Dominant Decay Mechanism Research Articles

Normal form expansions and thermal decay rates of Bose-Einstein condensates with short- and long-range interaction

The thermally induced coherent collapse of Bose-Einstein condensates at finite temperature is the dominant decay mechanism near the critical scattering length in condensates with at least partially attractive interaction. The collapse dynamics out of the ground state is mediated by a transition state whose properties determine the corresponding decay rate or lifetime of the condensate. In this paper, we perform normal form expansions of the ground and the transition state of condensates with short-range scattering interaction as well as with anisotropic and long-range dipolar interaction in a variational framework. This method allows one to determine the local properties of these states, i.e. their mean-field energy, their normal modes, the coupling between different modes, and the structure of the reaction channel to any desired order. We discuss the physical interpretation of the transition state as a certain density distribution of the atomic cloud and the behavior of the single normal form contributions in dependence on the s-wave scattering length. Moreover, we investigate the convergence of the local normal form when using extended Gaussian variational approaches, and present the condensate’s decay rate.

Carrier trapping study on a Ge nanocrystal by two-pass lift mode electrostatic force microscopy

Trapped charges inside an isolated germanium nanocrystal (Ge NC) have been studied by two-pass lift mode electrostatic force microscopy (EFM) measurements at room temperature. From visualized EFM images, electrons and holes were proven to be successfully injected and trapped in the Ge NC and distributed homogenously at the edge of its truncated spherical morphology. The Ge NC is found to have iso-potential surface and behave as a conductive material after being charged. It is also shown that the dominant charge decay mechanism during discharging of Ge NCs is related to the leakage of these trapped charges. A truncated capacitor model is used to approximate the real capacitance between the tip and Ge NC surface and to quantitatively study these trapped charges. These investigations demonstrate the potential for Ge nanocrystal memory applications.

Platinum Oxide Growth on Pt/C Fuel Cell Catalysts Using Asymmetric Scan Electrochemical Quartz Crystal Nanogravimetry

The urban automotive drive cycle includes multiple rapid idle-to-peak power transients posing a significant durability challenge for proton exchange membrane fuel cells (PEMFCs) [1–4]. The power transients force the cathode catalyst to undergo potential transitions where Pt surface oxides are quickly formed and removed, resulting in Pt catalytic surface area loss and severe performance decay [5–7]. Considerable progress has been made to reduce performance loss by surface nanoengineering [8–14] and novel catalyst systems’ development [13, 15–23]. Despite all of the achievements in theoretical modeling of fuel cell processes [24, 25], the relative contributions of the known decay mechanisms and their dependence on the transient potential profile and operational conditions remain elusive and therefore limiting the development of mitigation strategies. The dominant decay mechanisms are Pt dissolution with or without redeposition [26–32], migration/ coalescence [33], catalyst nanoparticle size distribution change [34, 35], and detachment from the support due to carbon corrosion [36]. The factors influencing the transient potential profiles include transition rates, the upper and lower potential limits (range between 0.6 and 1.0 V), and time at either idle (∼1.0 V) or peak (∼0.6 V) power [37]. The transition rates allow for the extent to oxide formation and reduction that control surface reorganization [25]. Operational conditions that influence decay rates include temperature [38] and relative humidity (acidity) [39]. The electrochemical quartz crystal nanobalance (EQCN) technique has been developed to utilize the piezoelectric effect of a quartz crystal to quantify nanogram (<0.4 ng cm) mass changes [40, 41]. The sensitivity to mass change scales directly with the resonate frequency (ƒ0) of the crystal which is typically between 5 and 10 MHz. The mass change (Δm) is inversely proportional to the frequency change (−Δƒ) through the Sauerbrey equations, Δm=−Cf Δƒ, where (Cf) is the mass sensitivity coefficient. The mass change is complicated by the dynamics of the interfacial electrical double layer impacted by the orientation of the electrolyte and water dipoles that vary with potential, in addition to Pt dissolution and oxide formation and reduction at cathode relevant potentials. EQCN studies of Pt-coated quartz crystals have probed oxide formation during cyclic voltammetric scans at rates between 5 and 70 mV s [3, 42, 43], imposed perturbation profile at a single upper potential limit [3, 28], and mass loss over time during potentiostatic holds [30, 44]. A limited number of EQCN studies have been done on Pt/C nanoparticles limited to mass loss during potential holds [45, 46]. Within the present study, the impact of single perturbation sweep rates on commercial Pt/C, common to accelerated stress tests and instantaneous automotive idle-to-peak power C. A. Rice :D. Betancourt Center for Manufacturing Research and Department of Chemical Engineering, Tennessee Tech University, Cookeville, TN 38501, USA

Macroscopic quantum many-body tunneling of attractive Bose-Einstein condensate in anharmonic trap

We study the stability of attractive atomic Bose-Einstein condensate and the macroscopic quantum many-body tunneling (MQT) in the anharmonic trap. We utilize correlated two-body basis function which keeps all possible two-body correlations. The anharmonic parameter (λ) is slowly tuned from harmonic to anharmonic. For each choice of λ the many-body equation is solved adiabatically. The use of the van der Waals interaction gives realistic picture which substantially differs from the mean-field results. For weak anharmonicity, we observe that the attractive condensate gains stability with larger number of bosons compared to that in the pure harmonic trap. The transition from resonances to bound states with weak anharmonicity also differs significantly from the earlier study of [N. Moiseyev, L.D. Carr, B.A. Malomed, Y.B. Band, J. Phys. B 37, L193 (2004)]. We also study the tunneling of the metastable condensate very close to the critical number N cr of collapse and observe that near collapse the MQT is the dominant decay mechanism compared to the two-body and three-body loss rate. We also observe the power law behavior in MQT near the critical point. The results for pure harmonic trap are in agreement with mean-field results. However, we fail to retrieve the power law behavior in anharmonic trap although MQT is still the dominant decay mechanism.

Optically detected magnetic resonance studies of luminescence‐quenching processes in π‐conjugated materials and organic light‐emitting devices

AbstractIt is widely recognized that nonradiative quenching of excitons by other excitons and polarons become the dominant decay mechanism of these excitons at high excitation densities. These quenching processes cause the roll‐off in the efficiency of organic light‐emitting devices (OLEDs) and prevent lasing at high injection current densities. This review presents the optically‐detected magnetic resonance (ODMR) evidence for these photoluminescence‐ and electroluminescence‐quenching processes. And while it provides such evidence for quenching of singlet excitons by polarons and triplet excitons, it reveals the central role of the strongly spin‐dependent annihilation of triplet excitons by polarons, since under normal excitation conditions the steady‐state polaron and triplet exciton populations are 100–104 times the singlet exciton population. In addition, it also suggests that quenching of singlet excitons by bipolarons, likely stabilized by a counterpolaron or countercharge at specific sites, may also be a significant quenching mechanism that also affects the charge transport properties.

A power pulsed low-pressure argon microwave plasma investigated by Thomson scattering: evidence for molecular assisted recombination

A squared-wave power pulsed low-pressure plasma is investigated by means of Thomson scattering. By this method the values of the electron density and temperature are obtained, directly. The plasma is created by a surfatron launcher in pure argon at gas pressures of 8–70 mbar. Features of the pulse rise and decay are studied with microsecond time resolution. During the pulse rise we observe initial high temperature values, while the density is still rising. At power switch-off we find decay times of the electron density that are smaller than what is expected on the basis of diffusion losses. This implies that the dominant decay mechanism in the studied pressure regime is provided by molecular assisted recombination.

Triplet dynamics in fluorescent polymer light-emitting diodes

We report a study of the triplet exciton dynamics and their effect on the performance of fluorescent organic light-emitting diodes. These polymer light-emitting diodes comprise metal oxide, injection electrodes, and poly(9,9${}^{\ensuremath{'}}$-dioctylfluorene-co-benzothiadiazole) as the emissive material and exhibit external quantum efficiencies up to 6.5$%$. Transient optical absorption measurements following a short (0.5 to 50 $\ensuremath{\mu}$s) electrical drive pulse were used to monitor triplet dynamics during device operation. Triplet generation and decay processes were modeled, and we find that triplet-triplet annihilation is the dominant triplet decay mechanism. Singlet states, generated from triplet-triplet annihilation were monitored as delayed electroluminescence after the end of the drive pulse. From the delayed electroluminescence dynamics, we determine monomolecular as well as bimolecular triplet decay rates and estimate the triplet-charge annihilation rate. Singlet states generated from bimolecular triplet-triplet annihilation contribute up to 33$%$ of the total amount of singlets generated in these fluorescent devices. To model these results, we require that triplet states can undergo bimolecular annihilation several times. With this model, we show that singlets can reach a maximum fraction of 40$%$ of all excitons generated by charge recombination, without violating spin statistics. Singlet states generated from triplet-triplet annihilation are one important explanation for high external quantum efficiencies found in these fluorescent devices.

Measurements of the48Ca(γ,n)reaction

The 48Ca({\gamma},n) cross section was measured using {\gamma}-ray beams of energies between 9.5 and 15.3 MeV generated at the Triangle Universities Nuclear Laboratory (TUNL) high-intensity {\gamma}-ray source (HI{\gamma}S). Prior to this experiment, no direct measurements had been made with {\gamma}-ray beams of sufficiently low energy spread to observe structure in this energy range. The cross sections were measured at thirty-four different {\gamma}-ray energies with an enriched 48Ca target. Neutron emission is the dominant decay mechanism in the measured energy range that spans from threshold, across the previously identified M1 strength, and up the low-energy edge of the E1 giant dipole resonance (GDR). This work found B(M 1) = 6.8 \pm 0.5 {\mu}N2 for the 10.23 MeV resonance, a value greater than previously measured. Structures in the cross section commensurate with extended random-phase approximation (ERPA) calculations have also been observed whose magnitudes are in agreement with existing data.

Three-body photodissociation dynamics of [formula omitted](CO 2)

The three-body dissociation of I 2 - (CO 2) following excitation of the I 2 - chromophore to the repulsive A′ 2Π g,1/2 and B 2 Σ g , 1 / 2 + electronic states at 1.72 and 3.21 eV has been investigated using fast beam photofragment translational spectroscopy. The translational energy distributions for three-body dissociation provide a direct measurement of the CO 2 binding energy, yielding a value of 218 ± 10 meV. These distributions are vibrationally resolved and show that some CO 2 is produced with bend excitation. Dalitz plots show that the dominant three-body decay mechanism is asynchronous–concerted decay, in which the two bond cleavages are distinct but nearly simultaneous events.

Influence of the charge carrier tunneling processes on the recombination dynamics in single lateral quantum dot molecules

We report on the charge carrier dynamics in single lateral quantum dot molecules and the effect of an applied electric field on the molecular states. Controllable electron tunneling manifests itself in a deviation from the typical excitonic decay behavior in dot molecules. It results in a faster population decay and can be strongly influenced by the tuning electric field and intermolecular Coulomb energies. A rate equation model is developed and compared to the experimental data to gain more insight into the charge transfer and tunneling mechanisms. Nonresonant (phonon-mediated) electron tunneling which changes the molecular exciton character from direct to indirect, and vice versa, is found to be the dominant tunable decay mechanism of excitons besides radiative recombination.

EFM Study on Ge Island: Carrier Charge and Storage Effect

AbstractIn this paper, individual Ge nano island on top of a silicon dioxide layer of thermally grown on a n+ type doped silicon (001) substrate have been studied. The charging ability of an individual Ge island was evaluated by EFM two-pass lift mode measurement. Such Ge nano island becomes an iso-potential and behaves as a conductive material after being charged. These charges were directly injected and were trapped homogenous in the isolated Ge island. It is also shown that the dominant charge decay mechanism during discharging of nc-Ge is related to the leakage of these trapped charges. Further more, the retention time of these trapped charges was evaluated and the electrostatic force was also studied by using different tip bias during scan. Such a study should be very useful to the Ge-nc in memory applications.

Excited state reactions in fluorescent proteins

The green fluorescent protein is a key technology in bioimaging. In this critical review, we consider how its various applications can be tailored from knowledge of the excited state chemistry. The photophysics of the basic chromophore in solution are described in detail, and the dominant radiationless decay mechanism is characterised. The quite different photophysics of wild type GFP are described next. The unique excited state proton transfer reaction observed can be used to model proton transfer processes in proteins. Examples where the proton transfer is blocked, or redirected to occur over a low short barrier H-bond are discussed. Finally the photophysics underlying the new generation of photochemically active fluorescent proteins are discussed (155 references).

Optical Phonon Lifetimes in Single-Walled Carbon Nanotubes by Time-Resolved Raman Scattering

The lifetimes of optical phonons (OPs) in single-walled carbon nanotubes are determined by time-resolved incoherent anti-Stokes Raman scattering using a subpicosecond pump-probe method. Lifetimes in semiconducting and metallic nanotubes at room temperature are similar, 1.2 and 0.9 ps, respectively. The OP lifetimes decrease with increasing temperature, approximately scaling as approximately 1/T, consistent with anharmonic processes being the dominant decay mechanism for both semiconducting and metallic nanotubes.

Comparison of the low-energy decay mechanisms of C70+ and C70−

Low-energy multiple-collisional-excitation experiments have been performed on C70+ and C70− at the ClusterTrap apparatus. The ions are stored in a Penning trap where they are excited via radial dipolar excitation before undergoing collisions with neutral argon atoms. The dominant decay mechanism for C70+ (sequential loss of C2-units) is compared with the dominant decay process of C70− (thermionic electron emission). A simple model based on the decay rates of the clusters is found to be in reasonable agreement with the experimental data obtained for the fragmentation process of the cationic fullerenes. The same model, when applied to the anions is observed to be in less agreement with the experimental results.

The Influence of Surface Trapping and Dark States on the Fluorescence Emission Efficiency and Lifetime of CdSe and CdSe/ZnS Quantum Dots

To investigate the influence of surface trapping and dark states on CdSe and CdSe/ZnS quantum dots (QDs), we studied the absorption, fluorescence intensity and lifetime by using one-and two-photon excitation, respectively. Experimental results show that both one- and two-photon fluorescence emission efficiencies of the QDs enhance greatly and the lifetime increase after capping CdSe with ZnS due to the effective surface passivation. The lifetime of one-photon fluorescence of CdSe and CdSe/ZnS QDs increase with increasing emission wavelength in a supralinear way, which is attributed to the energy transfer of dark excitons. On the contrary, the lifetime of two-photon fluorescence of bare and core-shell QDs decrease with increasing emission wavelength, and this indicates that the surface trapping is the dominant decay mechanism in this case.

Metastable rovibrational f -symmetry levels of the I 1 Π g state of H2, HD and D2: experiment and theory

We present accurate experimental measurements of the lifetimes and term values of the metastable rovibrational f-symmetry levels of the I 1Πg state of HD. These levels lie above the second dissociation limit of hydrogen. The lifetimes were obtained directly from the observation of the time-dependent decay of the fluorescence. These levels decay by direct fluorescence and by predissociation after tunnelling through the potential energy barrier of the I 1Πg state. We present ab initio results for both of these decay processes for H2, HD and D2. The theoretical calculations provide an explanation of the rotational dependence of the observed lifetimes for HD, the lack of rotational dependence for D2, and the approximately equal lifetimes for N = 1 for HD and D2. The calculated lifetimes for the metastable levels of H2 explain why no experimental data is yet available for the parent isotopomer. However, the use of recently updated Hönl–London factors with published ab initio electronic transition moments reveals a larger than expected discrepancy with experiment for those levels of HD and D2 for which emission is the dominant decay mechanism.

Charge trapping properties at silicon nitride/silicon oxide interface studied by variable-temperature electrostatic force microscopy

Charge trapping properties of electrons and holes in ultrathin nitride-oxide-silicon (NOS) structures were quantitatively determined by variable-temperature electrostatic force microscopy (EFM). From charge retention characteristics obtained at temperatures between 250 and 370°C and assuming that the dominant charge decay mechanism is thermal emission followed by oxide tunneling, we find that there are considerable deep trap centers at the nitride-oxide interface. For electron, the interface trap energy and density were determined to be about 1.52eV and 1.46×1012cm−2, respectively. For hole, these are about 1.01eV and 1.08×1012cm−2, respectively. In addition, the capture cross section of electron can be extracted to be 4.8×10−16cm2. The qualitative and quantitative determination of charge trapping properties and possible charge decay mechanism reported in this work can be very useful for the characterization of oxide-nitride-silicon based charge storage devices.

Energy and Electron Transfer in β-Alkynyl-Linked Porphyrin−[60]Fullerene Dyads

Three porphyrin-fullerene dyads, in which a diyne bridge links C(60) with a beta-position on a tetraarylporphyrin, have been synthesized. The free-base dyad was prepared, as well as the corresponding Zn(II) and Ni(II) materials. These represent the first examples of a new class of conjugatively linked electron donor-acceptor systems in which pi-conjugation extends from the porphyrin ring system directly to the fullerene surface. The processes that occur following photoexcitation of these dyads were examined using fluorescence and transient absorption techniques on the femtosecond, picosecond, and nanosecond time scales. In sharp contrast to the photodynamics associated with singlet excited-state decay of reference tetraphenylporphyrins (ZnTPP, NiTPP, and H(2)TPP), the diyne-linked dyads undergo ultrafast (<10 ps) singlet excited-state deactivation in toluene, tetrahydrofuran (THF), and benzonitrile (PhCN). Transient absorption techniques with the ZnP-C(60) dyad clearly show that in toluene intramolecular energy transfer (EnT) to ultimately generate C(60) triplet excited states is the dominant singlet decay mechanism, while intramolecular electron transfer (ET) dominates in THF and PhCN to give the ZnP(*+)/C(60)(*-) charge-separated radical ion pair (CSRP). Electrochemical studies indicate that there is no significant charge transfer in the ground states of these systems. The lifetime of ZnP(*+)/C(60)(*-) in PhCN was approximately 40 ps, determined by two different types of transient absorption measurement in two different laboratories. Thus, in this system, the ratio of the rates for charge separation (k(CS)) to rates for charge recombination (k(CR)), k(CS)/k(CR), is quite small, approximately 7. The fact that charge separation (CS) rates increase with increasing solvent polarity is consistent with this process occurring in the normal region of the Marcus curve, while the slower charge recombination (CR) rates in less polar solvents indicate that the CR process occurs in the Marcus inverted region. While photoinduced ET occurs on a similar time scale in a related dyad 15 in which a diethynyl bridge connects C(60) to the para position of a meso phenyl moiety of a tetrarylporphyrin, CR occurs much more slowly; i.e., k(CS)/k(CR) approximately equal to 7400. Thus, the position at which the conjugative linker is attached to the porphyrin moiety has a dramatic influence on k(CR) but not on k(CS). On the basis of electron density calculations, we tentatively conclude that unfavorable orbital symmetries inhibit charge recombination in 15 vis a vis the beta-linked dyads.

Adjustment of the Southern Ocean to Wind Forcing on Synoptic Time Scales

Abstract This study addresses the response of the Southern Ocean to high-frequency wind forcing, focusing on the impact of several barotropic modes on the circumpolar transport. A suite of experiments is performed with an unstratified model of the Southern Ocean, forced with a stochastic wind stress that contains a large range of frequencies with synoptic time scales. The Southern Ocean adjustment displays a different character for frequencies below and above 0.2 cpd. The low-frequency range is dominated by an “almost-free-mode” response in the region where contours of f /H are obstructed by only a few bathymetric features; the truly free mode only plays a minor role. Topographic form stress, rather than friction, is the dominant decay mechanism of the Southern Mode. It leads to a spindown time scale on the order of 3 days. For the high-frequency range, the circumpolar transport is dominated by the resonant excitation of oscillatory modes. The “active” response of the ocean leads to strong changes and even discontinuities in the phase relation between transport and wind stress.

The rare decays of D mesons

The flavor changing transitions in the c → uγ, c → uγγ and c → ul + l − offer the possibility to search for new physics in the charm sector. We investigate dominant decay mechanisms in the radiative decays D → Vγ, D → P( V) l + l −, D → γγ and we discuss chances to see physics beyond the standard model in these decays. In addition, we analyze Cabibbo allowed D → Kπγ decays with nonresonant Kπ, and we probe the role of light vector mesons in these decays.