Time-dependent Unrestricted Hartree-Fock Research Articles

Saturable absorption behavior of (5,10,15,20-tetraphenylporphyrinato)-silver crystal thin film at wavelength 532 nm

A single crystal of (5,10,15,20-tetraphenylporphyrinato)-silver was obtained and its structure reported. Using direct growth on quartz substrates method, a crystal thin film was obtained and the film’s morphology was investigated by atomic force microscopy. The nonlinear absorption properties of the film were studied using open aperture [Formula: see text]-scan technique by picosecond laser pulses at different fluence at wavelength 532 nm, nonlinear saturable absorption behavior was observed. Time-dependent unrestricted Hartree-Fock with the basis set LanL2DZ was used in computing the linear and non-linear optical properties of (5,10,15,20-tetraphenylporphyrinato)-silver.

Third-order nonlinear effects on femtosecond x-ray-absorption near-edge spectra

Reduced density matrix equations combined with a cluster molecular-orbital method are formulated to describe resonant core-valence electron excitation by an intense femtosecond x-ray pulse. The electron-hole interaction is treated by the time-dependent unrestricted Hartree-Fock approximation supplemented by a screening correction in the Born approximation. Nonlinear $K$-shell absorption spectra of metallic copper are thereby computed. Numerical results indicate that, in the third-order nonlinear regime, where the fraction of excited electrons increases in proportion to the x-ray intensity, the absorption spectra above the $K$ edge undergo shifts toward high-energy side. The origin of this third-order nonlinearity can be traced to a negative renormalization of the core orbital energy due to strong electron-hole attraction, which offers a sharp contrast to the usual optical nonlinearity due simply to the increment of upper-state populations. For higher intensities, broadening of the absorption edge and saturation of absorption are predicted. These trends are qualitatively consistent with nonlinear spectroscopy experiments using the SACLA x-ray free-electron laser.

Cobalt and zinc tetraphenylporphyrins thin films for all optical switching application

Abstract Cobalt and zinc tetraphenylporphyrins have been obtained and characterized by X-ray single crystal diffraction. Their crystal thin films were prepared using direct growth on quartz substrates. The surface morphology of the thin films were studied by atomic force microscopy. The nonlinear optical refraction properties of the thin films were investigated by using closed aperture Z-scan technique with 20 ps (ps) pulses at wavelength 1064 nm. Time-dependent unrestricted Hartree-Fock (TDUHF) with the basis set LanL2DZ were used in computing the linear and non-linear optical properties.

Density matrix formalism for femtosecond x-ray absorption and its excitonic enhancement

A reduced density matrix (RDM) formalism to simulate the dynamics of core-valence electronic excitation in a metal induced by a femtosecond x-ray laser pulse is presented. The theory is based on the time-dependent unrestricted Hartree–Fock (TDUHF) equation for a one-electron RDM combined with an off-diagonal collision term in the Born approximation (BA), where molecular orbitals (MOs) of a model cluster are used as a basis set. These MOs are calculated with a newly developed semiempirical method in the neglect of diatomic differential overlap approximation that takes into account atomic core structures and valence-orbital hybridization. The off-diagonal component of core-valence RDM is decomposed into a rapidly oscillating factor synchronizing with the x-ray field and a slowly varying envelope; time evolution of the latter yields induced electric polarization and complex electric susceptibility. As a numerical test, K-shell absorption near-edge spectra of copper are computed for a single-shot, weak femtosecond x-ray pulse. It is thereby shown that the attractive interaction between an excited electron and a core hole, described by the exchange part of the self-energy matrix, gives rise to excitonic enhancement of x-ray near-edge absorption. The TDUHF approximation significantly overestimates the excitonic enhancement; additional consideration of the screening of electron-hole interaction by valence electrons in BA leads to a prediction of absorption coefficients comparable to the existing experimental values.

Bloch equations for intense x-rays

Density matrix equations for ultrafast electron-hole dynamics in solid target under intense x-rays are presented. The theory is an extension of the usual optical Bloch equations to the case of intense x-ray free-electron lasers, where the creation of multiple core holes and the associated orbital relaxation dynamics are additionally incorporated through the cluster model combined with the time-dependent unrestricted Hartree–Fock approximation. Numerical examples are demonstrated for K-edge excitation of sodium by a femtosecond x-ray pulse; nonlinear dynamics of absorption, orbital energy shifts and induced electric polarization are discussed.

Assessment of Time-dependent Unrestricted Hartree-Fock Method for Electron Dynamics in Intense Laser Fields

Assessment of Time-dependent Unrestricted Hartree-Fock Method for Electron Dynamics in Intense Laser Fields

Interchain interaction effects on polaron–bipolaron transition on conducting polymers

We investigate the effects of interchain interaction on the polaron–bipolaron transition on conjugated polymer. We use a Su-Schrieffer-Heeger model modified to include interchain interaction, an external electric field and electron–electron interaction via extended Hubbard terms. We study the dynamics within the time-dependent unrestricted Hartree-Fock approximation. We find that adding a hole in interacting conducting polymer chains bearing a single positively charged polaron leads to the direct transition of polaron to bipolaron state. The transition which is produced is single-polaron to bipolaron transition whose excitation spectrum explains the experimental data. The competing mechanism of two polarons merging to form a bipolaron is also observed under special circumstances. We also find that depending on how fast the hole is inserted, a structure that contains a bipolaron coupled to a breather is created. The bipolaron-breather pair can be decoupled under the action of an external electric field.

Dynamical evolution of polaron to bipolaron in conjugated polymers

We investigate the insertion of holes in conjugated polymers bearing polarons. We use a Su-Schrieffer-Heeger model modified to include electron-electron interactions via an extended Hubbard Hamiltonian, a Brazovskii-Kirova symmetry breaking term, and an external electric field. We study the dynamics performing numerical calculations within the time-dependent unrestricted Hartree-Fock approximation. We find that adding a hole in a conducting polymer bearing a single positively charged polaron leads to the direct transition of polaron to bipolaron state. The transition produced is a single-polaron to biplaron transition whose excitation spectrum explains the experimental data. The competing mechanism of two polarons merging to form a bipolaron is not observed. We also find that depending on how fast the hole is inserted, a structure that contains a bipolaron coupled to a breather is created. The bipolaron-breather pair can be decoupled under the action of an external electric field. We determine the value of the critical electric field to untrap the bipolaron from the breather.

Photo-induced phase transition in charge ordered perovskite manganites

Photo-irradiation effects on the charge and spin ordered manganites at quarter filling are investigated. Time dependence of spin and charge states after irradiation is calculated by utilizing the time-dependent unrestricted Hartree–Fock method in a finite size cluster. It is shown, by analyzing charge distribution, energy gap and magnetization, that the transition from the charge ordered phase associated with the antiferromagnetic order to the ferromagnetic metallic one occurs. Melting of the charge order and growing of the magnetization occur, cooperatively.

Semiempirical time-dependent unrestricted Hartree–Fock approach for the determination of molecular linear and nonlinear optical properties

The time-dependent unrestricted Hartree–Fock (TDUHF) iterative procedure has been implemented at the semiempirical level in the MOPAC 2000 package to determine the (non)linear optical responses of open-shell systems. In addition to providing the dynamic quantities, this procedure enables the decomposition of the total (hyper)polarizabilities into their spin contributions. Its application to reference molecules using AM1 level is reported to illustrate the capabilities of the method compared to ab initio and experimental results. Although for small systems the semi-empirical scheme fails in reproducing the highly-correlated ab initio values, the TDUHF/AM1 approach appears as an efficient tool for studying larger conjugated systems.

Use of polarons and bipolarons in logical switches based on conjugated polymers

Considering impurity molecules functioning as switches, the charge-transport process on a single conducting polymer chain is analyzed. The dynamics of polarons and bipolarons is studied with the time-dependent unrestricted Hartree-Fock approximation using a combination of the Su-Schrieffer-Heeger and extended Hubbard models, extended to include the potential due to the dopants. A Brazovskii-Kirova-type symmetry-breaking interaction is inserted in the Hamiltonian model. The polarons and bipolarons are put into motion by a spatially uniform external electric field introduced through a time-dependent vector potential. The process of charge transport through the sites with radicals that work like a switch is analyzed by the numerical resolution of the equations of motion. The use of polarons and bipolarons as information carriers is analyzed. The results show that the impurity molecule can work as a gate to the passage of charged polarons or bipolarons (i.e., it can control of charge transport). The polarons and/or bipolarons can be trapped by the impurity molecule or can pass through it, resulting in two configurations: closed and open logical switches. Such results show that polarons and bipolarons provide good candidates to logical switching in molecular circuits of conducting polymers. The energies involved are also discussed.

Dynamical effects on the competition between polarons and bipolarons in conjugated polymers

The dynamics of polarons and bipolarons is numerically studied using the Su–Schrieffer–Heeger model combined with the Pariser–Parr–Pople model extended to include an external electric field and a Brazovskii–Kirova symmetry-breaking interaction. The charge transport on a single conducting polymer chain is investigated. The time-dependent unrestricted Hartree–Fock approximation is used. It is found that dynamical effects do not reverse the stability relation between polarons and bipolarons. On the other hand, as the electric field intensity gets stronger the moving polarons structure is blurred (destroyed) faster than the structure of moving bipolarons. This effect is reduced with increasing doping concentration. Therefore, it is shown within the present model that the applied external electric field as well as the doping concentration are fundamental in determining the relative prevalence of polarons or bipolarons.

Competition between polarons and bipolarons — doping and electric field dynamical effects

The dynamics of polarons and bipolarons is numerically studied using the Su-Schrieffer-Heeger model combined with the Pariser-Parr-Pople model extended to include an external electric field and a Brazovskii-Kirova (BK) symmetry breaking interaction. The time-dependent unrestricted Hartree-Fock approximation is used. It is found that dynamical effects do not reverse the stability relation between polarons and bipolarons. On the other hand, as the electric field intensity gets stronger the moving polarons structure is blurred (destroyed) faster than the structure of moving bipolarons. This effect is reduced with increasing doping concentration. Therefore, it is shown within the present model that the applied external electric field as well as the doping concentration are fundamental in determining the relative prevalence of polarons or bipolarons.

Photogeneration Dynamics of Nonlinear Excitations in Polyacetylene

Dynamical process of the formation of nonlinear excitations such as charged or neutral solitons from a photogenerated electron-hole pair in polyacetylene is studied numerically, based on Su-Schrieffer-Heeger's model extended to involve on-site as well as nearest-neighbor electron-electron Coulomb repulsion terms. The electron-electron interactions are treated within the time-dependent unrestricted Hartree-Fock approximation. Lattice fluctuations and a weak local disorder are introduced in order to trigger the formation and to represent many different initial conditions. Starting from an initial configuration with an electron at the bottom of the conduction band and a hole at the top of the valence band, separated by the Peierls gap, the time dependent Schrodinger equation for the electron wave functions and the equation of motion for the lattice displacements are solved numerically. It is not systematic but rather stochastic whether nonlinear excitations are created or not. We have obtained branching rati...

Electric-field effects on the competition between polarons and bipolarons in conjugated polymers

The charge transport on a single conducting polymer chain is investigated. The dynamics of polarons and bipolarons is numerically studied using the Su-Schrieffer-Heeger model combined with the Pariser-Parr-Pople model extended to include an external electric field and a Brazovskii-Kirova symmetry breaking interaction. The time-dependent unrestricted Hartree-Fock approximation is used. It is found that dynamical effects do not reverse the stability relation between polarons and bipolarons. On the other hand, as the electric field intensity gets stronger the moving polarons structure is blurred (destroyed) faster than the structure of moving bipolarons. This effect is reduced with increasing doping concentration. Therefore, it is shown within the present model that the applied external electric field as well as the doping concentration are fundamental in determining the relative prevalence of polarons or bipolarons.

Ab initio coupled Hartree–Fock study of the Bloembergen effect on paramagnetic systems: SiH3 radical

The Bloembergen effect on 29Si and 1H nuclei in SiH3 radical has been investigated by an ab initio time-dependent unrestricted Hartree–Fock method with the use of a double zeta plus polarization basis set. The largest effect is seen on the isotropic part of the hyperfine interaction at 29Si nucleus. The first-order electric field-induced hyperfine coupling coefficients for 1H nuclei exhibit dependence on the spatial position of the atom in a molecule-fixed coordinate system. Implications of this and other interesting features of the Bloembergen effect on structural characterization of point defects in solid environment are discussed. ©1999 John Wiley & Sons, Inc. J Comput Chem 20: 1274–1280, 1999

A new model for intense laser-atom interactions in helium

I present a theoretical justification of the `Crapola' model, which is used to describe the approximate behaviour of two-electron atoms in an intense laser field. Time-dependent unrestricted Hartree-Fock (TDUHF) is first developed from the variational principle. I then show, by means of a series of approximations, that this leads to the Crapola model. The approximations are clarified and the region in which they are accurate is determined. I include a comparison of using unrestricted and restricted Hartree-Fock for estimating the ground-state energy of the helium atom. This conception of the ground state plays an important part in understanding how the Crapola model arises. A new modified version of the Crapola model, which is expected to have a wider range of validity, is developed and discussed.

Dynamics of charge transfer in molecular switches II. Effective potentials and Coulomb interaction

The charge transport on a single conducting polymer chain, with impurity molecules working as switches, is investigated. The dynamics of solitons through the sites that work like a switch is numerically studied using the Su—Schrieffer—Heeger model extended to include short-range Coulomb interactions. The time-dependent unrestricted Hartree—Fock approximation is used. The considered molecular switches are single and pairs of donor—acceptor molecules bonded to the chain. The influence of the parameters of the molecules on the charge transport and the role of the length of separation between the sites where the donor and acceptor molecules bond are determined. The effects of the inclusion of short-ranged Coulomb interactions are analyzed. We write effective potentials determining the soliton behavior for each case, where the switching sites allow the soliton to pass, be reflected or trapped.

Double multiphoton ionization of a model atom

A direct solution of the time-dependent Schr\"odinger equation (TDSE) for a two-electron model atom in a strong electromagnetic field is used to extract probabilities for both single and double electron ionization. For three-photon ionization at a moderately strong intensity, the TDSE calculations find single ionization at 62%, single ionization with excitation at 10%, and double ionization at 3%. The sequential process of escape from an already once ionized atom is found to make a substantial contribution to the double-ionization probability. Multiphoton-ionization probabilities are also obtained from a solution of the time-dependent unrestricted Hartree-Fock (TDUHF) equations for the same model atom. Although in qualitative agreement with the TDSE results, the TDUHF results show significantly less single ionization with excitation and have a different angular distribution for double ionization.

Spin-unrestricted time-dependent Hartree–Fock theory of frequency-dependent linear and nonlinear optical properties

A self-consistent time-dependent unrestricted Hartree–Fock (TDUHF) theory of linear and nonlinear optical properties is presented. Expressions are derived to calculate the elements of the linear polarizability tensor α, the first-hyperpolarizability tensor β, and the second-hyperpolarizability tensor γ, in terms of spin-adapted perturbed density matrices. For the hyperpolarizability tensors, β and γ, expressions are also derived from the lower-order solutions to the TDUHF equations. A novel feature of the present formulation is that it automatically allows for the separation of the contributions to (hyper)polarizability tensors from individual spin. Results obtained from the calculations of α(ωσ;αa) for H, C, N, O, F, Si, P, S, Cl, O2, NO, and OH and of β(−ωσ;ωa,ωb) corresponding to various second-order nonlinear optical processes for NO and OH radicals are presented. The present results for α show excellent agreement with the literature data. The calculated result for β∥ in the case of the NO radical is too small and has a different sign in comparison to the published experimental data. For both NO and OH, the β tensor for different second-order nonlinear optical processes show the order: β(−2ω;ω,ω)≳β(−ω;0,ω) ≡β(0;ω,−ω)≳β(0;0,0).