Investigation of sensitized anti-stokes phosphors using sinusoidal modulation of the pump intensity

We describe a simple and relatively inexpensive method of investigating the processes by which incident IR radiation is converted to visible emission in anti-Stokes phosphors by multiphoton excitation. The procedure involves sinusoidal modulation of the intensity of the IR exciting radiation and analysis of the harmonic content of the visible output from the phosphor. A two-photon process, for example, results in some second harmonic modulation of the visible emission. The harmonic content depends on the relaxation times involved and on the nature of the conversion process. The experiments thus enable one to establish the order of the conversion process, to distinguish between possible mechanisms and to determine certain relaxation times. Representative results for the ytterbium-sensitized anti-Stokes phosphors Y₂O₂S-Ho-Yb and Y₂O₂S-Er-Yb are described.     

Relaxation Processes at High Levels of Vibrational Excitation of Polyatomic Molecules in the Ground Electronic State

By the spectral and kinetic characteristics of the luminescence of vapors of polyatomic molecules (anthracene, anthraquinone, fluorenone) initiated by selective IR multiphoton excitation (IR MPE) of molecules in the ground electronic state S ₀ the relaxation processes proceeding under vibrational excitation of molecules to energies exceeding the energies of the lower excited electronic states have been investigated. The changes in the spectral and kinetic characteristics with increasing CO₂ laser energy density and vapor P _v and foreign gas pressure P _FG are analyzed. They are similar to the characteristics obtained for normal fluorescence of these molecules with changing vibrational energy E _vib content. On the basis of experimental data and model calculations it has been concluded that at the laser radiation densities used in the case of IR MPE the molecules reach energies considerably exceeding the energies of the electronic levels. It is shown that a nonadiabatic connection between the electronic states leads to the population of mixed electronic states isoenergetic to the vibrational levels of the ground electronic state and to emission of delayed luminescence spectrally identical to the normal luminescence of these molecules. It has been found that when high vibrational levels are populated, new relaxation channels, such as reverse electron relaxation, emission from high vibrational levels of the ground electronic state, and multiquantum vibrational energy transfer at collisions leading to a rapid establishment of vibrational equilibrium become important.     

Upconversion emission enhancement of TiO2 coated lanthanide-doped Y2O3 nanoparticles

To investigate the upconversion emission,this paper synthesizes Tm3+ and Yb3+ codoped Y2O3 nanoparticles,and then coats them with TiO2 shells for different coating times.The spectral results of TiO2 coated nanoparticles indicate that upconversion emission intensities have respectively been enhanced 3.2,5.4,and 2.2 times for coating times of 30,60 and 90 min at an excitation power density of 3.21×102 W.cm 2,in comparison with the emission intensity of non-coated nanoparticles.Therefore it can be concluded that the intense upconversion emission of Y2O3:Tm3+,Yb3+ nanoparticles can be achieved by coating the particle surfaces with a shell of specific thickness.     

Renormalization group analysis of polymer cyclization with non-equilibrium initial conditions

We develop a renormalization group approach for cyclizing polymers for the case when chain ends are initially close together (ring initial conditions). We analyze the behavior at times much shorter than the longest polymer relaxation time. In agreement with our previous work (Europhys. Lett. 73, 621 (2006)) we find that the leading time dependence of the reaction rate k(t) for ring initial conditions and equilibrium initial conditions are related, namely k _ring(t) ∝ t ^-δ and k _eq(t) ∝ t ^1-δ for times less than the longest polymer relaxation time. Here δ is an effective exponent which approaches δ = 5/4 for very long Rouse chains. Our present analysis also suggests a “sub-leading” term proportional to (ln t)/t which should be particularly significant for smaller values of the renormalized reaction rate and early times. For Zimm dynamics, our RG analysis indicates that the leading time dependence for the reaction rate is k(t) ∼ 1/t for very long chains. The leading term is again consistent with the expected relation between ring and equilibrium initial conditions. We also find a logarithmic correction term which we “exponentiate” to a logarithmic form with a Landau pole. The presence of the logarithm is particularly important for smaller chains and, in the Zimm case, large values of the reaction rate.     

Dielectric relaxation spectroscopy during isothermal curing of semi-interpenetrating polymer networks

Dielectric relaxation spectroscopy measurements have been performed during isothermal curing of semi-interpenetrating polymer networks (semi-IPNs) and the related pure networks at frequencies between 3 kHz and 3 MHz and curing temperatures between 313 and 393 K. The pure networks consist of diglycidyl ether of bisphenol A (DGEBA) cross-linked with diaminodiphenylmethane (DDM) and the semi-IPNs contain in addition 10 or 20 wt% of polysulfone PSn) as the linear component. Temperature dependent dielectric measurements have been performed on the fully cured samples in the temperature range from 130 to 550 K for the same frequencies. For the pure networks, the imaginary part of the complex permittivity ?″ shows a decrease with curing time t _cure followed by a peak. For the semi-IPNs this peak is much broader for all cases and can be resolved into two maxima for several curing conditions. The decrease in ?″ is connected to the decrease in the dc-conductivity due to gelation, whereas the peak is related to the relaxation of dipoles. The existence of two maxima in the time dependence of ?″ is an indication for two different relaxations in a phase separated structure. This is supported by temperature dependent measurements on the fully cured samples. The characteristic relaxation times ι, which are extracted from the maxima of ?″(t _cure) for different frequencies f using the relation ωτ = 1(ω = 2πf), increase during curing by several orders of magnitude for both the pure networks and the semi-IPNs. In order to extract the characteristic times for gelation t _g and the times t ₀ where the dc-conductivity σ_dc approaches a singularity, σ_dc was fitted by a power law, σ_dc = σ₀[(t _g - t)/t _g]^p, and an exponential function, σ_dc = A exp [B/(t - t _o)], respectively. In the error limits of the experiments both procedures lead to similar results. The t _g and t _o values are in good agreement with those measured mechanically and no significant differences between pure networks and semi-IPNs cured at the same temperature were found.     

Luminescence and origin of lead-related centers in single crystalline films of Y2SiO5 and Lu2SiO5

In the temperature range 4.2–350 K, the steady-state and time-resolved emission and excitation spectra and luminescence decay kinetics are studied for the undoped Y₂SiO₅ and Lu₂SiO₅ single crystalline films grown by liquid phase epitaxy method from the PbO-based flux and, owing to that, containing lead ions substituting for Y³⁺ or Lu³⁺ ions. Luminescence characteristics of Pb-related centers of different types are identified. On the basis of the results obtained, we suggest that the ultraviolet emission of Pb-related centers arises from the Pb²⁺ ions substituting for Y³⁺ or Lu³⁺ ions in the Y1 and Lu1 lattice sites of the X₂ structure. Possible hypotheses on the origin of the intense complex lead-related blue emission are discussed. We propose phenomenological models describing the excited-state dynamics of the studied luminescence centers. We also determine characteristic parameters of the corresponding relaxed excited states, in particular, the energy separations between the excited states and the rates of the radiative and non-radiative transitions from these states.     

Structure and Curie temperature of Y<Subscript>2</Subscript>Fe<Subscript>17−<Emphasis Type="Italic">x</Emphasis></Subscript>Cr<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>

The structures of Y₂Fe_17−x Cr _x are simulated by theab initio potentials. The site preference of Cr atom in Y₂Fe₁₇ is evaluated and the order is determined as 4f, 12j, which is close to the experimental result. Based on the site preference behavior, the calculated parameters and the atom sites of Y-Fe-Cr system are studied. The result corresponds well to observed data. Further, the DOS of the relaxed structures are calculated and the variation in Curie temperature is explained qualitatively by the spin-fluctuation theory.     

The role of excited species in UV-laser materials ablation

The stability of a planar surface upon pulsed UV-laser irradiation is studied with special emphasis on polymer ablation. Here, we consider a two-level system in which the excitation energy is dissipated via stimulated emission, non-radiative transitions, and activated desorption of excited species. With thermal relaxation times t _T10^–10 s the ablation front turns out to become stable. This could explain the smooth surfaces obtained after pulsed UV-laser ablation of pure and stress free organic polymers. The situation is quite different for materials, for example metals, where fast thermal relaxation of the excitation energy within times, typically, t _T<10^–11 s, gives rise to instabilities which result in surface roughening.On leave from the Institute of General Physics, Russian Academy of Sciences, 117942 Moscow, RussiaOn leave from the Institute of Applied Physics, Russian Academy of Sciences, 603600 Nishnii Novgorod, Russia     

Non-linear scission/recombination kinetics of living polymerization

Living polymers are formed by reversible association of primary units (unimers). Generally the chain statistical weight involves a factor σ < 1 suppressing short chains in comparison with free unimers. Living polymerization is a sharp thermodynamic transition for σ ≪ 1 which is typically the case. We show that this sharpness has an important effect on the kinetics of living polymerization (one-dimensional association). The kinetic model involves i) the unimer activation step (a transition to an assembly-competent state); ii) the scission/recombination processes providing growth of polymer chains and relaxation of their length distribution. Analyzing the polymerization with no chains but unimers at t = 0 , with initial concentration of unimers M ≳ M ^* (M^* is the critical polymerization concentration), we determine the time evolution of the chain length distribution and find that: 1) for M ^* ≪ M ≪ M ^*/σ the kinetics is characterized by 5 distinct time stages demarcated by 4 characteristic times t₁, t₂, t₃ and t^*; 2) there are transient regimes (t ₁ ≲ t ≲ t ₃) when the molecular-weight distribution is strongly non-exponential; 3) the chain scissions are negligible at times shorter than t₂. The chain growth is auto-accelerated for t ₁ ≲ t ≲ t ₂ : the cut-off chain length (= polymerization degree 〈n〉_w N ₁ ∝ t ² in this regime. 4) For t ₂ < t < t ₃ the length distribution is characterized by essentially 2 non-linear modes; the shorter cut-off length N₁ is decreasing with time in this regime, while the length scale N₂ of the second mode is increasing. (5) The terminal relaxation time of the polymer length distribution, t^*, shows a sharp maximum in the vicinity of M^*; the effective exponent is as high as ∼ σ^-1/3 just above M^*.     

The effects of thermal annealing on the relaxation of rubbing-induced birefringence in polystyrene

We have conducted a systematic study on the effects of post rubbing annealing on the relaxation of rubbing-induced birefringence of polystyrene. It is found that annealing at T₀ only affects the relaxation up to T₀ + T_Lag, where T_Lag is proportional to the logarithm of the annealing time t_A. A theoretical model based on the distribution of relaxation times due to the individual birefringence elements is proposed. To remove its contribution to the net birefringence each element must overcome an energy barrier E = (317 + 1.17ξ)×10³ J/mol, and therefore must have a characteristic relaxation time τ which depends on temperature T and a barrier height which ranges from 340.4 kJ/mol to 445.7 kJ/mol. The relaxation of birefringence is expressed by the equation NB(T, t) = N(ξ)e^-t/τ(T,ξ)dξ, in which both the relaxation time τ(T,ξ) and the distribution function N(ξ) can be extracted from experimental data. The predictions of the model agree well with all the experimental results presented in this work. The differences and similarities of the relaxation of birefringence with respect to the physical aging of quenched PS are discussed. In particular, similarities in terms of the general temperature lag phenomena are noted.     

Nonexponential Photoluminescence Dynamics in an Inhomogeneous Ensemble of Excitons in WSe2 Monolayers

The spectral and spatiotemporal dynamics of photoluminescence in monolayers of transition metal dichalcogenide WSe₂ obtained by mechanical exfoliation on a Si/SiO₂ substrate is studied over a wide range of temperatures and excitation powers. It is shown that the dynamics is nonexponential and, for times t exceeding ∼50 ps after the excitation pulse, is described by a dependence of the form 1/(t + t₀). Photoluminescence decay is accelerated with a decrease in the temperature and in the energy of emitting states. It is shown that the observed dynamics cannot be described by a bimolecular recombination process, such as exciton—exciton annihilation. A model that describes the nonexponential photoluminescence dynamics by taking into account the spread of radiative recombination times of localized exciton states in a random potential gives good agreement with experimental data.

     

Influence of the nematic order on the rheology and conformation of stretched comb-like liquid crystalline polymers

We have studied the rheology and the conformation of stretched comb-like liquid-crystalline polymers. Both the influence of the comb-like structure and the specific effect of the nematic interaction on the dynamics are investigated. For this purpose, two isomers of a comb-like polymetacrylate polymer, of well-defined molecular weights, were synthesized: one displays a nematic phase over a wide range of temperature, the other one has only an isotropic phase. Even with high degrees of polymerization N, between 40 and 1000, the polymer chains studied were not entangled. The stress-strain curves during the stretching and relaxation processes show differences between the isotropic and nematic comb-like polymers. They suggest that, in the nematic phase, the chain dynamics is more cooperative than for a usual linear polymer. Small-angle neutron scattering has been used in order to determine the evolution of the chain conformation after stretching, as a function of the duration of relaxation t _r. The conformation can be described with two parameters only: , the global deformation of the polymer chain, and p, the number of statistical units of locally relaxed sub-chains. For the comb-like polymer, the chain deformation is pseudo-affine: is always smaller than (the deformation ratio of the whole sample). In the isotropic phase, has a constant value, while pincreases as tr. This latter behavior is not that expected for non-entangled chains, in which p varies as t _r ^1/2 (Rouse model). In the nematic phase, decreases as a stretched exponential function of t _r, while p remains constant. The dynamics of the comb-like polymers is discussed in terms of living clusters from which junctions are produced by interactions between side chains. The nematic interaction increases the lifetime of these junctions and, strikingly, the relaxation is the same at all scales of the whole polymer chain. Received 5 May 1999 and Received in final form 18 October 1999     

Functionals of paths of a diffusion process and the Onsager-Machlup function

The Onsager-Machlup function for a one-dimensional diffusion processX _t is obtained by means of the Radon-Nikodym derivative of the measure in function space induced byX _t with respect to the measure corresponding to an auxiliary diffusion processY _t.     

A μSR Study of Er Spin Dynamics in (Y1−x Er x )Ni2B2C Superconductors

Zero field μSR has been used to probe rare earth spin dynamics in the magnetic superconductors, Y_1−x Er _x Ni₂B₂C. The muon spin relaxation function is stretched exponential, exp (−(λt)^β), in form, as usually found for spin glass systems above the glass temperature. However, the Y_1−x Er _x Ni₂B₂C compounds show no evidence of coexisting superconducting and static spin glass ground states even at concentrations below the critical value (x=0.6) for long range antiferromagnetic order. The temperature dependence of both the muon spin relaxation rate λ and the exponent β suggests that Er spin dynamics change significantly at the superconducting transition temperature. This revised version was published online in September 2006 with corrections to the Cover Date.     

Optical study of single crystals grown by the Czochralski method from Yb3+-doped (Gd1−x Y x )2SiO5 solid solution

Yb³⁺-doped crystals were grown by the Czochralski method from (Gd_0.5Y_0.5)₂SiO₅ melt employing seeds cut out from single crystals of Yb:Gd₂SiO₅ and Yb:Y₂SiO₅. XRD analysis revealed that the structure of the solid solution crystals grown is consistent with the structure of the seed, namely C2/c for the Yb:Y₂SiO₅ seed and P2₁/c for the Yb:Gd₂SiO₅ seed. Optical absorption spectra, emission spectra and luminescence decay curves were recorded at 10 K and 300 K. Analysis of gathered spectroscopic data made it possible to evaluate radiative transition rates for the ²F_5/2 multiplet of Yb³⁺ in the two structures and to assess crystal field splitting of multiplets involved in radiative transitions. It has been concluded that transition intensities and relaxation dynamics of Yb³⁺ in the systems studied are similar but the intensity distribution of emission and absorption bands depends significantly on the system structure. This feature, combined with inhomogeneous broadening of spectral lines, may be advantageous for the tailoring of lasers employing ytterbium-doped crystals for specific applications.     

Dipolar interaction and incoherent quantum tunneling: a Monte Carlo study of magnetic relaxation

We study the magnetic relaxation of a system of localized spins interacting through weak dipole interactions, at a temperature large with respect to the ordering temperature but low with respect to the crystal field level splitting. The relaxation results from quantum spin tunneling but is only allowed on sites where the dipole field is very small. At low times, the magnetization decrease is proportional to as predicted by Prokofiev and Stamp, and at long times the relaxation can be described as an extension of a relaxed zone. The results can be directly compared with very recent experimental data on Fe₈ molecular clusters. Received 9 February 1999     

Enthalpy Relaxation Study of Poly(Vinyl Chloride) Films Based on the Time-Temperature Superposition Principle

Abstract

In the enthalpy relaxation of poly(vinyl chloride), a decrease in enthalpy upon the isothermal ageing was measured using the differential scanning calorimetry method as a function of ageing time (t_A) and ageing temperature. The range of the ageing temperature was from 56?°C (T_g ? 25?°C) to 72?°C (T_g ? 9?°C) where T_g denotes the glass transition temperature. The limiting value of the decrease in enthalpy was determined by applying a stretched exponential function to the measured enthalpy data. The relaxation function (?) was derived from the measured enthalpy and the construction of a master curve was tried by shifting the ? ? t_A curves of the respective ageing temperatures horizontally. Although there was no agreement between the shift factors (a_T) and the relaxation times of the ? ? t_A curves, the superposition was successfully constructed and the a_T values obtained for the poly(vinyl chloride) sample were found to be comparable to those reported for viscoelastic experiments over a broad temperature range above and below T_g carried out for different polymers. The origin of the decrease in enthalpy was briefly discussed in terms of the chain dynamics in the isothermal condition.     

Foreign gas broadening of an IR absorption line of formaldehyde

The theory of nuclear spin state relaxation of symmetrical molecules like formaldehyde contains a collision time t_c that is interpreted as time between rotationally inelastic molecular collisions. This time traditionally is determined from measurements of pressure broadening of spectral lines. In order to test whether these collision times, which determine spin relaxation rate constants and line broadening coefficients, respectively, are the same or at least related to another, we have performed systematic measurements of the broadening of an IR line of formaldehyde by other gases of different pressures.     

Stretched exponential relaxation in the Coulomb glass

The relaxation of the specific heat and the entropy to their equilibrium values is investigated numerically for the three-dimensional Coulomb glass at very low temperatures. The long time relaxation follows a stretched exponential function, f (t) = f ₀exp - (t/τ)^β , with the exponent β increasing with the temperature. The relaxation time diverges as an Arrhenius law when T→ 0. Received 24 May 2001 and Received in final form 12 September 2001     

Joint Optimization of the Number of Pulses and the Relaxation Pause in the Combined Multiple-Pulse Spin-Locking Sequence for Detecting Explosives by NQR

The problem has been solved for the joint optimization of the number N of pulses and the relaxation pause t _p in the combined multiple-pulse spin-locking sequence, which is successfully used for detecting some explosives by nuclear quadrupole resonance (NQR). Initially, the formula is derived for the signal-to-noise ratio provided by this sequence. Using the condition that an explosive detection time T _det is constant (it is typical in practice), from this expression the set of two interrelated equations is obtained for optimum values of N and t _p, which provide the maximum signal-to-noise ratio. These values are determined by both effective transverse T _2e and longitudinal T ₁ relaxation times and sequence parameters, and do not depend on the number m of sequence repetitions. The m value is calculated as the quotient of T _det by the duration of the onefold sequence with optimum values of N and t _p. This set of equations is shown to be mutually compatible at all values of sequence parameters and relaxation times T ₁ and T _2e, which occur in actual practice. The results of the experiments, which have been carried out on cyclotrimethylene trinitramine (RDX) at a ¹⁴N NQR frequency of 3.41 MHz and temperatures of 26 and −3°C, are in good agreement with the theoretical ones.