Observed Decay Time Research Articles

Lengthening of the Sm2+ 4f55d → 4f6 decay time through interplay with the 4f6[5D0] level and its analogy to Eu2+ and Pr3+

Recent research activity on Sm2+-doped compounds has significantly increased the amount of available data on 4f55d → 4f6 decay times. This enabled the systematic comparison of spectroscopic and time resolved luminescence data to theoretical models describing the interplay between the 4f55d and 4f6[5D0] excited states on the observed decay time. A Boltzmann distribution between the population of the excited states is assumed, introducing a dependence of the observed 4f55d → 4f6 decay time on the energy gap between the 4f55d and 4f6[5D0] levels and temperature. The model is used to interpret the origin of the large variation in reported 4f55d → 4f6 decay times through literature, and links their temperature dependence to applications such as luminescence thermometry and near-infrared scintillation. The model is further applied to the analogous situation of close lying 4fn-15d and 4fn states in Eu2+ (6P7/2) and Pr3+ (1S0).

Impact of Molecular Surface Diffusion on the Physical Stability of Co-Amorphous Systems.

In this study, surface diffusion of l-aspartic acid-carvedilol (ASP-CAR) co-amorphous systems at different ASP concentrations is measured and correlated with their physical stability. ASP-CAR films at ASP concentrations of 1-5% (w/w) were prepared by a newly developed method based on a vacuum compression molding process. Surface diffusion measurements were conducted on these systems based on the surface grating decay method using atomic force microscopy (AFM). The results demonstrate that a small amount of ASP (i.e., ≤ 5% w/w) in the co-amorphous systems could significantly slow down the grating decay process compared with that of pure amorphous CAR, indicating a reduced surface diffusion of CAR molecules. The decay time gradually increased in co-amorphous systems with increasing ASP concentration from 1 to 5% (w/w), with the longest observed decay time of around 800 h for the 5%ASP-CAR system, which was more than 200 times longer compared to the decay time of pure amorphous CAR (approximately 3 h). A good correlation between the decay constants of the pure amorphous CAR and co-amorphous films at ASP concentrations of 1-5% (w/w) and the physical stability of corresponding amorphous powder samples was found. Overall, this study provides a new method to prepare co-amorphous films for surface property measurements and reveals the impact of surface diffusion on the physical stability of co-amorphous systems.

Ultrafast dynamics of proflavine bound to poly (methacrylic acid) in aqueous solution

The excited-state dynamics of fluorescent probes attached to polymer segments under un-entangled conditions unveil the internal dynamics and interior polarity of both synthetic and natural polymers. Synthetic polyelectrolytes have been considered as a simple model system to circumvent the structural and dynamic complexity of bio-macromolecules on interaction with solvent molecules in the vicinity. In this study, femtosecond dynamics of proflavine cation, a molecular probe with well-established excited state dynamics have been explored by steady-state and time-resolved fluorescence to uncover structural dynamics of polymethacrylic acid at the sub-picosecond to the picosecond time domain. The femtosecond fluorescence transients display a bi-exponential decay. The shorter decay time of 1–5 ps represents the reorientation of the water molecules during the excited state of the fluorophore. The increase in the lifetime of this component from 1.0 ps to 5.0 ps on increasing the pH of the solution from 2.0 to 5.0 is ascribed to the reorientation of water molecules due to the compact and hydrophobic nature of the macromolecule. The second decay component of proflavine bound to PMAA increases from 70 ps to 180 ps on increasing the pH of the solution from 2.5 to 5.0. The hydrophobic part of the polyelectrolyte present under acidic conditions restricts the solvent reorganization during the solvation of the covalently bound dye molecule and the stretched linear chains of the polyelectrolyte do not influence the solvation and the observed decay time is identical to that of free dye in aqueous solution.

Simulation of UV absorption spectra and relaxation dynamics of uracil and uracil-water clusters.

Despite many studies, the mechanisms of nonradiative relaxation of uracil in the gas phase and in aqueous solution are still not fully resolved. Here we combine theoretical UV absorption spectroscopy with nonadiabatic dynamics simulations to identify the photophysical mechanisms that can give rise to experimentally observed decay time constants. We first compute and theoretically assign the electronic spectra of uracil using the second-order algebraic-diagrammatic-construction (ADC(2)) method. The obtained electronic states, their energy differences and state-specific solvation effects are the prerequisites for understanding the photodynamics. We then use nonadiabatic trajectory-surface-hopping dynamics simulations to investigate the photoinduced dynamics of uracil and uracil-water clusters. In contrast to previous studies, we found that a single mechanism - the ethylenic twist around the C[double bond, length as m-dash]C bond - is responsible for the ultrafast component of the nonradiative decay, both in the gas phase and in solution. Very good agreement with the experimentally determined ultrashort decay time constants is obtained.

Determination of Transport Coefficients by Coronal Seismology of Flare-induced Slow-mode Waves: Numerical Parametric Study of a 1D Loop Model

Abstract Recent studies of a flaring loop oscillation event on 2013 December 28 observed by the Atmospheric Imaging Assembly of the Solar Dynamics Observatory have revealed the suppression of thermal conduction and significant enhancement of compressive viscosity in hot (∼10 MK) plasma. In this study we aim at developing a new coronal seismology method for determining the transport coefficients based on a parametric study of wave properties using a 1D nonlinear magnetohydrodynamics (MHD) loop model in combination with the linear theory. The simulations suggest a two-step scheme: we first determine the effective thermal conduction coefficient from the observed phase shift between temperature and density perturbations as this physical parameter is insensitive to the unknown viscosity; then from the loop model with the obtained thermal conduction coefficient, we determine the effective viscosity coefficient from the observed decay time using the parametric modeling. With this new seismology technique we are able to quantify the suppression of thermal conductivity by a factor of about 3 and the enhancement of viscosity coefficient by a factor of 10 in the studied flaring loop. Using the loop model with these refined transport coefficients, we study the excitation of slow magnetoacoustic waves by launching a flow pulse from one footpoint. The simulation can self-consistently produce the fundamental standing wave on a timescale in agreement with the observation.

Ultrafast Dynamics of Electronically Excited Host-Guest Complexes of Cucurbiturils with Styryl Dyes.

The relaxation mechanism of electronically excited states of host–guest complexes between cucurbiturils (CB) and pyridinium styryl dyes is considered in detail on the basis of the recent results obtained by the up-conversion fluorescence technique. The addition of CB to aqueous dye solutions increases the longest fluorescence decay times from about 50 ps for the free dyes to 100–150 ps for the bound ones. This is attributed to the braking of intramolecular rotations around the single bonds of the styryl moiety that is provided by guest’s displacement inside the cavity, whose driving force is a Coulombic interaction of the styryl dye cation and negatively charged CB portals. This displacement, a translational movement along the CB axis, is associated with the observed decay time of about 1 ps. There is also a characteristic time of about 100 fs, attributed to vibrational relaxation. In fact, such complexes can operate as a molecular machine, the molecular switch.

Decay of muons generated by laser-induced processes in ultra-dense hydrogen H(0)

This work reports identification of muons by their characteristic life-time of 2.20 μs after laser-induction of their precursor mesons, both kaons K± and KL0 and pions π± in ultra-dense hydrogen H(0). The pair-production signal from scattered muons at a metal converter in front of a photo-multiplier detector is observed with its decay. The observed signal intensity is decreased by a metal beam-flag which intercepts the meson and muon flux to the detector. Using D(0), the observed decay time is (2.23 ± 0.05) μs in agreement with the free muon lifetime of 2.20 μs. This signal is apparently due to the preferential generation of positive muons. Using p(0), the observed decay time is in the range 1–2 μs, thus shorter than the free muon lifetime, as expected when the signal is mainly caused by negative muons which interact with matter by muon capture.

Ultrafast Excited-State Dynamics of Hydrogen-Bonded Cytosine Microsolvated Clusters with Protic and Aprotic Polar Solvents.

Microsolvation effects on the ultrafast excited-state deactivation dynamics of cytosine (Cy) were studied in hydrogen-bonded Cy clusters with protic and aprotic solvents using mass-resolved femtosecond pump-probe ionization spectroscopy. Two protic solvents, water (H2O) and methanol (MeOH), and one aprotic solvent, tetrahydrofuran (THF), were investigated, and transients of Cy·(H2O)1-6, Cy·(MeOH)1-3, and Cy·THF microsolvated clusters produced in supersonic expansions were measured. With the aid of electronic structure calculations, we assigned the observed dynamics to the low-energy isomers of various Cy clusters and discussed the microsolvation effect on the excited-state deactivation dynamics. With the protic solvents only the microsolvated clusters of Cy keto tautomer were observed. The observed decay time constants of Cy·(H2O) n are 0.5 ps for n = 1 and ∼0.2-0.25 ps for n = 2-6. For Cy·(MeOH) n clusters, the decay time constant for n = 1 cluster is similar to that of the Cy monohydrate, but for n = 2 and 3 the decays are about a factor of 2 slower than the corresponding microhydrates. With the aprotic solvent, THF, hydrogen-bonded complexes of both keto and enol tautomers are present in the beam. The keto-Cy·THF shows a decay similar to that of the keto-Cy monomer, whereas the enol-Cy·THF exhibits a 2-fold slower decay than the enol-Cy monomer, suggesting an increase in the barrier to excited-state deactivation upon binding of one THF molecule to the enol form of Cy.

Spectral and temporal photoluminescence behavior of colloidal PbS quantum dots

We have studied a temporal and spectral behavior of the photoluminescence of the colloidal PbS quantum dots (QDs) with a controlled spacing between QDs. The high-speed streak camera system combined with a monochromator was used to obtain the time evolution of the luminescence spectrum itself.In the close-packed QD sample with a QD edge-to-edge spacing of 4.2nm, the two-step photoluminescence decay with time constants of 30 and 200ns was observed after initial fast change in the luminescence due to the fast energy transfer process. This two-step decay also occurred for the other QD samples with large QD edge-to-edge spacings, i.e., 5.8 and 12.5nm. Here, the observed decay time for these samples showed the same spectral behavior even though the distance between QDs is considerably different. Such characteristic photon energy dependence strongly suggests a decay mechanism that the decay consists of (1) fast non-radiative decay from the excited state to the ground state through surface defect states and (2) slow intrinsic radiative decay. Thus, these results indicate a new aspect of the photoluminescence decay characteristics, which shows the importance of the non-radiative energy transfer path in the PbS QD luminescence.

Growth and Luminescence Properties of Ce and Ca Co-Doped LiGdF<sub>4</sub>-LiF Eutectic Scintillator

Ce and Ca co-doped LiGdF4-LiF eutectics were grown and their directionally solidified eutectic (DSE) system has been investigated. In these eutectics, apart from the expected 305 nm emission of the regular Ce3+, 310 nm emission of the Gd3+ and Ce3+-perturbed emission around 320-380 nm was observed. Ce3+-perturbed emission at 340 nm shows the intrinsic decay time of about 27 ns. In photoluminescence mechanism the observed decay time shortening of the regular Ce3+ and Gd3+ centers point to the energy transfer in the sequence the regular Ce3+→ (Gd3+)n→ the perturbed Ce3+ sites.

Creation and dynamics of two-dimensional skyrmions in antiferromagnetic spin-1 Bose-Einstein condensates

We numerically simulate the creation process of two-dimensional skyrmionic excitations in antiferromagnetic spin-1 Bose--Einstein condensates by solving the full three-dimensional dynamics of the system from the Gross--Pitaevskii equation. Our simulations reproduce quantitatively the experimental results of [Choi et al., Phys. Rev. Lett. 108, 035301 (2012)] without any fitting parameters. Furthermore, we examine the stability of the skyrmion by computing the temporal evolution of the condensate in a harmonic potential. The presence of both the quadratic Zeeman effect and dissipation in the simulations is vital for reproducing the experimentally observed decay time.

Light-matter coupling in ZnTe-based micropillar cavities containing CdTe quantum dots

We study the coupling of CdTe quantum dots emission with ZnTe-based micropillar cavity modes. Nonresonant cavity mode feeding is reported together with an enhancement of the emission of a quantum dot thanks to resonant coupling with the cavity mode. The coupling is evidenced both in experiments with continuous and pulsed excitation. A theoretical Purcell factor is calculated and an experimental Purcell factor 5.7 is determined confirming the theoretical predictions. Additionally, we discuss the influence of the cascaded emission occurring under increased excitation power on the observed decay time of the excitonic transition.

Ultrafast optical Kerr effect of excitons weakly confined in GaAs thin films

AbstractWe investigate ultrafast response of optical Kerr effect (OKE) induced by excitons weakly confined in GaAs thin films. The observed decay time of optical response agrees approximately with incident pulse duration. At 100‐fs pulse irradiation the response time is estimated to be 170 fs, though the exciton lifetime is about 10 ns. It is considered that the ultrafast response of OKE is caused by not population relaxation but orientation relaxation of excitons. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Study of scintillation in helium mixed with xenon to develop thermal neutron detectors

We have studied the transfer of luminescence from He to Xe by adding a small amount of Xe to He. Luminescence spectra, scintillation yields and time profiles of the mixture were measured. The He/Xe mixture emitted scintillation photons in the 140–210 nm wavelength region, peaked around 173 nm. The scintillation yields and time profiles were measured using a PMT sensitive to photon wavelengths between 115 and 300 nm. The observed decay time of scintillation in the mixture He/Xe is shorter than that of He 2 * . The scintillation yield increased with increasing Xe fraction. However, without recombination light it is saturated above a Xe pressure of 0.06 MPa.

Direct observation and reactions of Cl3 radical

The broad absorption of Cl3 radical was observed between 1150 and 1350 nm using cavity ring-down spectroscopy at 213-265 K and 50-200 Torr with He, N2, Ar, or SF6 diluents. The absorption intensity of Cl3 increased at lower temperature and higher pressure. SF6 was the most efficient diluent gas. The temperature dependent equilibrium constants for Cl3 formation from Cl+Cl2 were theoretically calculated at the MP4SDQ6-311+G(d) level. Observed decay time profiles of Cl3 and the pressure dependence of Cl3 formation are explained by the equilibrium reaction and a decay reaction of Cl+Cl3.

Temperature dependence of carrier lifetimes in InN

AbstractTime‐resolved pump–probe transmission measurements were used to determine the temperature dependence of carrier lifetime for InN epilayers with unintentionally doped levels from 1018 to 1019 cm–3. The observed decay time at 20 K is well explained by a dominating radiative interband recombination, while at room temperature it is attributed to a defect related nonradiative recombination channel. The temperature dependence of the radiative lifetime is deduced from the measurements of both differential transmission decay time and PL intensity. For the best quality sample, we find the radiative lifetime increases proportionally to T3/2, as theory predicts when a k‐selection rule holds, which suggests that the radiative band‐to‐band transition accounts for the observed infrared photoluminescence over the entire temperature. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Quantum Turbulence in SuperfluidHe3Illuminated by a Beam of Quasiparticle Excitations

We have measured directly the Andreev scattering of a controllable beam of quasiparticle excitations by a localized tangle of quantum vortices in superfluid 3He-B at low temperatures. We present a microscopic description of the Andreev scattering from a vortex line allowing us to estimate the vortex separation scale in a dilute tangle of vortices, providing a better comparison of the observed decay time of the turbulence with recent numerical simulations. The experiments also suggest that below 200 microK we reach the low temperature limit for turbulent dynamics.

Structure Dependent Ultrafast Relaxation Time of Photo-excited Carriers in SiC

The ultrafast inter-conduction band carrier dynamics in 6H-SiC and 4H-SiC was observed by using the pump and probe transient absorption technique. The observed decay times of transmission changes are 1.25 ps for 6H-SiC and 630 fs for 4H-SiC, respectively. Dependence of the transmission change on probe wavelength and polarization were analyzed by using the steady-state absorption profiles. From this analysis, the transmission change was ascribed to the decrease of electron population in the lowest conduction band. The experimental results show that the decay of the transmission change was dominated by the inter-band electron phonon scattering. The inter-band deformation potential in 4H-SiC, deduced from the observed decay time, was 1.25 times larger than that in 6H-SiC. This polytype dependence of inter-band deformation potential is discussed in terms of the hexagonality and band-gap energies.

Strong temperature dependence of photoluminescence properties in visible InAlAs quantum dots

Strong temperature dependence of optical properties has been studied in visible InAlAs/AlGaAs quantum dots, by employing photoluminescence (PL) and time-resolved photoluminescence (TRPL) measurements. The fast redshift of the exciton emission peak was observed at much lower temperature range compared to that observed in the InAs/GaAs QDs. In TRPL we did not observe the constant decay time even at low temperature. Instead, the observed decay time increases quickly with increasing temperature, showing 2D properties in the transient dynamic process. We attributed our results to the strong lateral coupling effect, which results in the formation of the local minibands or extended states from the discrete energy levels.

Microscopic Models for Ultrafast Photoinduced Solvent to Dye Electron Transfer in DMA/Oxazine Solution

A microscopic model for the ultrafast photoinduced electron-transfer process in solutions of oxazine-1 in N,N-dimethylaniline (DMA) is presented. Translational and rotational motions of the solvent molecules are treated classically by molecular dynamics simulations. Along the trajectory, the energy gaps between the optical excitation and the charge transfer states involving the first solvation shell are calculated quantum mechanically, together with the corresponding electron-transfer matrix elements. The first solvation shell consists of ∼20 DMA molecules and is rather stable with a decay time of ∼20 ps. The calculated fluctuations of energy gaps and couplings occur on a time scale of about 0.3−1 ps, considerably slower than the fastest experimentally observed decay time of 70 fs. At most points along the trajectory, at least one DMA molecule is in a position and orientation that gives large electron transfer matrix elements up to 0.08 eV. The coupling of the electron transfer to intermolecular vibration...