Andrey Pereverzev

Time-convolutionless master equation for mesoscopic electron-phonon systems.

The time-convolutionless master equation for the electronic populations is derived for a generic electron-phonon Hamiltonian. The equation can be used in the regimes where the golden rule approach is not applicable. The equation is applied to study the electronic relaxation in several models with the finite number of normal modes. For such mesoscopic systems the relaxation behavior differs substantially from the simple exponential relaxation. In particular, the equation shows the appearance of the recurrence phenomena on a time scale determined by the slowest mode of the system. The formal results are quite general and can be used for a wide range of physical systems. Numerical results are presented for a two level system coupled to Ohmic and super-Ohmic baths, as well as for a model of charge-transfer dynamics between semiconducting organic polymers.

Energy and charge-transfer dynamics using projected modes

For electron-phonon Hamiltonians with the couplings linear in the phonon operators, we construct a class of unitary transformations that separate the normal modes into two groups. The modes in the first group interact with the electronic degrees of freedom directly. The modes in the second group interact directly only with the modes in the first group but not with the electronic system. These transformations can be carried out independently for different types of phonon modes, e.g., high- versus low-frequency phonon bands. This construction generalizes recently introduced transformations for systems exhibiting a conical intersection topology. The separation of the normal modes into several groups allows one to develop new approximation schemes. We apply one of such schemes to study electronic relaxation at a semiconducting polymer interface.

Correlation functions in quantized Hamilton dynamics and quantal cumulant dynamics

Quantized Hamilton dynamics (QHD) [O. V. Prezhdo and Y. V. Pereverzev, J. Chem. Phys. 113, 6557 (2000)] and quantal cumulant dynamics (QCD) [Shigeta et al., J. Chem. Phys. 125, 244102 (2006)] are used to obtain a semiclassical description of two-time correlation functions (CFs). Generally, lower-order CFs couple to higher-order CFs. The infinite hierarchy is terminated by a closure, which neglects higher-order irreducible correlators and provides an efficient approximation to quantum mechanics. The approach is illustrated with a simple nonlinear system, for which the real part of the classical CF continues a perfect oscillation and the imaginary part is identically zero. At little computational expense, the second-order QHD/QCD approximation reproduces the real and imaginary parts of the quantum-mechanical CF.

Molecular dynamics study of the pressure-dependent terahertz infrared absorption spectrum of α- and γ-RDX

Terahertz infrared absorption spectra of the α and γ polymorphs of 1,3,5-trinitro-1,3,5-triazacyclohexane (RDX) were predicted using two different theoretical approaches based on molecular dynamics simulations. The thermodynamic conditions studied were T = 298 K and hydrostatic pressures P = 0.0, 1.0, and 2.0 GPa for α-RDX and P = 3.0, 5.2, and 7.0 GPa for γ-RDX. The spectra obtained using the two methods are similar but not identical. In the case of α-RDX for pressure P = 0.0 GPa both spectra agree reasonably well with experimental data. The predicted spectra for α-RDX exhibit red-shifting (mode softening) of the main absorption peak with increasing pressure while for γ-RDX the spectra exhibit overall blue-shifting with increasing pressure.

Terahertz normal mode relaxation in pentaerythritol tetranitrate

Normal vibrational modes for a three-dimensional defect-free crystal of the high explosive pentaerythritol tetranitrate were obtained in the framework of classical mechanics using a previously published unreactive potential-energy surface [J. Phys. Chem. B 112, 734 (2008)]. Using these results the vibrational density of states was obtained for the entire vibrational frequency range. Relaxation of selectively excited terahertz-active modes was studied using isochoric-isoergic (NVE) molecular dynamics simulations for energy and density conditions corresponding to room temperature and atmospheric pressure. Dependence of the relaxation time on the initial modal excitation was considered for five excitation energies between 10 and 500 kT and shown to be relatively weak. The terahertz absorption spectrum was constructed directly using linewidths obtained from the relaxation times of the excited modes for the case of 10 kT excitation. The spectrum shows reasonably good agreement with experimental results. Dynamics of redistribution of the excited mode energy among the other normal modes was also studied. The results indicate that, for the four terahertz-active initially excited modes considered, there is a small subset of zero wave vector (k = 0) modes that preferentially absorb the energy on a few-picosecond time scale. The majority of the excitation energy, however, is transferred nonspecifically to the bath modes of the system.

Terahertz spectrum and normal-mode relaxation in pentaerythritol tetranitrate: Effect of changes in bond-stretching force-field terms

Terahertz (THz) active normal-mode relaxation in crystalline pentaerythritol tetranitrate (PETN) was studied using classical molecular dynamics simulations for energy and density conditions corresponding to room temperature and atmospheric pressure. Two modifications to the fully flexible non-reactive force field due to Borodin et al. [J. Phys. Chem. B 112, 734 (2008)] used in a previous study of THz-active normal-mode relaxation in PETN [J. Chem. Phys. 134, 014513 (2011)] were considered to assess the sensitivity of the earlier predictions to details of the covalent bond-stretching terms in the force field. In the first modification the harmonic bond-stretching potential was replaced with the Morse potential to study the effect of bond anharmonicity on the THz-region mode relaxation. In the second modification the C-H and nitro-group N-O bond lengths were constrained to constant values to mimic lower quantum occupation numbers for those high-frequency modes. The results for relaxation times of the initially excited modes were found to be insensitive to either force-field modification. Overall time scales for energy transfer to other modes in the system were essentially unaffected by the force-field modifications, whereas the detailed pathways by which the energy transfer occurs are more complicated for the Morse potential than for the harmonic-bond and fixed-bond cases. Terahertz infrared absorption spectra constructed using calculated normal-mode frequencies, transition dipoles, and relaxation times for THz-active modes were compared to the spectra obtained from the Fourier transform of the dipole-dipole time autocorrelation function (DDACF). Results from the two approaches are in near agreement with each other and with experimental results in terms of main peak positions. Both theoretical methods yield narrower peaks than observed experimentally and in addition predict a weaker peak at ω ∼ 50 cm(-1) that is weak or absent experimentally. Peaks obtained using the DDACF approach are broader than those obtained from the normal-mode relaxation method.

Ultrafast Photophysics of Organic Semiconductor Junctions

This contribution gives an overview of our recent studies of the electronic structure and ultrafast photophysics of semiconductor polymer junctions. We focus on the phonon-assisted exciton dissociation at donor-acceptor heterojunctions, using state-of-the-art electronic structure methods in conjunction with vibronic coupling models and multiconfigurational quantum dynamical techniques. The decay of the photogenerated exciton towards an interfacial charge-separated state is an ultrafast (femtosecond to picosecond scale) process which precedes photocurrent generation. We describe this process using a linear vibronic coupling model parametrized for two to three electronic states and 20-30 phonon modes. Several representative interface configurations are considered, which are shown to differ significantly in their cross-chain interactions but exhibit an efficient exciton dissociation in all cases investigated. The exciton decay depends critically on the presence of intermediate states and on the dynamical interplay between high-frequency (C=C stretch) and lowfrequency (ring-torsional) modes. The resulting molecular-level picture of exciton dissociation could contribute to the design of efficient polymer junctions.

Dissipation of classical energy in nonlinear quantum systems

We show using two simple nonlinear quantum systems that the infinite set of quantum dynamical variables, as introduced in quantized Hamilton dynamics [O. V. Prezhdo and Y. V. Pereverzev, J. Chem. Phys. 113, 6557 (2000)], behave as a thermostat with respect to the finite number of classical variables. The coherent classical component of the evolution decays by coupling to the chaotic quantum reservoir. The classical energy, understood as the part of system energy expressible through the average values of coordinates and momenta, is transferred to the quantum energy expressible through the higher moments of coordinates and momenta and other quantum variables. At long times, the classical variables reach equilibrium, and the classical energy fluctuates around the equilibrium value. These phenomena are illustrated with the exactly solvable Jaynes-Cummings model and a nonlinear oscillator.

Effect of vacancy defects on the terahertz spectrum of crystalline pentaerythritol tetranitrate

Molecular dynamics was used to study the effect of molecular vacancies on the terahertz (THz) infrared (IR) absorption spectrum of crystalline pentaerythritol tetranitrate (PETN). Threedimensionally periodic simulation supercells containing either 10 percent or 20 percent vacancies were created by randomly removing molecules from the perfect crystal supercell. Following re-equilibration of the defective materials, which remained crystalline on the 170 ps time scale of the simulations, THz spectra were determined from isochoric-isoergic trajectories as the Fourier transform of the ensembleaveraged equilibrium dipole-dipole time autocorrelation function. The calculated spectra exhibit substantial peak broadening and increase in the low-frequency absorption intensity with increasing vacancy concentration. Spectral redshifting with increasing vacancy concentration was also observed, in particular for the higher-frequency member of the pair of peaks comprising the main THz-region IR absorption in crystalline PETN.

Calculation of anharmonic couplings and THz linewidths in crystalline PETN

We have developed a method for calculating the cubic anharmonic couplings in molecular crystals for normal modes with the zero wave vector in the framework of classical mechanics, and have applied it, combined with perturbation theory, to obtain the linewidths of all infrared absorption lines of crystalline pentaerythritol tetranitrate in the terahertz region (<100 cm(-1)). Contributions of the up- and down-conversion processes to the total linewidth were calculated. The computed linewidths are in qualitative agreement with experimental data and the results of molecular dynamics simulations. Quantum corrections to the linewidths in the terahertz region are shown to be negligible.