Population Transfer Rates Research Articles

Exciton energy transfer inside cavity-A benchmark study of polaritonic dynamics using the surface hopping method.

Strong coupling between the molecular system and photon inside the cavity generates polaritons, which can alter reaction rates by orders of magnitude. In this work, we benchmark the surface hopping method to simulate non-adiabatic dynamics in a cavity. The comparison is made against a numerically exact method (the hierarchical equations of motion) for a model system investigating excitonic energy transfer for a broad range of parameters. Surface hopping captures the effects of the radiation mode well, both at resonance and off-resonance. We have further investigated parameters that can increase or decrease the rate of population transfer, and we find that surface hopping in general can capture both effects well. Finally, we show that the dipole self-energy term within our parameter regime does not significantly affect the system's dynamics.

Analysis of a diffusive epidemic model with a zero-infection zone

In this paper, we propose and analyze a diffusive epidemic model with a zero-infection zone for the susceptible population. A distinct feature of the model is that the infected people are assumed to not able to cross the boundary of the zero-infection zone while the susceptible people cross the boundary at a prescribed rate. We investigate the global existence and boundedness of the solutions of the model and the existence and uniqueness of the disease-free equilibrium. We define the basic reproduction number and show that it is a threshold parameter for the disease dynamics. Moreover, we study the impact of the population transfer rate at the zero-infection zone boundary on the magnitude of the basic reproduction number. Finally, numerical simulations are performed as a supplement to our theoretical results.

Role of Spatial Impurity Spread on the Transition Dynamics of Doped GaAs Quantum Dot in Presence of Noise

The present study thoroughly explores the time‐average population transfer rate (TAPTR) of impurity‐doped GaAs quantum dot (QD) pursuing the change in the spatial impurity spread (SIS). The said excitation rate is studied under the influence of Gaussian white noise (GWN). The rise of the ground‐state electronic density takes place due to different types of time‐changing fluctuations, viz. simple sinusoidal field, time‐dependent confinement potential, and time‐dependent magnetic field. GWN couples with the QD by additive and multiplicative modes. In this work, the joint influence of SIS and GWN and its pathway of inclusion and the nature of the time‐dependent perturbations are examined on the attributes of the TAPTR. The TAPTR curves are composed of steadfast rise, steadfast diminish, maximization (relevant to generation of large nonlinear optical properties), minimization, and saturation (suggesting dynamic freezing). The findings elucidate the means of fine‐tuning the TAPTR among the doped GaAs QD eigenstates in presence of noise, when the SIS undergoes a gradual change.

Exploiting line-mixing effects for laser absorption spectroscopy at extreme combustion pressures

A unique spectroscopic strategy has been developed for laser absorption sensing of carbon monoxide (CO) and carbon dioxide (CO2) at extreme pressures (P > 50 atm) relevant to modern combustion devices. The strategy exploits the band narrowing effects of line mixing, which acutely impact spectrally dense regions, such as bandheads, where line spacing is small. Line mixing is shown to counter collisional line-broadening effects that reduce differential absorption at elevated pressures and often limit the pressure range of laser absorption methods. In this work, the R-branch bandheads of CO and CO2, which are only observed at high temperatures relevant to combustion, are targeted near 2.3 µm and 4.2 µm, respectively. Spectral line-mixing models were developed for each bandhead region to account for the collision-induced population transfer rates between rotational energy states over a wide range of elevated temperatures and pressures. Modified-exponential-gap models using the relaxation matrix formalism were shown to capture the thermodynamic dependence of the population transfer rates and enabled scaling. Differential absorption at the bandheads was observed to increase by up to a factor of ten at high gas densities, due to line-mixing effects, enabling detection with relatively narrow-band tunable semi-conductor lasers. With refined spectroscopic models, laser absorption measurements of temperature, CO, and CO2 were demonstrated over a range of high pressures (up to 104 atm) in a sub-scale rocket combustor operated with kerosene and supercritical methane.

Theoretical study of the selective control of photoionization and photodissociation

Present work have investigated the selective control of the optimal path and the population transfer rate for photoionization and photodissociation of CO and NO by using time-dependent wave packet dynamics method. Combing the calculated transition properties, it has determined the optimal route of photoionization that is CO X1Σ+→E1П(v’ = 0)→CO+ X2Σ+ by two-photon, which the laser wavelength corresponds to 110 nm and 202 nm, respectively. By changing the laser parameters, we found that, when the intensity is 6.7 × 1013W/cm2 and duration is 100fs, not only the transfer rate of ionized population reaches maximum 99.1%, but also the purity is very high. For photodissociation of NO molecule, the projection of initial wave function of ground state on the quantum state v’ = 8 of excited state A2Σ+ is calculated. It showed that the yield of photodissociation is about 43% by using guassian pulse with intensity 1 × 1014W/cm2 and duration 56fs. However, not only promoting earlier t = 25fs of reaction time but also making the yield increases up to 57% by the positive chirp pulse. These main information suggest a scheme for the feasibly photochemical reaction and will provide an important reference for further theoretical and experimental researches.

Dissipation and dynamics in ultrafast intersystem crossings.

The effects of dynamics and dissipation on ultrafast intersystem crossings are studied for a dissipative two-level system coupled to a local vibronic mode. A method of amplitude damping of the wave packet is presented that accounts better for the position of the wave packet and avoids spurious transitions between potential wells. It is demonstrated that Fermi's golden rule, the typical semiquantitative approach to extract population transfer rates from potential landscapes, only holds under limited conditions. Generally, the effects of dynamics and dissipation lead to deviations from the expected exponential population transfer, strong changes in transfer times and total population transfer, and significant recurrence or "spill back" of the wave packet.

Equilibration Dependence of Fucoxanthin S1 and ICT Signatures on Polarity, Proticity, and Temperature by Multipulse Femtosecond Absorption Spectroscopy.

To demonstrate the value of the multipulse method in revealing the nature of coupling between excited states and explore the environmental dependencies of lowest excited singlet state (S1) and intramolecular charge transfer (ICT) state equilibration, we performed ultrafast transient absorption pump-dump-probe and pump-repump-probe spectroscopies on fucoxanthin in various solvent conditions. The effects of polarity, proticity, and temperature were tested in solvents methanol at 293 and 190 K, acetonitrile, and isopropanol. We show that manipulation of the kinetic traces can produce one trace reflecting the equilibration kinetics of the states, which reveals that lower polarity, proticity, and temperature delay S1/ICT equilibration. On the basis of a two-state model representing the S1 and ICT states on the same S1/ICT potential energy surface, we were able to show that the kinetics are strictly dependent on the initial relative populations of the states as well as the decay of the ICT state to the ground state. Informed by global analysis, a systematic method for target analysis based on this model allowed us to quantify the population transfer rates throughout the life of the S1/ICT state as well as separate the S1 and ICT spectral signatures. The results are consistent with the concept that the S1 and ICT states are part of one potential energy surface.

Rate equation reformulation including coherent excitation: application to periodic protocols based on spectral hole-burning

A large number of signal-processing protocols are based on recording a spectral pattern via spectral hole-burning in an inhomogeneously broadened absorption profile. We present a simulation method specifically designed for periodic excitation sequences leading to the creation of a spectral pattern. This method is applicable to any multilevel atomic structure. The atomic variables’ coherent dynamics are solved for a single temporal excitation step. The result is expressed as an equivalent population transfer rate. This way, the whole sequence is described as a matrix product and the steady state of the system under periodic excitation is easily derived. The propagation through the atomic medium is fully decoupled from the temporal evolution. We apply this method to the engraving of a spectral grating in a large-absorption Tm:YAG sample for wideband spectral analysis.

Mathematical Modeling And Numerical Simulation of Dropwise Condensation on an Inclined Circular Tube

Dropwise condensation can improve heat transfer process and, consequently, leads to considerable reduction in size and weight of condensers as well as improvement in the dehumidification process in many applications, especially in civil transport aircraft. It can also be used as an efficient cooling tool for electronics and electrical systems in aircraft engineering and aerospace technology. In this paper, the stable dropwise condensation on an inclined tube is mathematically analyzed. To do this, the population of small droplets is estimated by population balance theory while an empirical correlation is used for large droplets. To calculate heat transfer across each droplet, sum of temperature drops due to droplet curvature, phase change at droplet-vapor interface, conduction through the droplet and promoter layer, are equated with surface subcooling. The total heat transfer is calculated with the given droplets population and heat transfer through single droplet. Subsequently, effects of various parameters, including surface subcooling, contact angle and contact angle hysteresis on the growth rate, maximum radius of droplet, droplets population, and total heat transfer rate, are investigated. Results show that growth rate and heat flux of small droplets are much higher than those of the larger ones; hence, surface with small droplets is preferred for dropwise condensation purposes. Droplets with low contact angle and contact angle hysteresis have higher heat transfer rates. Increasing the inclination of tube improves heat transfer process to such an extent that vertical tubes have higher heat transfer rate than the horizontal ones. This fact indicates that vertical tubes must be used for designing condensers with dropwise condensation, which is quite the opposite for condensers designed based on filmwise condensation.

A two-stage scheduling method for global humanitarian logistics in large-scale disaster response

Large-scale disaster response is a global issue. Planners must make effective logistics plans to deliver vital first-aid resources (e.g., medicine, food, clothing, machinery) to affected areas from all over the world. Furthermore, planners must consider surging demand and uncertain transportation situations. To explain the efficiency of distribution plans, this study addresses the resource allocation effectiveness losses, which are the losses caused by the mismatch between supply and demand in affected areas, and the emergency logistics time losses, which are the time losses caused by logistics processes under emergency conditions. A two-stage scheduling method is then proposed that considers random demand and travel time. In consideration of the incompleteness or lack of information in real situations, the scheduling method with Bayesian information updates is proposed by assuming that demand and transportation uncertainty are represented by population transfer rate (PTR) and road affected level (RAL), respectively. Next, a simulation study on Wenchuan earthquake response is conducted to illustrate the proposed method. Finally, the insights derived from the study are provided in the conclusion.

A two-stage scheduling method for global humanitarian logistics in large-scale disaster response

Large-scale disaster response is a global issue. Planners must make effective logistics plans to deliver vital first-aid resources (e.g., medicine, food, clothing, machinery) to affected areas from all over the world. Furthermore, planners must consider surging demand and uncertain transportation situations. To explain the efficiency of distribution plans, this study addresses the resource allocation effectiveness losses, which are the losses caused by the mismatch between supply and demand in affected areas, and the emergency logistics time losses, which are the time losses caused by logistics processes under emergency conditions. A two-stage scheduling method is then proposed that considers random demand and travel time. In consideration of the incompleteness or lack of information in real situations, the scheduling method with Bayesian information updates is proposed by assuming that demand and transportation uncertainty are represented by population transfer rate (PTR) and road affected level (RAL), respectively. Next, a simulation study on Wenchuan earthquake response is conducted to illustrate the proposed method. Finally, the insights derived from the study are provided in the conclusion.

A Study on Probe Transmission through an Inverted Y-type Atomic System in Presence of Three Coherent Laser Fields

A four-level inverted Y-type atomic system is subjected to three coherent laser fields simultaneously. The system has two closely spaced ground energy levels, one intermediate energy level and an uppermost energy level. Transition from the two ground levels to the intermediate level as well as from the intermediate level to the uppermost level are dipole allowed. Transition from the two ground energy levels to the uppermost level is dipole forbidden. A weak laser beam, known as the probe beam, acts between one of the ground levels and the intermediate level. A strong laser field, known as the control or pump field, couples the intermediate level with the uppermost level. A repump filed of intensity usually higher than the probe field acts between the other ground level and the intermediate level. The probe transmission through such a medium is investigated at different values of the control and repump Rabi frequencies under Doppler free condition as well as with thermal averaging. The nonradiative population transfer rates between the ground energy levels have been incorporated.

FORMATION OF FIELD INDUCED ABSORPTION IN THE PROBE RESPONSE SIGNAL OF A FOUR-LEVEL ‘V’ TYPE ATOMIC SYSTEM: A THEORETICAL STUDY

A density matrix based analytical model is developed to study the coherent probe field propagation through a four-level V type system in presence of a coherent control field. The model allows coupling of the probe field from the upper ground level to both of the excited levels keeping the control field locked to a particular transition. The addition of an extra ground level to a conventional three-level V type system creates extra decay paths to the ground levels for the upper level population. A set of sixteen density matrix based equations are formed and then solved analytically under rotating wave approximation to study the probe response under steady state condition. The simulated probe absorption spectra shows absorption dip at the centre of a transparency window only under Doppler broadened condition although the conventional EIT window appears under Doppler free condition. The dependence of the field induced absorption signal on the Rabi frequency of the control field, population transfer rate among the ground levels and temperature of the vapour medium has been studied in details.

Influence of Coherent Tunneling and Incoherent Hopping on the Charge Transfer Mechanism in Linear Donor-Bridge-Acceptor Systems.

The mechanism of charge transfer has been observed to change from tunneling to hopping with increasing numbers of DNA base pairs in polynucleotides and with the length of molecular wires. The aim of this paper is to investigate this transition by examining the population dynamics using a tight-binding Hamiltonian with model parameters to describe a linear donor-bridge-acceptor (D-B-A) system. The model includes a primary vibration and an electron-vibration coupling at each site. A further coupling of the primary vibration with a secondary phonon bath allows the system to dissipate energy to the environment and reach a steady state. We apply the quantum master equation (QME) approach, based on second-order perturbation theory in a quantum dissipative system, to examine the dynamical processes involved in charge-transfer and follow the population transfer rate at the acceptor, ka, to shed light on the transition from tunneling to hopping. With a small tunneling parameter, V, the on-site population tends to localize and form polarons, and the hopping mechanism dominates the transfer process. With increasing V, the population tends to be delocalized and the tunneling mechanism dominates. The competition between incoherent hopping and coherent tunneling governs the mechanism of charge transfer. By varying V and the total number of sites, we also examine the onset of the transition from tunneling to hopping with increasing length.

Quasi-Markov Model for Extrapolating Observed Discrete Distributions

We present a quasi-Markov model of population dynamics calibrated to repeated cross-sectional data. While sharing the attractive features of classic Markov models, it avoids the practical problems associated with longitudinal data, and --- due to its focus on population transfer rates between discrete states --- it is particularly well adapted to microsimulation modelling. The proposed method of calculating the model estimation error based on information theory aims at recovering the maximum amount of information contained in the data. The application of the model to data on BMI in the UK and marijuana use in the USA produces reasonable extrapolations and confidence bounds, yielding important observations about the analysed variables. The results and properties of the model are compared with the commonly used multinomial logistic regression method. The thorough cross-validation of the models supports the advantages of using the proposed new approach.

Shortcuts to adiabatic passage for population transfer and maximum entanglement creation between two atoms in a cavity

We use the approach of "transitionless quantum driving" proposed by Berry to construct shortcuts to the population transfer and the creation of maximal entanglement between two $\Lambda $-type atoms based on the cavity quantum electronic dynamics (CQED) system. An effective Hamiltonian is designed by resorting to an auxiliary excited level, a classical driving field and an extra cavity field mode to supplement or substitute the original reference Hamiltonian, and steer the system evolution along its instantaneous eigenstates in an arbitrarily short time, speeding up the rate of population transfer and creation of maximal entanglement between the two atoms inside a cavity. Numerical simulation demonstrates that our shortcuts' performance is robust against the decoherences caused by atomic spontaneous emission and cavity photon leakage.

Parity-dependent oscillations in collisional polarization transfer: CN(A2Π, v = 4) + Ar

We report the first systematic experimental and theoretical study of the state-to-state transfer of rotational angular momentum orientation in a (2)Π-rare gas system. CN(X(2)Σ(+)) was produced by pulsed 266 nm photolysis of ICN in a thermal bath (296 K) of Ar collider gas. A pulsed circularly polarized tunable dye laser prepared CN(A(2)Π, v = 4) in two fully state-selected initial levels, j = 6.5 F1e and j = 10.5 F2f, with a known laboratory-frame orientation. Both the prepared levels and a range of product levels, j' F1e and j' F2f, were monitored using the circular polarized output of a tunable diode laser via cw frequency-modulated (FM) spectroscopy in stimulated emission on the CN(A-X) (4,2) band. The FM Doppler lineshapes for co-rotating and counter-rotating pump-and-probe geometries reveal the time-dependence of the populations and orientations. Kinetic fitting was used to extract the state-to-state population transfer rate constants and orientation multipole transfer efficiencies (MTEs), which quantify the degree of conservation of initially prepared orientation in the product level. Complementary full quantum scattering (QS) calculations were carried out on recently computed ab initio potential energy surfaces. Collision-energy-dependent tensor cross sections for ranks K = 0 and 1 were computed for transitions from both initial levels to all final levels. These quantities were integrated over the thermal collision energy distribution to yield predictions of the experimentally observed state-to-state population transfer rate constants and MTEs. Excellent agreement between experiment and theory is observed for both measured quantities. Dramatic oscillations in the MTEs are observed, up to and including changes in the sign of the orientation, as a function of even/odd Δj within a particular spin-orbit and e/f manifold. These oscillations, along with those also observed in the state-to-state rate constants, reflect the rotational parity of the final level. In general, parity-conserving collisions conserve rotational orientation, while parity-changing collisions result in large changes in the orientation. The QS calculations show that the dynamics of the collisions leading to these different outcomes are fundamentally different. We propose that the origin of this behavior lies in interferences between collisions that sample the even and odd-λ terms in the angular expansions of the PESs.

Peak shape analysis of diagonal and off-diagonal features in the two-dimensional electronic spectra of the Fenna–Matthews–Olson complex

We have recorded a series of two-dimensional electronic spectra of the Fenna-Matthews-Olson (FMO) complex from Prosthecochloris aestuarii, with several crosspeaks sufficiently resolved to permit a quantitative analysis of both the amplitude and the two-dimensional peak shape. The exponential growth and/or decay of peaks on and off the main diagonal provides information on population transfer rates between pairs of excitons. Quantum beats observed in the amplitudes and shapes of these peaks persist throughout the relaxation process, indicating that energy transfer in FMO involves both incoherent and coherent dynamics. By comparing the oscillations in the amplitude and shape of crosspeaks, we confirm theoretical predictions regarding their correlation and identify previously indistinguishable combinations of nonlinear response pathways that contribute to the signal at particular positions in the spectra. Such analysis is crucial to understanding the enormous amount of information contained in two-dimensional electronic spectra and offers a new route to uncovering a complete description of the energy transfer kinetics in photosynthetic antennae.

Nonlinear dimensionality reduction for nonadiabatic dynamics: The influence of conical intersection topography on population transfer rates

Conical intersections play a critical role in the nonadiabatic relaxation of excited electronic states. However, there are an infinite number of these intersections and it is difficult to predict which are actually relevant. Furthermore, traditional descriptors such as intrinsic reaction coordinates and steepest descent paths often fail to adequately characterize excited state reactions due to their highly nonequilibrium nature. To address these deficiencies in the characterization of excited state mechanisms, we apply a nonlinear dimensionality reduction scheme (diffusion mapping) to generate reaction coordinates directly from ab initio multiple spawning dynamics calculations. As illustrated with various examples of photoisomerization dynamics, excited state reaction pathways can be derived directly from simulation data without any a priori specification of relevant coordinates. Furthermore, diffusion maps also reveal the influence of intersection topography on the efficiency of electronic population transfer, providing further evidence that peaked intersections promote nonadiabatic transitions more effectively than sloped intersections. Our results demonstrate the usefulness of nonlinear dimensionality reduction techniques as powerful tools for elucidating reaction mechanisms beyond the statistical description of processes on ground state potential energy surfaces.

Correlated intermolecular coupling fluctuations in photosynthetic complexes

The functioning and efficiency of natural photosynthetic complexes is strongly influenced by their embedding in a noisy protein environment, which can even serve to enhance the transport efficiency. Interactions with the environment induce fluctuations of the transition energies and couplings between the chlorophyll molecules, and due to the fact that different fluctuations will partially be caused by the same environmental factors, correlations between the various fluctuations will occur. We argue that fluctuations of the couplings should, in general, not be neglected, as these have a considerable impact on population transfer rates, decoherence rates, and the efficiency of photosynthetic complexes. Furthermore, while correlations between transition energy fluctuations have been studied, we provide the first quantitative study of the effect of correlations between coupling fluctuations and transition energy fluctuations, and of correlations between the various coupling fluctuations. It is shown that these additional correlations typically lead to changes in interchromophore transfer rates and population oscillations and can lead to a limited enhancement of the light harvesting efficiency.