre in CaO there are two lines adjacent to that associated with the 3P 0 state and although they do not exhibit linear Zeeman splittings, there are quadratic Zeeman shifts due to large off-diagonal terms. Again the related levels are attributed to 1I 6 states. The centre is shown to have orthorhombic symmetry and the energy levels for this centre are determined.

Optical hole-burning within the 574.2 nm line of the F centre in CaO

The authors report the observations of long-lived optical holes burned in the 1A1g to 3T1u zero-phonon line of the F center in CaO. Splitting of the holes in a magnetic field is shown to be consistent with the data obtained from optically detected magnetic resonance measurements of the 3T1u state. Stark measurements are reported but not all the electric field broadening is found to be reversible and this effect is not explained. The Stark shifts are minimal and inconsistent with values given previously. Hole-burning in the 571 nm zero-phonon line of an orthorhombic centre is briefly reported.

The reinterpretation of the zero-field double resonance spectra of the lowest excited 3T 1u state of the F center in CaO in terms of random internal strain of E g symmetry

The zero-field double resonance spectra of the relaxed excited 3T 1u state of the F center in CaO were determined and interpreted previously. In the present paper the phosphorescence microwave double resonance (PDMR) spectra are reinterpreted in terms of random internal strain of E g symmetry. The strain variables of the subensemble of F centers responding to a particular microwave-induced transition are exclusively determined by the microwave and the phosphorescence frequency. Consequently, the level structure in this subensemble can be calculated. To each PMDR spectrum, two graphical representations describing the possible spread in the phosphorescence frequency and the vibronic level structures of the F centers contributing to the PMDR signal are related. From these graphical representations, the occurring radiationless transitions can be traced. It is shown that, at the low energy side, the bounds of the PMDR lines are determined by the random strain distribution whereas the upper bounds are determined by fast radiationless transitions or by the lack of a population difference between the spin levels involved in the microwave-induced transition.

Optically induced vibronic relaxation in the 3T 1u state of the F center in CaO

The static and dynamic properties of the phosphorescent triplet state of the F center in CaO in the presence of a [101] stress have been investigated by optically detected magnetic resonance. From optical polarizations and results of transient-nutation experiments it is concluded that both the E g and the T 2g strain components induced by the stress determine the structure of the 3T 1u state. By means of time-resolved ODMR experiments it is demonstrated that the spin—lattice relaxation and spin dephasing proceed by Orbach-type relaxation processes between the stress-split vibronic states, induced by time-dependent E g and T 2g strains resulting from lattice vibrations. The influence of the temperature and the excitation intensity on the relaxation rates shows that at 1.2 K the dynamics in this system is governed by phonons that are generated in the optical-excitation cycle.

On the interpretation of g values of the lowest excited 3T 1u state of the f center in CaO

In order to interpret experimental data obtained from observations on the lowest excited 3T 1u state of the F center in CaO in terms of physical entities, a set of assumptions is applied which depends on the type of the data. The agreement between the values of physical entities calculated by using distinct sets of assumptions is poor. In the present work the origin of this discrepancy is investigated. Hereto the measured g values are compared with the corresponding mathematical expressions, which are derived by using four currently applied assumptions, and it turns out that no consistent fit is obtainable. The analysis of the inconsistency revealed that the assumption that the Jahn-Teller coupling can be described by using one single effective frequency, is not applicable. Furthermore, arguments are given that in the calculation the mixing of the 3T 1u state with other electronic states probably plays a significant role.

Spin-selective electron tunneling from excited F A centers to F+ centers in CaO

From pulsed phosphorescence microwave double resonance experiments it is established that electron tunneling between F A and F + centers in CaO occurs from F A centers in the excited 3 A 1 state. Only the radiative sub-levels within this 3 A 1 state act as precursor states in the electron transfer process. The tunneling rate at 1.2 K is determined to be 1.5±0.2 s -1.

Laser excitation and spin coherence of phosphorescent F 2+2 centers in CaO

Spin coherence in the laser-excited triplet state of F 2+ 2 centers in CaO is studied. Selective probing of the F +-center local properties is provide by the measurement of the F 2+ 2-center spin-echo decay at different excitation wavelengths. In addition, spin-echo beat phenomena arising at high-power pulsed-laser excitation are reported.

Determination of the g and D values of a spin triplet in case of misalignment

The effect of misalignment of the sample with respect to the magnetic field direction, on the angular dependence of the resonant magnetic fields of a 3T 1u state is analyzed. From these results two methods are derived to determine the degree of misalignment and to correct the g and D values for misalignment. One method is applied on the lowest excited 3T 1u state of the F center in CaO. The influence of random internal strain on the determination of the g values is taken into account. The results of the misalignment determination are compared with those obtained from X-ray diffraction. The agreement is satisfactory.

Site-selective dephasing of photo-excited triplet spins in CaO

Electron spin dephasing in the photo-excited 3B1 state of the F22+ centre in CaO is studied from optically detected spin echoes after laser-selective optical pumping. The results show site-selective homogeneous broadening of the zero-field triplet spin transitions. Local concentration variations of nearby F+ centres are held responsible for inhomogeneities in the F22+-centre optical absorption and triplet-spin dynamics.

Energy and phase relaxation of phosphorescent F centers in CaO

In this paper we study the temperature-induced homogeneous broadening of the no-phonon line in the emission spectrum of the F center in CaO. The linewidth can be fitted to n̄(n̄+1), where n̄ is the thermally averaged occupation number of phonons with a frequency of 90 cm−1. The results are characteristic of elastic scattering of pseudolocalized phonons at the defect site. These phonons also appear to dynamically couple the Jahn–Teller components of the F center in the photoexcited 3T1u state and thus give rise to a temperature dependence of the lifetime of this phosphorescent state. Finally, from experiments using laser-selective excitation it is concluded that the zero-phonon emission peaking at 571.1 nm does not originate in the F center.

Li tunneling effects on spin-lattice relaxation rates of a color center in CaO:Li

Low-temperature spin-lattice relaxation rates of the ${[{F}_{\mathrm{Li}}]}^{0}$ center in single crystals of CaO:Li at $X$- and $\mathrm{Ku}$-band microwave frequencies exhibit an anisotropy in the functional form of the temperature dependence: from $T$ in the [100] direction to csch ($\frac{25}{T}$) in the [111] direction. A model involving longitudinal and transverse tunneling of a Li ion adjacent to the trapped electron can qualitatively explain this and most of the other characteristics of the relaxation data, with the exception of the hyperfine dependence.

Divalent manganese in an axial symmetry site of additively coloured CaO single crystals

Electron paramagnetic resonance data dealing with the first observation of a Mn2+ axial center in CaO are reported. The tetragonal EPR spectrum was found in additively coloured crystals at 100K the best-fit parameters to the spin-Hamiltonian were found to be : g/sub ///=2.0015 (5), gperpendicular to =2.0020 (5), b20=+132.6 (4)*10-4 cm-1, b40=+3.5 (3)*10-4 cm-1, b44=+11.7 (8)*10-4 cm-1, and A=-67.2 (5)*10-4 cm-1. The hyperfine coupling constant is anomalously small and points towards a considerable covalent bonding. The analysis of the data presented in this paper strongly suggests that the axial spectrum is due to Mn2+ ions perturbed by an adjacent F centre. Assuming this model, the reduction in the value of the hyperfine interaction relative to that of the free ion was estimated in terms of the approach of Simanek and Sroubek which puts the main emphasis of such a reduction on the covalent effects of the 4s orbitals of the Mn2+ ion and the result compares reasonably well with the experimental determination.

Electron transfer from excited F centres to F +, F +A and F +AA centres in CaO studied by optically detected magnetic resonance (ODMR)

ODMR measurements via the magnetic circular polarization of the emission (MCPE) of F A(Mg) centres in CaO at 1.6 K showed besides the known F A-ODMR lines a non zero MCPE, which vanishes at g ≅ 2. From the dynamical behaviour of this MCPE and the F A luminescence is shown that an electron tunneling takes place from the excited T 1u F centre state to F + A centres forming excited F A centres. The ground state back tunneling rate is 10 7 times smaller and cannot explain the observed electron transfer cycle. The back tunneling is also going via excited states. The results can be understood assuming a random centre distribution. A new luminescence band at 785 nm could be identified by ODMR as being due to tetragonal F AA centres. The electron transfer is also observed between F and F + and F + AA centres.

ODMR study of electron transfer from excited F centres to F +, F +A and F +AA centres in CaO

The optically detected magnetic resonance spectra of F and F A centres in CaO:Mg show additional narrow lines at g = 2. From the dynamical behaviour of these lines and their microwave power dependence it is shown, that they are due to the magnetic resonance in the ground state of the F + and F + A centres, respectively. This is direct evidence for an electron transfer taking place between the F centre triplet state and the F + A centre ground state. In the same crystal a new luminescence band at 785 nm was found which could be identified by ODMR as being due to a tetragonal F AA centre (i.e. two neighbouring Mg ++ ions replacing Ca ++ on opposite sides of the vacancy). Also to those centres the electron transfer was observed.

Optically detected spin dephasing in ionic solids

Spin coherence transients in the photo-excited 3B 1-state of F 2+ 2 centers in CaO were studied at low magnetic field strengths. An enhanced coherence decay rate is observed when cross-relaxation with F + centers occurs. The results are representative for a rapid contact between a triplet spin packet and the homogeneously narrowed S = 1 2 spin reservoir.