Fluorescence enhancement of Eu3+ by Tb3+ in dimethylsulfoxide (DMSO)

Photoexcitation of EuCl₃.6H₂O and TbCl₃.6H₂O mixtures in DMSO at various wavelengths brings about a decrease in the fluorescence intensity of Tb³⁺ and a subsequent enhancement in the fluorescence intensity of Tb³⁺ provided that [Tb³⁺] < [Eu³⁺]. The average value of the electronic excitation energy transfer rate constant k₁₀ which was found to be independent of the excitation wavelength, was determined to be about 1.50 × 10³ M^-1 s^-1. Photoexcitation of Tb³⁺ and subsequent population of high energy excited states is accompanied by rapid nonradiative de-excitation processes to the lowest excited state ⁵D₄, which is the origin of the energy transfer process. A lower limit for the value of the reaction rate constant, associated with the transition ⁵D₃ ? ⁵D₄, namely k₅, is of the order of 10⁵ ?10⁶ s^-1. Excitation at conditions leading to the exclusive population of the ⁵D₄ state of Tb³⁺ gave rise to a value of k₁₀ equal to (2.2 ± 0.4) × 10³ M^-1 s^-1 and a critical separation (R₀)_exp between Tb³⁺ and Eu³⁺ of about 13 Å. A theoretical value of R₀ equal to 14.2 Å was calculated. The energy transfer process does not appear to take place via clear cut dipole-dipole interactions but rather via complex multipole and/or exchange interactions.     

Effect of higher harmonics in quarter-filled Su-Schrieffer-Heeger model

The role of the higher harmonics of the order parameter in the ground state of the quarter-filled Su-Schrieffer-Heeger model is investigated. The lowest energy is obtained when the 4k_F order parameter Δ₂ vanishes. Furthermore, as far as the uniform excitations are concerned, the energy of the system has a minimum at a finite value of Δ₂ when the phase of the fundamental (or 2k_F) order parameter deviates from its ground state values.     

Intrinsic surface state on Be(0 0 0 1)

Using monochromatized synchrotron radiation in the range 24–30 eV, we have recorded angle-resolved photoemission spectra from a clean Be(0 0 0 1) crystal face. A surface state located in a band gap around Γ with an initial state energy of ?2.8 eV in normal emission was found. For k_∥ along the $\text{Γ}$$\text{M}$ line the surface state disperses upwards and passes E_F at about 55% of the distance to the surface Brillouin zone boundary.     

Experimental study of the reactions of Si+(2P) ions with several small organic molecules at near thermal energies

The kinetics of the reactions of ground state silicon ions Si⁺(²P) with C₂H₆, CH₃OH, and CH₃CH₂OH has been investigated using a Selected Ion Flow Drift Tube technique (SIFDT). The reaction rate coefficients and the product distributions have been determined as functions of the reactant ion/reactant neutral average centre-of-mass kinetic energy (E _CM). The reactions studied are fast at thermal and near thermal energies and the reaction rate coefficients decrease by nearly one order of magnitude withE _CM increasing up to a few eV. It is shown that the energy dependences of the measured reaction rate coefficients can be described by the function of the formk ≈ 1/[1+ (E _CM/E _CM1) ^m ], whereE _CM1 andm are parameters which can be determined from the experimental data. The results are interpreted in terms of a simple model assuming the reactions to proceed via the formation of long-lived complexes. These intermediate complexes decompose with unimolecular rate coefficientsk _?1 andk ₂ back to the reactants and forward to the products, respectively. The ratiosk _?1/k ₂ can be approximated for reactions studied by a power law functionk _?1/k ₂ ∞ (E _CM) ^m .     

Förster-type energy transfer mechanism in PF2/6 to MEH-PPV conjugated polymers

Energy transfer mechanism between Poly[9,9-di-(2′-ethylhexyl)fluorenyl-2,7-diyl] (PF_2/6) as a donor and poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) as an acceptor in different solvents has been studied using steady-state emission measurements. Four different solvents namely, tetrahydrofuran (THF), toluene, chlorobenzene (C.B) and benzene have been used in this study. The absorption and luminescence behaviors of the samples are measured at a fixed concentration of donor (0.1 μM) while the concentrations for acceptor are kept in the range of 0.1–1.0 μM. Based on these measurements, the energy transfer properties namely quenching rate constant (k_SV), energy transfer rate constant (k_ET), energy transfer probability (P_DA), transfer efficiency (η) and critical distance of energy transfer (R_o) are calculated. The use of THF resulted in the highest energy transfer. Long range dipole–dipole interaction between the excited donor and ground state acceptor molecules is the dominant mechanism responsible for the energy transfer as proven by the large values of R_o.     

Energy spectra of developed superfluid turbulence

Turbulence spectra in superfluids are modified by the nonlinear energy dissipation caused by the mutual friction between quantized vortices and the normal component of the liquid. We have found a new state of fully developed turbulence, which occurs in some range of two Reynolds parameters characterizing the superfluid flow. This state displays both the Kolmogorov-Obukhov 5/3-scaling law E_k ∝ k^?5/3 and a new “3-scaling law” E_k ∝ k^?3, each in a well-separated range of k.     

Luminescence properties of some platinum (II) complexes. Counter-ion and molecular geometry effects

Reflectance and luminescence spectra, and emission lifetimes of 14 charged and neutral Pt (II) complexes in the solid crystalline state are reported. The lifetimes (in the range of some tens of microseconds) indicate that the emissions are due to a spin-forbidden process. On the basis of spectral correlations, the phosphorescence is tentatively identified as due to the lowest d-d ligand field transition when the bonding of the ligand is essentially σ in character, and to a π1 → d charge-transfer transition for those complexes in which the ligands themselves have π orbital systems. Both the radiative (k_r) and non-radiative (k_n) rate constants are sensitive to changes in molecular geometry (cis, trans isomers) and counter-ions. Assuming unitary efficiency for the intersystem crossing to the emitting state, the counter-ion appears to predominantly affect k_n through vibrational coupling of the chromophore with the lattice. For the cis forms, both k_r and k_n are affected in a complex manner, with metal-metal interactions playing an important role. For the trans forms, however, the constancy of the quantum yield with respect to temperature suggests that k_n is negligible in comparison to k_r, and therefore the trans chromophores behave as isolated systems within the crystalline lattice.     

Angular resolved photoemission from intrinsic surface states on Al (100)

Using photon energies of 11.7, 16.8 and 21.2 eV, we have recorded angular resolved photoemission spectra from clean Al (100). A dominant, surface sensitive peak is interpreted as emission from a two-dimensional band of surface states. The band mass is $\text{m}{}^1\text{=(1.03±0.10)m}$, and the band minimum, (2.8±0.2) eV below E_F for surface momentum k₆=0, is located within the bulk band gap. The observed existence of the peak for large values of k₆, indicates a transition from a true surface state to a surface resonance.     

Rotational excitation of HD by collisions with He

The information theoretic surprisal analysis is employed to characterise the recent quantal close-coupling computations of rotational excitation rates by Green. The analysis is based on comparing the actual rates to the priors k⁰ which depend only on the temperature and the reduced “energy defect” ΔE_R/kT. The surprisal representation of the computed rates as k=k⁰ x exp (-I₀ - θ_R6ΔE_R6/kT is accurate throughout the temperature range considered by Green and is found to reduce in the low and high temperature limits to functional forms previously employed to characterise the rotational energy transfer rates in H₂.     

Inductive-resonant theory of nonradiative transitions in lanthanide and transition metal ions (review)

The review is devoted to the theory of nonradiative transitions in tricharged ions of lanthanides and transition metals in the condensed phase, which was proposed in 1971. The theory is based on the phenomenon of nonradiative energy transfer from an electronically excited ion to surrounding molecular groups with excitation of resonant vibrational states and makes it possible to calculate the nonradiative transition rate constant (k _nr) by a formula that is similar to the Förster formula. The primary emphasis is placed on recent experimental works that directly confirm the proposed theory. It is shown that the theory satisfactorily quantitatively accounts for (i) the effect of deuteration of molecular groups surrounding ions on k _nr, (ii) the energy gap law, and (iii) the dependence of k _nr on the distance between the ion and deactivating groups. Furthermore, it is shown that (iv) the theory makes it possible to satisfactorily quantitatively calculate in the dipole-dipole approximation the constant k _nr of the electronic transition based on the knowledge of the radiative rate constant and the vibrational absorption spectra of molecular groups in the range of overlap with the luminescence spectrum of the ion; (v) the temperature dependence of k _nr; and (vi) the anomalously low k _nr in the case where the corresponding radiative transition is caused by the magnetic rather than the electric dipole. Literature data are presented that directly experimentally support the proposed theory of nonradiative transitions. In addition, works where this approach is used to calculate k _nr of transitions in laser media are described.     

Ground state energy of spin polarized hard core neutron matter

The expansion of the ground state energy of spin polarized hard core neutron matter in powers of x =k_Fc (k_F = Fermi momentum, c = hardcoreradius) is calculated up to terms ≈x⁸.     

First principles simulations of energy and polarization dependent angle-resolved photoemission spectra of Bi2212

We discuss first-principles simulations of angle-resolved photoemission (ARPES) intensity in Bi2212 where the photoexcitation process is modeled realistically by taking into account the full crystal wavefunctions of the initial and final states in the presence of the surface. Some recent results aimed at understanding the effects of the energy and polarization dependencies of the ARPES matrix element are presented. The nature of the Fermi surface (FS) maps obtained via ARPES by holding the initial state energy fixed at the Fermi energy (E_F) is clarified. The theoretically predicted FS map at 21 eV photon energy displays a remarkable level of agreement with the corresponding ARPES spectrum taken over a large area of the (k_x,k_y) plane. Our analysis shows how the ARPES matrix element can help disentangle closely spaced energy levels and FS sheets and highlight different aspects of the electronic spectrum in complex materials under various experimental conditions.     

Ab initio chemical kinetics for the reactions of HNCN with O(P) and O2

The kinetics and mechanisms of the reactions of cyanomidyl radical (HNCN) with oxygen atoms and molecules have been investigated by ab initio calculations with rate constant prediction. The doublet and quartet state potential energy surfaces (PESs) of the two reactions have been calculated by single-point calculations at the CCSD(T)/6-311+G(3df, 2p) level based on geometries optimized at the CCSD/6-311++G(d, p) level. The rate constants for various product channels of the two reactions in the temperature range of 300-3000 K are predicted by variational transition state and RRKM theories. The predicted total rate constants of the O(³P) + HNCN reaction at 760 Torr Ar pressure can be represented by the expressions k_total (O + HNCN) = 3.12 × 10⁻¹⁰ × T^−0.05 exp (−37/T) cm³ molecule⁻¹ s⁻¹ at T = 300-3000 K. The branching ratios of primary channels of the O(³P) + HNCN are predicted: k₁ for producing the NO + CNH accounts for 0.72-0.64, k₂ + k₉ for producing the ³NH + NCO accounts for 0.27-0.32, and k₆ for producing the CN + HNO accounts for 0.01-0.07 in the temperature range studied. Meanwhile, the predicted total rate constants of the O₂ + HNCN reaction at 760 Torr Ar pressure can be represented by the expression, k_total(O₂ + HNCN) = 2.10 × 10⁻¹⁶ × T^1.28exp (−12200/T) cm³ molecule⁻¹ s⁻¹ at T = 300-3000 K. The predicted branching ratio for k₁₁ + k₁₃ producing HO₂ + ³NCN as the primary products accounts for 0.98-1.00 in the temperature range studied.     

UV-photoemission at constant wavevector component k6: Application to copper

Angle-resolved energy distribution curves of photoelectrons obtained at constant wavevector component k₆ have been presented for the surface of Cu(111). The main observed transitions can be interpreted as due to direct interband transitions between the APW initial states and free-electron-like final states which are calculated simply in one-dimension along k_⊥ lines parallel to the surface normal.     

Tunneling measurements on amorphous Si1−xAux: Metak-insulator transition and superconductivity

The metal- insulator transition and superconductivity of amorphous Si_1−xAu_x was studied by tunneling measurements. A zero bias anomaly was observed in the density of states in the normal state, in agreement with the theory of electron interaction. The energy gap was obtained in the superconducting state. The value of the energy gap at 0 K, Δ₀ was found smaller than 1.76 k_BT_C predicted by the BCS theory.     

Perpendicular anisotropy and magneto-optical Kerr effect of (Ni/Pd) multilayers

Nickel film, with total thickness t_Ni in the range 1000-2000 Å, is known to exhibit perpendicular magnetic anisotropy (PMA), if the film has been deposited at room temperature. This phenomenon is due to the magneto-elastic (ME) effect. The same is also true for the (Ni/Pd)_n multilayers, where n is the period (n≥3). In this paper, we have made two kinds of multilayers: one, which does not have a Pd cap layer, belongs to the A-group, and the other, which has, belongs to the B-group. The polar Kerr rotation θ_k, the polar Kerr ellipticity ε_k, and the figure of merit (θ_k)²R, where R is the reflectance, were measured for the two wavelengths, i.e. λ=633 and 442 nm, respectively. The effective PMA energy K_⊥ was measured from the vibrating sample magnetometer. It was found that the most favorable multilayer for the magneto-optical (MO) application exists among the A-group samples: i.e. the t_Ni=1300 Å, t_Pd=50 Å (seed layer), and n=1 sample. We obtained θ_k=−9.76 min, ε_k=−9.13 min, (θ_k)²R=1.51 (rad)² at λ=442 nm, and K_⊥=3.21×10⁶ erg/cc for this optimal multilayer. Finally, the effects of the Pd seed layer on PMA and MO are also studied.     

On perturbation theory for the gr2N anharmonic oscillator

Perturbation series (PS) in powers of the coupling constant g for the D-dimensional anharmonic oscillator with power anharmonicity gr^2N is considered. The high order PS coefficients ?_k for the ground state energy are calculated explicitly with the help of recurrence relations among intergers. The rate of approach ?_k to their asymptotics $\text{?}{}_{\mathrm{k}}$ as a function of space dimension D is discussed.     

Superfluidity of neutron matter: (II). Triplet pairing

The possibility of neutron triplet pairing and superfluidity in neutron star matter is investigated, and the energy gap and corresponding critical temperature is calculated or estimated as a function of Fermi momentum or density. The calculations are performed for a “one-pion-exchange gaussian” potential, and compared with the results for neutron and proton singlet pairing and superfluidity calculated earlier.The results indicate that neutron superfluidity, corresponding specifically to ³P₂ state pairing, may exist in a high-density shell in the nuclear-matter region of a neutron star, i.e. for 1.6 × 10¹⁴g/cm³ < ρ < 1.4 × 10¹⁵g/cm³, and the maximum self-consistent energy gap is Δ⁰₁k_F ≈ 0.6 MeV and Δ⁰₃(k_F) ≈ 0.1 MeV for an effective mass $\text{m}{}^1\text{≈ 0.75}\text{and}\text{k}{}_{\mathrm{<mtext>F</mtext>}}\text{≈ 2.1}\text{fm}{}^{\mathrm{?1}}$, i.e. for a mas ? ≈ 5.2 × 10¹⁴g/cm³. For $\text{m}{}^1\text{= 1.0}$ we get correspondingly Δ⁰₁(k_F) ≈ 3.3 MeV and Δ⁰₃(k_F) ≈ 0.6 MeV for k_F ≈ 2.2 fm^?1.     

Regularities of energy transfer in aqueous solutions of Ln(III) ions upon formation of their bridged complexes via the CO 3 2− and OH− anions

The influence of the solvent pH and of the presence of carbonic acid anions on the energy transfer from the Eu(III) and Tb(III) ions to a large namber of Ln(III) ions, as well as on the concentration quenching of the Dy(III) ions, is studied. It is shown that, when the anions are present in solution at a concentration by 2–4 orders of magnitude lower than that of the lanthanide ions, the energy transfer rates increase by 2–4 orders of magnitude, but the luminescence decay remains exponential. It is established that the rate constant for energy transfer (k _t ) via the hydroxyl bridge increases with decreasing concentration of lanthanide ions in solution. In an alkalinized solution, (k _t ) depends weakly on the initial water pH, because the concentration of hydroxo groups is governed by hydrolysis of water under the action of the lanthanide ions introduced into it. It is found that, at the 10^?2 M concentration of lanthanide ions in solution, the values of (k _t ) change by almost two orders of magnitude depending on the choice of an ion pair; however, these changes in no way correlate with the overlap integrals of spectra, calculated by adopting the Forster mechanism, and the inclusion of an interaction of higher multipoles does not improve the agreement with the experimental data. It is shown that, when the energy is transferred via the OH^? bridge in solutions with a total concentration of Ln(III) ions of 10^?4 M, the value of (k _t ) increases up to 10⁷ M^?1 s^?1 and becomes independent of the choice of pairs of Ln(III) ions. The dependence of (k _t ) on the ratio of the dissociation constant for a binuclear complex and the probability of energy transfer between the ions within this complex are discussed. It is concluded that the change in this ratio explains the disappearance of the dependence of (k _t ) on the choice of ions as their concentration in solution decreases, but does not explain all the observed changes in (k _t ) if only the dipole-dipole mechanism of energy transfer between the ions in bridged complexes is considered.     

The forbidden rotational Q branch of CH3SiF3: A study in internal rotation

The pure rotational spectrum driven by the small dipole moment produced perpendicular to the symmetry axis by centrifugal distortion has been investigated for CH₃SiF₃ in the ground vibrational state using a Fourier transform waveguide spectrometer. Between 10.9 and 17.0 GHz, four (k + 3 ← k) series in the Q branch have been measured in the lowest torsional state v₆ = 0 for k = 4, 5, 6, and 7 with 54 ? J ? 65. In each transition, the quantum number σ = 0, +1, −1 labelling the different torsional sub-levels is conserved. For given (J,k), splittings from ∼10 to ∼45 MHz have been observed between lines with different values of σ. The global data set includes the anticrossing molecular beam energy differences of [W.L. Meerts, I. Ozier, Chem. Phys. 71 (1982) 401-415] as well as the mm-wave R branch frequencies and (A₁ − A₂) splittings of [P. Dréan, J.-M. Colmont, J. Demaison, L. Dore, C. Degli Esposti, J. Mol. Spectrosc. 176 (1996) 23-27]). A good fit was obtained by varying 15 molecular parameters characterizing the torsion-rotation Hamiltonian H_TR for the vibrational ground state. Because of the strong correlation between two of the quartic torsion-distortion parameters (F_0,3K and D_0,Km) and a redundancy connecting the centrifugal distortion constants, four models were obtained yielding comparable fits. In each case, effective values were determined for the A-rotational constant and the height of the potential hindering the internal rotation. A high precision determination of the structural parameter ρ was made that is the same in all four models. For the off-diagonal quartic centrifugal distortion constant ε₀ and the sextic constants H_0,J, H_0,JK, H_0,KJ, and h_0,3, the differences in the values obtained in the two different reductions used have been explained in terms of the redundancy connecting these parameters. For σ = 0, +1, −1, the energy level pattern for (|k| = 3) is discussed for the case where the pure torsional energy splitting and the matrix elements off-diagonal in k are of comparable magnitude. A method is described of using an R branch study of the resulting σ-splittings for (|k| = 3) to probe the zeroth-order torsional Hamiltonian.