Electron-vibrational Interaction Research Articles

Phonon-assisted optical bands of nanosized powdery SrAl2O4:Eu2+ crystals: Evidence of a multimode Pekarian

A stoichiometric powder composed of nanosized grains of SrAl2O4:Eu2+ was synthesized by combustion method at 500 °C with the subsequent calcination at 1000 °C. The zero-phonon line position, parameter of the Stokes shift, heat release factor and effective phonon energy were studied experimentally and analyzed in the framework of the multimode Pekar–Huang–Rhys model. Experimental data show that the optical 4f–5d transitions in Eu2+ ion exhibit a broad asymmetric electron–vibrational bands with a pronounced structure near the maxima. The form-function of the absorption and luminescence bands are theoretically analyzed in the framework of the model of the linear electron–vibrational interaction assuming strong coupling with the local vibration (estimated Pekar–Huang–Rhys parameter a=2S=10 and frequency ℏω=509 cm−1) and relatively weak interaction with the crystal phonons. The last results in an effective temperature dependent broadening of the discrete lines corresponding to the local vibrations and to a specific shape of the whole phonon assisted band (multimode Pekarian). Providing specific interrelation between the key parameters the calculated absorption and luminescence bands exhibit peculiar temperature dependent structured peaks in a qualitative agreement with the experimental data.

Electron–vibrational interaction in 5d state of Ce3+ ion in (LiMg/Li2Na/Li3)BF6 phosphors

The electron–vibrational interaction (EVI) parameters such as Huang–Rhys factor, effective phonon energy and zero phonon line position were estimated using data from our recent reported work, on photoluminescence in rare earth doped complex hexafluorides phosphors LiMgBF6, Li2NaBF6 and Li3BF6 Validity of results were established by modeling the emission line which was found to be in good agreement with experimental photoluminescence spectra.

Electron–vibrational interaction in 5d state of Eu2+ ion in LiMgBF6, Li2NaBF6 and Li3BF6:Eu2+ phosphors

Recently reported photoluminescence in Eu2+ ions activated complex hexafluorides phosphors LiMgBF6, Li2NaBF6 and Li3BF6 was studied to estimate electron–vibrational interaction (EVI) parameters such as Huang–Rhys factor, effective phonon energy, Stokes shift and zero phonon line position. Validity of results was established by modeling the emission line which was found to be in good agreement with experimental photoluminescence spectra.

A Model of Magnetic and Relaxation Properties of the Mononuclear [Pc2Tb]−TBA+ Complex

The present work is aimed at the elaboration of the model of magnetic properties and magnetic relaxation in the mononuclear [Pc(2)Tb](-)TBA(+) complex that displays single-molecule magnet properties. We calculate the Stark structure of the ground (7)F(6) term of the Tb(3+) ion in the exchange charge model of the crystal field, taking account for covalence effects. The ground Stark level of the complex possesses the maximum value of the total angular momentum projection, while the energies of the excited Stark levels increase with decreasing |M(J)| values, thus giving rise to a barrier for the reversal of magnetization. The one-phonon transitions between the Stark levels of the Tb(3+) ion induced by electron-vibrational interaction are shown to lead to magnetization relaxation in the [Pc(2)Tb](-)TBA(+) complex. The rates of all possible transitions between the low-lying Stark levels are calculated in the temperature range 14 K<T < 40 K. With the purpose of calculation of the temperature dependence of the relaxation time of magnetization, we solve the set of master equations for the populations of the Stark levels. The relaxation time is shown to diminish from 3.2 × 10(-2) s to 1.52 × 10(-4) s as the temperature increases from 27 K to 40 K. The obtained values of the relaxation time are in satisfactory agreement with the observed ones. The developed model also provides satisfactory description of the dc-magnetic data and paramagnetic shifts.

Photoluminescence and electron-vibrational interaction in 4fn−15d states of Ce3+ or Pr3+ ions doped LnBO3 (Ln=Lu, Y, La) orthoborates materials

Calcite, vaterite and aragonite type rare earth LnBO3 (Ln=Lu, Y, La) orthoborate powders, doped with 1% cerium or praseodymium, were prepared by the classical solid state reaction method. The structure and the morphology of these powder materials were checked by X-ray Diffraction, Fourier Transform Infra Red Spectroscopy (FTIR) and Scanning Electron Microscopy. Room temperature excitation and emission spectra of four compounds: LuBO3 with calcite and vaterite structure, YBO3 (vaterite type structure) and LaBO3 with aragonite, doped with 1% cerium or praseodymium ions, have been measured and investigated. The effect of Ce3+ and Pr3+ crystalline environment in these compounds on the position of their 5d (4f5d) levels has been discussed. This work is also devoted to the problem of the electron-vibrational interaction (EVI) in 4f–5d optical transitions in orthoborate materials. The emission and excitation 4f–5d transitions resulted in broad vibronic bands whose shape functions were described and found. The main EVI parameters, such as the Huang-Rhys factor, effective phonon energy, and zero-phonon line position, were estimated. These values are checked by modeling the Ce3+ 5d–4f and Pr3+ 4f5d–4f2 emission lines shapes, in which good agreement with experimental spectra confirms validity of the performed analysis.

Ab initio, crystal field and experimental spectroscopic studies of pure and Ni2+-doped KZnF3 crystals

Pure and Ni2+ doped KZnF3 single crystals were studied using the combination of the DFT-based ab initio methods, crystal field theory and experimental spectroscopic techniques. The electronic, optical and elastic properties have been calculated and compared with available experimental data and good agreement was achieved. Elastic anisotropy of pure KZnF3 was modeled; calculations of the sound velocity, Debye temperature, Grüneisen parameter and specific heat capacity were performed. Comparison of the calculated results for the pure and doped material, which is reported for the first time for the considered material, enabled to identify the changes in the optical and electronic properties, which are due to the introduced nickel impurity ions. In particular, it was shown that the lowest Ni 3d states appear in the host's band gap at about 1.0 eV above the valence band. The changes of the electron density distribution after doping were also shown. Microscopic analysis of the crystal field effects based on the performed ab initio calculations of the Ni2+ density of states at different external pressures enabled to estimate the constants of the electron–vibrational interaction, Huang-Rhys factor, Stokes shift and local bulk modulus around impurity ions. The crystal field calculations of the Ni2+ energy levels were performed to analyze and assign the experimental absorption spectrum. Such a combination of the ab initio and semi-empirical calculating techniques leads to a complementary picture of the physical properties of KZnF3:Ni2+ and can be applied to other doped crystals.

Temperature dependence for the rate of hole transfer in DNA: Nonadiabatic regime

The positive charge transfer in DNA is investigated, using the first principle treatment of the electron-vibrational interaction. We show that rearrangements of atoms belonging to base pairs induced by charge transfer are essentially quantum mechanical in nature. Particularly at room temperature, around half of the rearrangements occur via quantum tunneling, while the other half takes place via thermally activated transitions. This effect reduces activation energies for charge transfer between both AT and GC pairs by a factor of two compared to their classical values. These behaviors are described within small polaron theory for the non-adiabatic charge transfer and compared to the experimental data and previous theoretical studies.

Influence of the Jahn–Teller effect on the g-tensor of the spin-orbit quartet in three-coordinate D3h structures

Theoretical calculations of g-tensor components for the spin–orbit quartet, which arises as the ground state in three-coordinate d9 complexes and low-spin d7 complexes of D3h symmetry, have been made on the assumption that the spin–orbit interaction is commensurable with the electron-vibrational interaction. The calculations were carried out within the framework of crystal field theory using representations of the hole formalism. The analytical expressions for g-tensor components were obtained limited to first-order terms. It was shown that the account of the electron–vibrational interaction in the excited quartet only provides three-axial anisotropy for the g-tensor. It was shown that the g-tensor rotates in the plane of the three-coordinate structure with consensual motion of the atoms. The resulting expressions for the g-factor components are in good agreement with experimental data. Being universal for a wide range of contributions of the vibronic and spin–orbit interactions, these expressions essentially fill the gap in studying structures of coordination compounds.

Topological modeling of the reactive capacity and biological activity of some amino-polysaccharides

The work is devoted to a microscopic analysis of the reactive capacity of chitin. An algorithm for modeling the deacetylation reaction in a monomeric unit of chitin is described. The reaction coordinate and the potential energy surface topography are determined taking into account the electron-vibrational interaction and low-symmetry perturbations within Jahn-Teller theory. Based on this algorithm, the topological modeling of the deacetylation process is performed for the first time and a mechanism of the biological activity of chitosan is proposed.

Exchange charge model and analysis of the microscopic crystal field effects in KAl(MoO 4) 2:Cr 3+

In the present paper, we report on consistent crystal field calculations of the Cr 3+ ions energy levels in KAl(MoO 4) 2 using actual D 3d site symmetry of the Cr 3+ position and employing the exchange charge model (ECM) of the crystal field. In addition to the energy level calculations, the Huang–Rhys factor S=5.7 and effective phonon energy ℏ ω=268 cm –1 were evaluated in the single configurational coordinate model. Detailed treatment of the microscopic crystal field effects in the ECM framework allowed to obtain analytical dependence of the crystal field strength 10 Dq on the Cr–O interionic distance and extracting from it the values of some parameters of the electron–vibrational interaction (EVI) in the KAl(MoO 4) 2:Cr 3+ system. All obtained results are compared with experimental data and discussed; agreement between the calculated and experimental parameters is good.

Photoluminescence of lanthanide-doped CaSi 2O 2N 2 phosphors and the energy-level diagram of lanthanide ions in CaSi 2O 2N 2

Lanthanide-doped CaSi 2O 2N 2 phosphors were synthesized by solid-state reaction. Their photoluminescence (PL) properties were investigated and analyzed systematically. Energy transitions attributed to electron transfers from 4f to 4f (ff), from 4f to 5d (fd), from 5d to 4f (df) and charge transfer (CT) transition were obtained and classified. Finally, an energy-level diagram of lanthanide-ions in CaSi 2O 2N 2 was proposed. The diagram matches perfectly with the experimental results and explains the luminescence properties of lanthanide-doped CaSi 2O 2N 2 phosphors. The electron-vibrational interaction in 4f–5d optical transitions of CaSi 2O 2N 2:Eu 2+ was also studied.

Electron–vibrational interaction in the 5d states of Ce 3+ ions in halosulphate phosphors

Using the literature data on excitation and luminescence spectra of Ce 3+ ions in NaMgSO 4F, Na 3SO 4F, and KZnSO 4Cl halosulphate phosphors [S.C. Gedam, S.J. Dhobe, S.V. Moharil, J. Lumin. 126 (2007) 121], we performed comparative studies of electron–vibrational interaction (EVI) of the cerium 5d states with host lattices. The main EVI parameters, such as the Huang-Rhys factor, effective phonon energy, and zero-phonon line position, were estimated and reported for these systems for the first time. The obtained results were checked by modeling the Ce 3+ 5d–4f emission line shapes; good agreement with experimental spectra confirms validity of the performed analysis.

Depolarization of the fluorescence in donor-acceptor pair under periodic electric field

The donor-acceptor pair in the external periodic electric field is under studying. The molecular system is excited by the short impulse of the polarized light, and the electronic excitation initially localized on the donor and interacts only with full symmetrical vibration of the ligand environments, thus the vibrational subsystem forms the package in the form of the coherent state. Donor and acceptor molecules are assumed as identical type of the molecules. The time dependence of the depolarization of the fluorescence is numerically calculated on the basis of the semi-phenomenological model. Numerical estimates are given for cases of the strong and weak electron-vibrational interaction. Transfer of the excitation in the system is defined by the non-linear properties of the system, especially in the case of strong electron-vibrational interaction. The depolarization of the fluorescence is consequence of the such transport and contains the interference addends which conduct to the quantum beats in the time dependence.

Studies of variation of interionic distances and crystal field effects in ZnS:V 2+ and MgO:Cr 3+

A detailed microscopic study of the crystal field strength 10 Dq for different interionic distances in cubic ZnS:V 2+ and MgO:Cr 3+ single crystals was performed. The exchange charge model of crystal field was used to calculate the 10 Dq values for different distances between the impurity ions and ligands. The obtained results were extrapolated by the power laws 10 Dq ∼ 1/ R n , with n = 4.4208 for ZnS:V 2+ and n = 6.4009 for MgO:Cr 3+. With these 10 Dq( R) dependencies, a number of important physical quantities describing the optical and dynamical properties of impurity centers (such as the constants of the electron-vibrational interaction, Huang–Rhys parameters, Stokes shifts, Jahn–Teller stabilization energies) were estimated and compared with available literature data.

Preparation and spectroscopic studies of the Mg xSr 1−xAl 2O 4:Eu, Dy ( x = 0.05–0.25) persistent phosphors

Mg x Sr 1− x Al 2O 4:Eu, Dy ( x = 0.05–0.25) phosphors were prepared by the solid-state reaction method; their excitation and emission spectra corresponding to the transitions between the ground 4f 7 and excited 4f 65d 1 configurations of Eu 2+ ion were studied. Broad band UV excited luminescence attributed to Eu 2+ ions was observed around 19,500 cm −1 (513 nm). From the comparison between the excitation and emission spectra, the crystal field splitting of the Eu 2+ 5d states and the parameters of electron–vibrational interaction, such as the Huang–Rhys factor, effective phonon energy, and zero-phonon line position for the studied compound were estimated for the first time.

Exchange with temperature of the electron-vibrational mode interaction between thienylene-phenylene copolymer rings

Thienylene―dialkoxyphenylene copolymers with polar side chains are soluble in most organic solvents and exhibit excellent glass adhesion properties, allowing the formation of homogeneous thin films on glass, quartz, and other substrates by spin coating. The electrochemical and optical properties of thin films of this material are studied in this work. The calculated band gap derived from electrochemical data (2.34 eV) is in good agreement with that observed from the UV―vis spectrum edge (2.25 eV). The analysis of the emission properties reveals the strong dependence on temperature of the electron coupling with different vibrational modes in the formation of the vibronic bands.

Studies of electron-vibrational interaction and crystal field splitting in 5d states of Eu2+ in CaAl2O4 co-doped with Eu2+ and Er3+

CaAl2O4 co-doped with Eu2+ and Er3+ were prepared by the solid-state reaction method and investigated for their photoluminescence properties. Broad band UV excited luminescence was observed for CaAl2O4 : Eu2+, Er3+ in the blue region due to transitions from the 4f65d1 to the 4f7 configuration of the Eu2+ ion. From the analysis of excitation and emission spectra, the crystal field splitting of the 5d states of Eu2+and the parameters of electron-vibrational interaction, such as the Huang–Rhys factor, effective phonon energy and zero-phonon line position of this system, were estimated for the first time.

Crystal field effects and electron–phonon interaction in K 2LiAlF 6:Cr 3+

The crystal field effects and electron–phonon interaction in K 2LiAlF 6:Cr 3+ elpasolite crystal were studied using the exchange charge model (ECM) of crystal field and harmonic approximation of lattice vibrations. The crystal field parameters (CFP) and energy level scheme for the [CrF 6] 3− cluster were calculated. As a further application of the ECM, the crystal field strength 10 Dq was studied for different “impurity ion–ligand” distances R. By fitting these values to the power law 10 Dq≈ R − n of coordinate R, the value of n was obtained as 4.56796. Several physical quantities (such as the electron–vibrational interaction constants, Stokes shifts, Huang–Rhys factors and Jahn–Teller stabilization energy) associated with the two normal vibrational modes a 1 g and e g of the [CrF 6] 3− complex coupled with the Cr 3+ electronic states, were calculated. The obtained results are compared with experimental data and discussed.

Photoluminescence in MgxSr1−xAl2O4:Eu2+, Nd3+and electron–vibrational interaction in the Eu2+ 5d states

Green phosphor compositions MgxSr1−xAl2O4:Eu, Nd (with x=0.05–0.25) were prepared by solid state reaction method. The effect of Mg substitution on photoluminescence characteristics was investigated. The photoluminescence show intense green emission for MgSrAl2O4:Eu2+, Nd3+ with long persistence. This green emission corresponds to transitions from 4f65d1 to 4f7 of Eu2+ ion. Comparative analysis of the excitation and emission spectra were used to evaluate the crystal field splitting of the 5d states of Eu2+ and the parameters of electron–vibrational interaction, such as Huang–Rhys factor, effective phonon energy, and zero-phonon line position.

Microscopic analysis of the crystal field strength and electron-vibrational interaction in cubicSrTiO3 dopedwith Cr3+, Mn4+ and Fe5+ ions

A detailed microscopic study of the crystal field strength10Dq for different interionic distances in cubicSrTiO3 doped with threeisoelectronic ions (Cr3+, Mn4+ and Fe5+) was performed. The exchange charge model of the crystal field was used to calculate the10Dq values atdifferent distances between impurity ions and ligands. The obtained results were represented by the powerlaws 1/Rn, withn = 4.9050, 5.7990 and6.5497 for Cr3+, Mn4+ and Fe5+, respectively. For the first time the role of two different contributions (the point charge andexchange charge) into the total crystal field strength was studied separately. With the obtained10Dq(R) dependences, a number of important physical quantities describing the optical anddynamical properties of impurity centers (such as the constants of the electron-vibrationalinteraction, Huang–Rhys parameters, Stokes’ shifts, Jahn–Teller stabilizationenergies, changes of the chemical bond lengths due to the combined effect of thelocal vibrational normal modes, bulk modulus and Grüneisen constants for thea1g normal mode) were calculated. The obtained results are in good agreement with availableexperimental data and can be readily applied for analysis of the optical spectra,electron-vibrational interaction and pressure effects for these and other similar systems.