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Absolute state-selected total cross sections σv’, v’=0 and 1, for the reaction N+2(X,v’=0,1) +Ar(1S0)→N2(X,v)+Ar+(2P3/2,1/2) [reaction (1)] over the center-of-mass collisional energy (Ec.m.) range of 1.2–140 eV have been measured using the photoionization mass spectrometric and radio frequency ion guide methods. These measurements, together with the relative values for σv’, v’=0–2, and spin-orbit-state distributions of product Ar+ ions determined using the crossed ion-neutral beam photoionization apparatus, allow the determination of the absolute values for σ2 and partial state-to-state cross sections σv’→J, v’=0–2, for reaction (1). Absolute values for σv’, v’=0–2, at Ec.m.=8 and 20 eV are in good agreement with those determined previously by the threshold photoelectron secondary ion coincidence method. Absolute values for σv’→J, v’=0–2, at Ec.m.=8 and 20 eV are also found to be in satisfactory accord with the predictions of the semiclassical multistate calculation which uses the ab initio potential energy surfaces of the [N2+Ar]+ system. Experimental state-to-state cross sections obtained in this study are consistent with those for the reaction Ar+(2P3/2)+N2(X,v=0)→Ar(1S0)+N+2 (X,v’) from the consideration of microscopic reversibility. [ABSTRACT FROM AUTHOR]

The vibrational state distributions of N+2(X,v’) ions resulting from the reactions, Ar+(2P3/2)+N2(X,v=0)→Ar(1S0) +N+2(X,v’) [reaction (1)] and Ar+(2P1/2)+N2(X,v=0)→Ar(1S0) +N+2(X,v’) [reaction (2)], over the center-of-mass collisional energy (Ec.m.) range of 0.25–41.2 eV in a crossed ion–neutral beam experiment have been probed by the charge exchange method. The experimental results obtained for reaction (1) are in accord with the predictions of the semiclassical multistate calculation of Spalburg and Gislason that N+2 ions are formed predominantly (>=85%) in the v’=1 state and that the production of N+2(X,v’=0) becomes more important as Ec.m. is increased. The experiment also supports the theoretical results for reaction (2) at Ec.m.=1.2 and 4.1 eV showing that >=80% of N+2 product ions are in the v’=2 state. However, the calculation is found to either over-estimate the populations for N+2(v’<2) or underestimate the populations for N+2(v’>2) resulting from reaction (2) at Ec.m.=10.3and 41.2 eV. Absolute spin-orbit-state-selected total cross sections for reactions (1) and (2), σ3/2 and σ1/2, respectively, at the Ec.m. range of 0.25–115.3 eV have also been measured using a tandem photoionization mass spectrometer which is equipped with a radio frequency (RF) octopole ion guide reaction gas cell. The measured values for σ3/2 at Ec.m.=4.1, 10.3, and 41.2 eV and σ1/2 at 41.2 eV are in reasonable agreement with the theoretical cross sections. However, the experimental values for σ3/2 at 1.2 eV and σ1/2 at 1.2, 4.1, and 10.3 eV are approximately a factor of 2 higher than the theoretical predictions. A model analysis, which takes into account possible collision-induced spin-orbit mixings of the reactant Ar+ states in the RF octopole gas cell, shows that the values for σ1/2/σ3/2 and σ1/2 determined using the ion beam–RF... [ABSTRACT FROM AUTHOR]

A new ion–molecule rection apparatus, which consists of a photoionization source, a tandem mass spectrometer, and a radio frequency octopole reaction cell is described. Using a quadrupole mass filter to reject H+3 background ions formed at the photoionization source, absolute total cross sections for the reaction H+2 (v’0) +H2 (v’0 =0)→H+3 +H, have been measured as a function of the vibrational state of reactant H+2, where v’0 =0–4, over the center-of-mass collision energy (Ec.m.) range of 0.04–15 eV. The experimental results are compared with phenomenological cross sections obtained in previous single gas cell studies, the quasiclassical trajectory calculations of Stine and Muckerman, and the recent similar calculations of Eaker and Schatz. The absolute total cross sections measured for v0 =0 and 3 at Ec.m. =0.5, 1, 3, and 5 eV are found to be in agreement with ‘‘trajectory surface hopping’’ calculations which include nonadiabatic surface hopping throughout the reaction. [ABSTRACT FROM AUTHOR]

We perform large-scale molecular dynamics simulations to study shock-induced melting transition of idealized hexagonal columnar nanocrystalline Cu. The as-constructed nanocrystalline Cu consists of unrotated (reference) and rotated columnar crystals, relative to the columnar axis. Shock loading is applied along three principal directions of the columnar Cu: two transverse (zigzag and armchair) and one longitudinal directions. Dynamic local melting processes are highly anisotropic with respect to the shock directions. For the transverse directions, hotspot effect and disparate dynamic responses of grains with different orientations may lead to partial or complete premelting of the initially rotated grains, which in turn leads to transient supercooling and heterogeneous recrystallization, and thus, the formation of nanocrystalline solids with modified grain structures or solid-liquid mixtures, depending on the extent of supercooling. With increasing shock strengths, the reference grains melt heterogeneously at interfaces and homogeneously inside. Conversely, 'bulk' premelting of the rotated grains is absent for the longitudinal direction, except for grain boundary melting. The progression of recrystallization or heterogenous melting diminishes and eventually eliminates the transient premelting or superheating of the system via latent heat and thermal diffusion. Premelting or superheating appears unlikely for bulk melting or well-defined Hugoniot states, if the thermal and mechanical equilibria are achieved, and the thermodynamic melting curve coincides with the partial melting Hugoniot states of a polycrystalline solid. [ABSTRACT FROM AUTHOR]

We propose a stochastic Schrödinger equation in which the momentum is coupled to a white Gaussian noise. In the stochastic representation, the kinetic energy representing the self-interaction of momentum is reduced to a linear term of momentum. As such, the quantum evolution operator factorizes into two contributions due to the momentum and the potential, respectively. The exact quantum propagator thereby becomes an expectation of the stochastic one in which the amplitude results from the potential with a fluctuating position: The particle moves with a constant velocity, subjected to a complex Brownian motion. We demonstrate that the stochastic Schrödinger equation can be feasibly used to derive the quantum propagators for the linear potential and the harmonic oscillator system. Novel semiclassical and other approximations may be developed from the new representation of quantum mechanics., (© 2023 Author(s). Published under an exclusive license by AIP Publishing.)

In this article, a systematic examination of the electronic and optical properties of partially fluorinated graphene is presented. In order to capture a large variety of fluorination degrees and configurations, different sizes of the supercell combining with various degrees of fluorination are considered. On top of periodic density functional theory, the G 0 W 0 method and the G 0 W 0 Γ method within many-body Green's function framework are employed. Including the description of electron-hole interactions, the optical spectra based on the Bethe-Salpeter equation are calculated. Two-sided fluorination with compact fluorination arrangements is energetically most favorable. The fluorination degree has a determined impact on the bandgap value in the system, while the fluorination pattern strongly influences the characteristics of the bands in the electronic structures. Depending on the polarization of the applied electromagnetic field, the optical absorption spectra of the same structure could vary significantly. These interesting results suggest the potential applications of partially fluorinated graphene as optoelectronic materials.

The stochastic differential equation is a powerful tool for describing the dynamics of a dissipative system in which noise characterizes the influence of the environment. For the Ornstein-Uhlenbeck noise, both the formula of differentiation and the hierarchy approach provide efficient numerical simulations, with the stochastic differential equation transformed into a set of coupled, linear ordinary differential equations. We show that while these two deterministic schemes result in different sets of equations, they can be regarded as two representations of an underlying linear-dynamics. Moreover, by manipulating the involved Ornstein-Uhlenbeck noise, we propose a unified algorithm that may reduce to the hierarchy approach or the formula of differentiation in different limits. We further analyze the numerical performance of this algorithm and find that the hierarchy approach appears to be more efficient for our numerical model studies.

Langevin dynamics has become a popular tool to simulate the Boltzmann equilibrium distribution. When the repartition of the Langevin equation involves the exact realization of the Ornstein-Uhlenbeck noise, in addition to the conventional density evolution, there exists another type of discrete evolution that may not correspond to a continuous, real dynamical counterpart. This virtual dynamics case is also able to produce the desired stationary distribution. Different types of repartition lead to different numerical schemes, of which the accuracy and efficiency are investigated through studying the harmonic oscillator potential, an analytical solvable model. By analyzing the asymptotic distribution and characteristic correlation time that are derived by either directly solving the discrete equations of motion or using the related phase space propagators, it is shown that the optimal friction coefficient resulting in the minimum characteristic correlation time depends on the time interval chosen in the numerical implementation. When the recommended "middle" scheme is employed, both analytical and numerical results demonstrate that, for good numerical performance in efficiency as well as accuracy, one may choose a friction coefficient in a wide range from around the optimal value to the high friction limit.

Feasibility of laser-cooling AlCl molecule is investigated using ab initio quantum chemistry. Potential energy curves, permanent dipole moments, and transition dipole moments for the X(1)Σ(+), a(3)Π, and A(1)Π states are studied based on multi-reference configuration interaction plus Davidson corrections (MRCI+Q) method with ACVQZ basis set, spin-orbit coupling effects are considered at the MRCI+Q level. Highly diagonally distributed Franck-Condon factors (f00 = 0.9988 and f11 = 0.9970) and branching ratios (R00 = 0.9965, R01 = 2.85 × 10(-3), R02 = 6.35 × 10(-4), and R03 = 2.05 × 10(-6)) for the A(1)Π1(ν(')=0)→X(1)Σ0(+) (+)(ν(″)=0) transition are determined. A sufficiently radiative lifetime τ (A(1)Π1) = 4.99 ns is predicted for rapid laser cooling. The proposed cooling wavelength is deep in the ultraviolet region at λ00 = 261.75 nm. Total emission rates for the a(3)Π0(+) →X(1)Σ0(+) (+), a(3)Π1→X(1)Σ0(+) (+), A(1)Π1 → a(3)Π0(+) , and A(1)Π1 → a(3)Π1 transitions are particularly small (∼10 s(-1)-650 s(-1)). The calculated vibrational branching loss ratio to the intermediate a(3)Π0(+) and a(3)Π1 states can be negligible. The results imply the probability of laser cooling AlCl molecule with three-electronic-level.

We present the ab initio study of spin-orbit coupling effects on laser cooling of BeI and MgI molecules. Potential energy curves for the X(2)Σ(+)(1/2), A(2)Π(1/2,3/2), and 2(2)Π(3/2,1/2) states are calculated using multi-reference configuration interaction method plus Davidson corrections. Spectroscopic parameters of BeI and MgI are in excellent agreement with available experimental and theoretical values. The A(2)Π(3/2) state of MgI is a repulsive state. It is an unsuitable scheme for the A(2)Π(3/2)(υ')← X(2)Σ(+)(1/2) (υ″) transition for laser cooling of MgI. Highly diagonally distributed Franck-Condon factors f00 for the A(2)Π(1/2,3/2) (υ' = 0) ← X(2)Σ(+)(1/2) (υ″ = 0) transitions and suitable radiative lifetimes τ for the A(2)Π(1/2,3/2) (υ' = 0) of BeI and MgI are obtained. Three laser wavelength drives are required for the A(2)Π(1/2,3/2)(υ')←X(2)Σ(+)(1/2) (υ″) transitions of BeI and MgI. The proposed cooling wavelengths of BeI and MgI are both in the violet region. The results imply the feasibility of laser cooling of BeI and MgI, and that laser cooling of BeI is more possible.

Ab initio calculations for three low-lying electronic states (X(2)Σ(+), A(2)Π, and 2(2)Π) of MgCl and MgBr molecules, including spin-orbit coupling, are performed using multi-reference configuration interaction plus Davidson correction method. The calculations involve all-electronic basis sets and Douglas-Kroll scalar relativistic correction. Spectroscopic parameters well agree with available theoretical and experimental data. Highly diagonally distributed Franck-Condon factors f00 for A(2)Π3/2,1/2 (υ' = 0) → X(2)Σ(+) 1/2 (υ″ = 0) are determined for both MgCl and MgBr molecules. Suitable radiative lifetimes τ of A(2)Π3/2,1/2 (υ' = 0) states for rapid laser cooling are also obtained. The proposed laser drives A(2)Π3/2 (υ' = 0) → X(2)Σ(+) 1/2 (υ″ = 0) transition by using three wavelengths (main pump laser λ00; two repumping lasers λ10 and λ21). These results indicate the probability of laser cooling MgCl and MgBr molecules.

We propose a semi-phenomenological Markovian Master equation for describing the quantum dynamics of atom-surface scattering. It embodies the Lindblad-like structure and can describe both damping and pumping of energy between the system and the bath. It preserves positivity and correctly accounts for the vanishing of the interaction of the particle with the surface when the particle is distant from the surface. As a numerical test, we apply it to a model of an Ar atom scattered from a LiF surface, allowing for interaction only in the vertical direction. At low temperatures, we find that the quantum mechanical average energy loss is smaller than the classical energy loss. The numerical results obtained from the space dependent friction master equation are compared with numerical simulations for a discretized bath, using the multi-configurational time dependent Hartree methodology. The agreement between the two simulations is quantitative.

The dynamics of a two-level system coupled to a spin bath is investigated via the numerically exact multilayer multiconfiguration time-dependent Hartree (ML-MCTDH) theory. Consistent with the previous work on linear response approximation [N. Makri, J. Phys. Chem. B 103, 2823 (1999)], it is demonstrated numerically that this spin-spin-bath model can be mapped onto the well-known spin-boson model if the system-bath coupling strength obeys an appropriate scaling behavior. This linear response mapping, however, may require many bath spin degrees of freedom to represent the practical continuum limit. To clarify the discrepancies resulted from different approximate treatments of this model, the population dynamics of the central two-level system has been investigated near the transition boundary between the coherent and incoherent motions via the ML-MCTDH method. It is found that increasing temperature favors quantum coherence in the nonadiabatic limit of this model, which corroborates the prediction in the previous work [J. Shao and P. Hanggi, Phys. Rev. Lett. 81, 5710 (1998)] based on the non-interacting blip approximation (NIBA). However, the coherent-incoherent boundary obtained by the exact ML-MCTDH simulation is slightly different from the approximate NIBA results. Quantum dynamics in other physical regimes are also discussed.

Thawed Gaussian wavepackets have been used in recent years to compute approximations to the thermal density matrix. From a numerical point of view, it is cheaper to employ frozen Gaussian wavepackets. In this paper, we provide the formalism for the computation of thermal densities using frozen Gaussian wavepackets. We show that the exact density may be given in terms of a series, in which the zeroth order term is the frozen Gaussian. A numerical test of the methodology is presented for deep tunneling in the quartic double well potential. In all cases, the series is observed to converge. The convergence of the diagonal density matrix element is much faster than that of the antidiagonal one, suggesting that the methodology should be especially useful for the computation of partition functions. As a by product of this study, we find that the density matrix in configuration space can have more than two saddle points at low temperatures. This has implications for the use of the quantum instanton theory.

Nonadiabatic dynamical simulations were carried out to study cis-to-trans isomerization of azobenzene under laser irradiation and/or external mechanical loads. We used a semiclassical electron-radiation-ion dynamics method that is able to describe the coevolution of the structural dynamics and the underlying electronic dynamics in a real-time manner. It is found that azobenzene photoisomerization occurs predominantly by an out-of-plane rotation mechanism even under a nontrivial resisting force of several tens of piconewtons. We have repeated the simulations systematically for a broad range of parameters for laser pulses, but could not find any photoisomerization event by a previously suggested in-plane inversion mechanism. The simulations found that the photoisomerization process can be held back by an external resisting force of 90-200 pN depending on the frequency and intensity of the lasers. This study also found that a pure mechanical isomerization is possible from the cis-to-trans state if the azobenzene molecule is stretched by an external force of approximately 1250-1650 pN. Remarkably, the mechanical isomerization first proceeds through a mechanically activated inversion, and then is diverted to an ultrafast downhill rotation that accomplishes the isomerization. Implications of these findings to azobenzene-based nanomechanical devices are discussed.

The zero-temperature dynamics of the spin-boson model with strong dissipation has been a challenging problem for more than 20 years. To solve this and quantum dynamics of dissipative systems at large, we recently proposed a mixed random-deterministic method. This scheme has been successfully used to simulate the time evolution of the spin-boson model at zero temperature for weak to moderate dissipation. For a better numerical performance, the approach is further modified so that it is flexible to convert a certain part of the random treatment to a deterministic one a la hierarchical equations. Applying the new method to the strong dissipated spin-boson model at zero temperature, we observe that the population in the localized state obeys a simple decay dynamics and the time scale is proportional to the reciprocal of the cutoff frequency.

Duschinsky rotation effect is a simple and effective way to characterize the difference between the ground state and excited state potential energy surfaces. For complex molecules, harmonic oscillator model is still the practical way to describe the dynamics of excited states. Based on the first-order perturbation theory a la Fermi golden rule, the authors have applied the path integral of Gaussian type for the correlation function to derive an analytic formalism to calculate the internal conversion rate process with Duschinsky rotation effect being taken into account. The validity of their formalism is verified through comparison with previous work, both analytically for the case of neglecting Duschinsky rotation and numerically for the ethylene molecules with two-mode mixing. Their expression is derived for multimode mixing.

Frantsuzov and Mandelshtam [J. Chem. Phys. 121, 9247 (2004)] have recently demonstrated that a time evolving Gaussian approximation (TEGA) to the imaginary time propagator exp(-betaH) is useful for numerical computations of anharmonically coupled systems with many degrees of freedom. In this paper we derive a new exact series representation for the imaginary time propagator whose leading order term is the TEGA. One can thus use the TEGA not only as an approximation but also to obtain the exact imaginary time propagator. We also show how the TEGA may be generalized to provide a family of TEGA's. Finally, we find that the equations of motion governing the evolution of the center and width of the Gaussian may be thought of as introducing a quantum friction term to the classical evolution equations.

A general formulation of nonperturbative quantum dynamics of solutes in a condensed phase is proposed to calculate linear and nonlinear vibrational line shapes. In the weak solute-solvent interaction limit, the temporal absorption profile can be approximately factorized into the population relaxation profile from the off-diagonal coupling and the pure-dephasing profile from the diagonal coupling. The strength of dissipation and the anharmonicity-induced dephasing rate are derived in Appendix A. The vibrational energy relaxation (VER) rate is negligible for slow solvent fluctuations, yet it does not justify the Markovian treatment of off-diagonal contributions to vibrational line shapes. Non-Markovian VER effects are manifested as asymmetric envelops in the temporal absorption profile, or equivalently as side bands in the frequency domain absorption spectrum. The side bands are solvent-induced multiple-photon effects which are absent in the Markovian VER treatment. Exact path integral calculations yield non-Lorentzian central peaks in absorption spectrum resulting from couplings between population relaxations of different vibrational states. These predictions cannot be reproduced by the perturbative or the Markovian approximations. For anharmonic potentials, the absorption spectrum shows asymmetric central peaks and the asymmetry increases with anharmonicity. At large anharmonicities, all the approximation schemes break down and a full nonperturbative path integral calculation that explicitly accounts for the exact VER effects is needed. A numerical analysis of the O-H stretch of HOD in D(2)O solvent reveals that the non-Markovian VER effects generate a small recurrence of the echo peak shift around 200 fs, which cannot be reproduced with a Markovian VER rate. In general, the nonperturbative and non-Markovian VER contributions have a stronger effect on nonlinear vibrational line shapes than on linear absorption., ((c) 2004 American Institute of Physics.)

Based on the Hubbard-Stratonovich transformation, the dissipative interaction between the system of interest and the heat bath is decoupled and the separated system and bath thus evolve in common classical random fields. This manipulation allows us to establish a novel theoretical methodology by which the reduced density matrix is formulated as an ensemble average of its random realizations in the auxiliary white noise fields. Within the stochastic description, the interaction between the system and the bath is reflected in the mutually induced mean fields. The relationship between the bath-induced field and the influence functional in the path integral framework is revealed. As a demonstration of this approach, we derive the exact master equations for two model systems., ((c) 2004 American Institute of Physics.)