Nonadiabatic Model Research Articles

Adsorption dynamics and effects of carbon to zeolite ratio of layered beds for multicomponent gas adsorption

To optimize the performance of an adsorption bed, layered beds of activated carbon and zeolite 5A are used for multicomponent gas separation (H2: 72.2%, CO2: 21.6%, CO: 2.03%, CH4: 4.17%). The adsorption dynamic characteristics were studied experimentally and theoretically using layered beds of activated carbon and zeolite 5A at 8 atm adsorption pressure and 16.67 L/min feed rate. Non-isothermal and non-adiabatic models, based on a linear driving force model and Dual-Site Langmuir adsorption isotherm model, were used. As the carbon ratio increased, the average velocity of CH4 wavefront became slow, and wavefronts of CO and CO2 propagated quickly on average.

Oscillatory thermal-diffusive instability of combustion waves in a model with chain-branching reaction and heat loss

In this paper we investigate the properties and the linear stability of premixed combustion waves in a non-adiabatic thermal-diffusive model with a two-step chain-branching reaction mechanism. Here we focus only on the emergence of the pulsating instabilities, and the stability analysis is carried out for Lewis numbers for fuel greater than one, and various values of Lewis number for radicals. We consider the problem in two spatial dimensions to allow perturbations of a multidimensional nature. It is demonstrated that the flame speed as a function of the parameters is a double-valued C-shaped function, i.e. for a given set of parameter values there are either two solutions, fast and slow solution branches, propagating with different speed, or the combustion wave does not exist. The extinction of combustion waves occurs at finite values of the parameters and non-zero flame speed. The flame structure demonstrates a slow recombination regime behaviour with negligible fuel leakage for the fast solution branch away from the extinction condition. For parameter values close to the extinction condition and on the slow solution branch, the fuel leakage is significant and a fast recombination regime is observed. It is demonstrated that two types of instabilities emerge in the model: the uniform planar and the travelling instability. The slow solution branch is always unstable due to the uniform perturbations. The fast solution branch is either stable or loses stability due to the travelling or uniform perturbations. The switching between the onset of various regimes of instability is due to the bifurcation of co-dimension two. In the adiabatic limit this bifurcation is found for Lewis number for fuel equal to one, whereas in the non-adiabatic case it moves towards values above unity. The properties of the travelling instability are studied in detail.

Nonadiabatic Excited-State Molecular Dynamics Modeling of Photoinduced Dynamics in Conjugated Molecules

Nonadiabatic dynamics generally defines the entire evolution of electronic excitations in optically active molecular materials. It is commonly associated with a number of fundamental and complex processes such as intraband relaxation, energy transfer, and light harvesting influenced by the spatial evolution of excitations and transformation of photoexcitation energy into electrical energy via charge separation (e.g., charge injection at interfaces). To treat ultrafast excited-state dynamics and exciton/charge transport we have developed a nonadiabatic excited-state molecular dynamics (NA-ESMD) framework incorporating quantum transitions. Our calculations rely on the use of the Collective Electronic Oscillator (CEO) package accounting for many-body effects and actual potential energy surfaces of the excited states combined with Tully's fewest switches algorithm for surface hopping for probing nonadiabatic processes. This method is applied to model the photoinduced dynamics of distyrylbenzene (a small oligomer of polyphenylene vinylene, PPV). Our analysis shows intricate details of photoinduced vibronic relaxation and identifies specific slow and fast nuclear motions that are strongly coupled to the electronic degrees of freedom, namely, torsion and bond length alternation, respectively. Nonadiabatic relaxation of the highly excited mA(g) state is predicted to occur on a femtosecond time scale at room temperature and on a picosecond time scale at low temperature.

Adsorption and thermal regeneration of acetone and toluene vapors in dealuminated Y-zeolite bed

Adsorption and thermal regeneration dynamics of acetone and toluene on dealuminated Y-zeolite (DAY-zeolite) were studied experimentally and theoretically. The isotherms of acetone on DAY-zeolite changed from Type-II to Type-III with an increase in temperature, whereas the isotherms of toluene were Type-I. The adsorption amount of acetone was higher than that of toluene at low temperatures, but its adsorption affinity was weaker. Due to the differences in isotherm shape, acetone and toluene showed different adsorption and desorption dynamic behaviors. The breakthrough point of acetone was achieved faster than that of toluene, but the saturation time was reached later. On the other hand, in the regeneration step via a hot nitrogen purge, acetone could be obtained more quickly and at higher concentration from the DAY-zeolite bed because its isotherm approached Type-III with a temperature increase. A non-equilibrium and non-adiabatic model, incorporating a linear driving force model, Langmuir model for toluene and Aranovich–Donohue model for acetone, were used to simulate the dynamic behaviors of concentration and temperature during the adsorption and thermal regeneration steps. The effects of operating variables such as purge gas velocity, regeneration temperature, and initial bed loading on the regeneration step were studied. Also, the specific energy requirement and purge gas consumption were evaluated to discuss the process efficiency of the DAY-zeolite bed.

Theoretical description of adiabatic laser alignment and mixed-field orientation: the need for a non-adiabatic model

We present a theoretical study of recent laser-alignment and mixed-field-orientation experiments of asymmetric top molecules. In these experiments, pendular states were created using linearly polarized strong ac electric fields from pulsed lasers in combination with weak electrostatic fields. We compare the outcome of our calculations with experimental results obtained for the prototypical large molecule benzonitrile (C(7)H(5)N) [J. L. Hansen et al., Phys. Rev. A, 2011, 83, 023406.] and explore the directional properties of the molecular ensemble for several field configurations, i.e., for various field strengths and angles between ac and dc fields. For perpendicular fields one obtains pure alignment, which is well reproduced by the simulations. For tilted fields, we show that a fully adiabatic description of the process does not reproduce the experimentally observed orientation, and it is mandatory to use a diabatic model for population transfer between rotational states. We develop such a model and compare its outcome to the experimental data confirming the importance of non-adiabatic processes in the field-dressed molecular dynamics.

The widespread occurrence of enzymatic hydrogen tunneling, andits unique properties, lead to a new physical model for the origins of enzyme catalysis

The investigation of C-H bond activation by enzymes over the past several decades has revealed a plethora of deviations from semi-classical kinetic models. Although the early enzymatic results were interpreted in the context of a tunneling correction, the emergent properties are now seen to be largely incompatible with this type of analysis as well. This chapter introduces some of the experimental data that form the basis for our present understanding. A vibronically nonadiabatic model, that has a number of features in common with the Marcus treatment for electron transfer, offers a robust physical picture for the hydrogen tunneling behavior seen in both native enzymes and in enzymes that have been perturbed either by site-specific mutagenesis or by perturbation of the reaction conditions. Native enzymes under optimal conditions most commonly show behavior that requires a heavy atom donor-acceptor distance that is in the range of 2.7Å. This compression beyond van der Waals distances is proposed to arise from the process of enzymatic conformational sampling. The absence of any evolutionary driving force to optimize tunneling for deuterium transfer (natural abundance < 0.02%), together with the frequent observation that the enthalpic barrier for deuterium transfer is the same or very similar to that for protium transfer, leads to the proposal that tunneling is a consequence of a generic property of enzyme function in which overall protein flexibility enables the generation of active sites that can be quite compressed.

Exact quantum statistics for electronically nonadiabatic systems using continuous path variables

We derive an exact, continuous-variable path integral (PI) representation of the canonical partition function for electronically nonadiabatic systems. Utilizing the Stock-Thoss (ST) mapping for an N-level system, matrix elements of the Boltzmann operator are expressed in Cartesian coordinates for both the nuclear and electronic degrees of freedom. The PI discretization presented here properly constrains the electronic Cartesian coordinates to the physical subspace of the mapping. We numerically demonstrate that the resulting PI-ST representation is exact for the calculation of equilibrium properties of systems with coupled electronic and nuclear degrees of freedom. We further show that the PI-ST formulation provides a natural means to initialize semiclassical trajectories for the calculation of real-time thermal correlation functions, which is numerically demonstrated in applications to a series of nonadiabatic model systems.

Kinetic electron emission from metals induced by impact of slow atomic particles

Available experimental data on the kinetic electron emission from metals bombarded by low energy atomic particles below the classical threshold were analyzed in terms of one-electron non-adiabatic model and in terms of phenomenological many-electron models. Total electron yields as a function of the particle velocity for several distinctly different substrate-particle systems were successfully interpreted using a phenomenological model. The essential microscopic mechanism of the model is the valence electron Auger interaction which leads to the Boltzmann – like energy distribution of the excited electrons in the impact zone.

Investigation on the structural and electrical properties of NdSrNi 1−xCr xO 4+δ (0.1≤ x≤0.9) system

The phases NdSrNi 1− x Cr x O 4+ δ (0.1≤ x≤0.9) have been synthesized by modified sol–gel method and subsequent annealing at 1250 °C in 1 atm of flowing argon. X-ray diffraction (XRD) analysis and electrical resistivity have been measured at room temperature. Rietveld refinement shows that all compositions with x>0.1 were found to crystallize in the tetragonal K 2NiF 4 type structure in the space group I4/ mmm, while for x=0.1, a mixture of two phases with the tetragonal space group I4/ mmm and the orthorhombic space group Fmmm. Variations of a and c parameters show a complex behavior with increasing chromium content. It was established that compounds with chromium content less then x≤0.5 are oxygen-deficient, while for x>0.5 the sample are oxygen-overstoichiometric. The NdSrNi 0.5Cr 0.5O 4+ δ compound exhibits semiconductive behavior and the electrical transport mechanism agrees with the non-adiabatic small polaron hopping model in the temperature ranges 298–493, 493–573 and 573–703 K separately.

Towards hot electron mediated charge exchange in hyperthermal energy ion–surfaceinteractions

We have made Na + and He + ions incident on the surface of solid state tunnel junctions and measured the energy lossdue to atomic displacement and electronic excitations. Each tunnel junction consists of anultrathin film metal–oxide–semiconductor device which can be biased to create a band ofhot electrons useful for driving chemical reactions at surfaces. Using the binary collisionapproximation and a nonadiabatic model that takes into account the time-varying natureof the ion–surface interaction, the energy loss of the ions is reproduced. The energy loss forNa + ions incident on the devices shows that the primary energy loss mechanism is the atomicdisplacement of Au atoms in the thin film of the metal–oxide–semiconductor device. Wepropose that neutral particle detection of the scattered flux from a biased device could be aroute to hot electron mediated charge exchange.

The nature of p-modes and granulation in HD 49933 observed by CoRoT

<i>Context. <i/>Recent observations of HD 49933 by the space-photometric mission CoRoT provide <i>photometric<i/> evidence of solar type oscillations in a star other than our Sun. The first published reduction, analysis, and interpretation of the CoRoT data yielded a spectrum of p-modes with <i>l<i/> = 0, 1, and 2.<i>Aims. <i/>We present our own analysis of the CoRoT data in an attempt to compare the detected pulsation modes with eigenfrequencies of models that are consistent with the observed luminosity and surface temperature. <i>Methods. <i/>We used the Gruberbauer et al. frequency set derived based on a more conservative Bayesian analysis with ignorance priors and fit models from a dense grid of model spectra. We also introduce a Bayesian approach to searching and quantifying the best model fits to the observed oscillation spectra.<i>Results. <i/>We identify 26 frequencies as radial and dipolar modes. Our best fitting model has solar composition and coincides within the error box with the spectroscopically determined position of HD 49933 in the H-R diagram. We also show that lower-than-solar Z models have a lower probability of matching the observations than the solar metallicity models. To quantify the effect of the deficiencies in modeling the stellar surface layers in our analysis, we compare adiabatic and nonadiabatic model fits and find that the latter reproduces the observed frequencies better.

Shaping of attosecond pulses by phase-stabilized polarization gating

We demonstrate that the characteristics of the high-order harmonic spectra generated by few-cycle carrier-envelope phase-stabilized pulses can be finely adjusted by controlling the time-dependent ellipticity. The experimental measurements show evidence for the generation of single, pairs, and trains of attosecond pulses by controlling the time window of linear polarization of the driving pulses. The influence of the carrier-envelope phase on the generation process in different confinement configurations is interpreted and analyzed using a nonadiabatic stationary phase model. We show that the xuv emission depends critically on particular aspects of the fundamental electric field that allows us to steer the electron trajectories on the time scale of tens of attoseconds.

Constraints on primordial isocurvature perturbations and spatial curvature by Bayesian model selection

We present posterior likelihoods and Bayesian model selection analysis for generalized cosmological models where the primordial perturbations include correlated adiabatic and cold dark matter isocurvature components. We perform nested sampling with flat and, for the first time, curved spatial geometries of the Universe, using data from the CMB anisotropies, the Union supernovae (SN) sample, and a combined measurement of the integrated Sachs-Wolfe effect. The CMB alone favors a 3% (positively correlated) isocurvature contribution in both the flat and curved cases. The nonadiabatic contribution to the observed CMB temperature variance is $0l{\ensuremath{\alpha}}_{T}l7%$ at 98% C.L. in the curved case. In the flat case, combining the CMB with SN data artificially biases the result towards the pure adiabatic $\ensuremath{\Lambda}\mathrm{CDM}$ concordance model, whereas in the curved case the favored level of nonadiabaticity stays at the 3% level with all combinations of data. However, the ratio of Bayes factors, or $\ensuremath{\Delta}\mathrm{ln}$ (evidence), is more than 5 points in favor of the flat adiabatic $\ensuremath{\Lambda}\mathrm{CDM}$ model, which suggests that the inclusion of the 5 extra parameters of the curved isocurvature model is not supported by the current data. The results are very sensitive to the second and third acoustic peak regions in the CMB temperature angular power: therefore a careful calibration of these data will be required before drawing decisive conclusions on the nature of primordial perturbations. Finally, we point out that the odds for the flat nonadiabatic model are 1:3 compared to the curved adiabatic model. This may suggest that it is not much less motivated to extend the concordance model with 4 isocurvature degrees of freedom than it is to study the spatially curved adiabatic model, though at the moment the model selection disfavors both of these models.

Phase component and conductivities of Co-doped BaTiO3 thermistors

BaTi1−xCoxO3−δ (0.01 ≤ x ≤ 0.4) ceramics were prepared by a wet chemical process polymerized with polyvinyl alcohol. The phases and related electrical properties of the ceramics were investigated. The phase component of the ceramics changes from a tetragonal phase to a hexagonal one with the Co concentration increase. A pure hexagonal phase formed in the ceramic with x = 0.2. The measurement of the temperature dependence of resistances revealed that the ceramic resistivities increase with temperature rising at the temperatures (T) lower than half of the related Debye temperature (ΘD), and the ceramics show a negative temperature coefficient (NTC) effect at T > ΘD/2. The material constants B50/120 of the BaTi1−xCoxO3−δ NTC thermistors were calculated to be 3,187, 2,968 and 2,648 K for x = 0.2, 0.3 and 0.4, respectively. Narrow-band conduction and non-adiabatic hopping models are proposed for the conduction mechanisms at T ΘD/2, respectively.

Subsurface excitations in a metal

We investigate internal hot carrier excitations in a Au thin film bombarded by hyperthermal and low energy alkali and noble gas ions. Excitations within the thin film of a metal-oxide-semiconductor device are measured revealing that ions whose velocities fall below the classical threshold given by the free-electron model of a metal still excite hot carriers. Excellent agreement between these results and a nonadiabatic model that accounts for the time-varying ion-surface interaction indicates that the measured excitations are due to semilocalized electrons near the metal surface.

Generating single attosecond pulse using multi-cycle lasers in a polarization gate

We analyze the macroscopic effects which are responsible for producing clean isolated pulses lasting few hundreds of attoseconds when starting from multi-cycle fundamental pulses. In particular, we consider a polarization gating scheme and show that, at high fundamental peak intensities, in the range 0.7-1 PWcm(-2), it usually produces three-four main attosecond pulses of radiation at single dipole level, just located in the leading edge of the laser pulse. We describe the physical mechanisms contributing to the formation of a single attosecond pulse by using a three dimensional non-adiabatic model and a quantum trajectory phase calculation. An analysis of the scheme optimization and stability against various parameters is performed in view of an experimental scheme implementation.

Proton emission, gamma deformation, and the spin of the isomeric state of 141Ho

The nonadiabatic quasiparticle model for triaxial shapes is used to perform calculations for decay of 141Ho, the only known odd-Z even-N deformed nucleus for which fine structure in proton emission from both ground and isomeric states was observed. All experimental data corresponding to this unique case namely, the rotational spectra of parent and daughter nuclei, decay widths and branching ratios for ground and isomeric states, could be well explained with a strong triaxial deformation γ∼20°. The recent experimental observation of fine structure decay from the isomeric state, can be explained only with an assignment of I=3/2+ as the decaying state, in contradiction with the previous assignment, of I=1/2+, based on adiabatic calculations. This study reveals that proton emission measurements could be a precise tool to probe triaxial deformations and other structural properties of exotic nuclei beyond the proton drip-line.

Electrochemical reduction of tert-butyl chloride—Computational study

In this work kinetics of the electrochemical reduction of the tert-butyl chloride molecule are studied. The two-dimensional potential surface E( x, y) was obtained from the Hamiltonian model used earlier [1] to study quantum effects on the reaction rate of the same reaction. In series of molecular dynamics simulations two different models, isotropic ( γ x = γ y ) and anisotropic ( γ x > γ y ), of the friction parameters for the solvent ( γ x ) and for the C–Cl bond ( γ y ) were considered. An influence of several factors, such as an overpotential, a solvent friction coefficient and a temperature, on the rate constants k obtained with the two models are presented and compared to the results obtained for the non-adiabatic mechanism. It is shown, that the anisotropic model predicts generally much smaller rates when compared to those calculated with the isotropic model and also with the non-adiabatic model. On the curves k( γ x ) the turnover region is very well defined for relatively low frictions in the isotropic case, while for the anisotropic case it is shifted towards much higher γ x values. In all simulations the saddle point avoidance phenomenon was observed and its extent for different models and reaction conditions is discussed. Transfer coefficients α obtained for adiabatic and non-adiabatic reaction mechanisms as a function of the overpotential and temperature are also presented.

Superconducting Bands Stabilizing Superconductivity in YBa2Cu3O7 and MgB2

The nonadiabatic Heisenberg model (NHM) proposed as an extension of the Heisenberg model makes a contribution to the eigenstate problem of superconductivity. The Hamiltonian H^n derived within this group-theoretical model has superconducting eigenstates if and only if the considered material possesses a narrow, roughly half-filled "superconducting" energy band of special symmetry in its band structure. This paper shows that the high-temperature superconductor YBa_2Cu_3O_7 possesses such a superconducting band. This new result together with previous observations about other superconductors and non-superconductors corroborates the theoretical evidence within the NHM that stable superconducting states are connected with superconducting bands. It is proposed that the type of superconductivity, i.e., whether the material is a conventional low-T_c or a high-T_c superconductor, is determined by the energetically lowest boson excitations that carry the crystal spin 1*hbar and are sufficiently stable to transport this crystal spin-angular momentum through the crystal. This mechanism provides the electron-phonon mechanism that enters the BCS theory in conventional superconductors.

Formalisms for electron-exchange kinetics in aqueous solution and the role of Ab initio techniques in their implementation

Formalisms suitable for calculating the rate of electron exchange between transition metal complexes in aqueous solution are reviewed and implemented in conjunction with ab initio quantum chemical calculations which provide crucial off-diagonal Hamiltonian matrix elements as well as other relevant electronic structural data. Rate constants and activation parameters are calculated for the hex-aquo Fe2 +-Fe3+ system, using a simple activated complex theory, a non-adiabatic semi-classical model which includes nuclear tunnelling, and a more detailed quantum mechanical method based on the Golden Rule. Comparisons are made between calculated results and those obtained by extrapolating experimental data to zero ionic strength. All methods yield similar values for the overall rate constant (a 0.1 L/(mol-sec)). The experimental activation parameters (δHd and δSd) are in somewhat better agreement with the semi classical and quantum mechanical results than with those from the simple activated complex theory, thereby providing some indication that non-adiabaticity and nuclear tunnelling may be important in the Fe2+/3+ exchange reaction. It is concluded that a model based on direct metal-metal overlap can account for the observed reaction kinetics provided the reactants are allowed to approach well within the traditional contact distance of 6.9 a. 6 figures, 7 tables.