Controlling molecular orientation through radiative rotational transitions in strong static electric fields

The recent experimental observations of Bloch oscillations and the emission of electromagnetic radiation1 in the terahertz regime in semiconductor superlattices subject to static uniform electric fields, suggest that these systems may be developed as useful THz generators. The Bloch period is given by τB = h/(eFa), where e and F denote the magnitude of the electron charge and the electric field, h is Planck’s constant, and a is the spatial period of the superlattice. Modeling these systems in terms of independent electrons in an idealized one-dimensional superlattice potential in the effective mass approximation, and using high-accuracy numerical methods to solve the time-dependent Schrodinger equation, the electron wave function has been found2–4 to exhibit long-lived Bloch oscillations for electric fields in the range used in experiment. We here present results for this model, obtained using the methods of Ref 2–4, for the regime where the electrons are subject to very strong static electric fields, so that they exhibit characteristics of acceleration as well as some residual but faint signs of Bloch oscillations. Some preliminary results for this regime have been given in Sec. IIID of Ref 3, while a very detailed analysis is presented in Ref 4.

This paper provides a general theoretical descriptionof a weakly bound atomic system (a negative ion)interacting simultaneously with two(generally strong) fields, a static electric field anda monochromatic laser field having an arbitrary elliptical polarization.The zero-range δ-potential is used to modelthe interaction of a bound electron in a negative ionas well as the interaction of a detached electron with the residualatom. Our treatment combines the quasistationary(complex energy) and quasienergy (Floquet) approaches.This quasistationary, quasienergy state (QQES) formalismis the most appropriate one for analysing a decaying quantum systemunderthe influence of a periodic external perturbation.Existing QQES theory is reviewed and somenew results are discussed:the Hellmann-Feynman theorem and the normalization procedure forQQES,and the definition of the dipole moment andthe dynamic polarizability for a decaying atomic system(in strong static electric and/or laser fields).These results are illustrated using analytical formulaeobtained from an exact solution of theQQES problem for a δ-model potential in two strong fields.Finally, from the imaginary part of the dynamic polarizabilitywe obtain analytic results (to first order in thelaser-field intensity) for the photodetachment cross section.Our results are then compared with those of previoustheoretical studies.

Stochastic equations of the hydrodynamic theory of plasma are presented in relation to strong external fields. It is shown that the use of these stochastic equations makes it possible to obtain new theoretical solutions for plasma as a result of its heating in a strong external electric field. Theoretical solutions for the conductivity of turbulent plasma when heated in an external electric field of 100 V/cm are considered. Calculated values for the electron drift velocity, electron mobility, electron collision frequency, and the Coulomb logarithm in the region of strong electric fields are obtained. Here we consider experiments on turbulent heating of hydrogen plasma in the range of electric field strength of 100 < E < 1000. The calculated dependences of plasma conductivity are in satisfactory agreement with experimental data for heating plasma in a strong electric field. It is shown that the plasma turbulence in the region of strong electric fields E ~1000 V/cm is close to 100%. For the first time, it is confirmed that the derived dependences for collision frequency, drift velocity, and other values include the degree of turbulence of plasma, which makes it possible to correctly describe experimental data for heating plasma even with strong electric fields. In addition, it was determined that the scatter of experimental data may be associated with the variability of the function in the expression for the heat flux density. For the first time, it is shown theoretically that the experimentally determined fact of the possibility of the existence of an approximate constancy of plasma conductivity in the region E = 100–1000 V/cm can occur with an error of ~30%. The results show significant advantages of the stochastic hydrodynamic plasma theory over other methods that are not yet able to satisfactorily as well as qualitatively and quantitatively predict long-known experimental data while taking into account the degree of turbulence.

We propose a scheme to generate electric dipole moments in homonuclear molecular cations by creating, with an ultrashort pump pulse, a quantum superposition of vibrational states on electronic states strongly perturbed by very strong static electric fields. By field-induced molecular stabilization, the dipoles can reach values as large as 50 Debyes and oscillate on a time-scale comparable to that of the slow vibrational motion. We show that both the electric field and the pump pulse parameters can be used to control the amplitude and period of the oscillation, while preventing the molecule from ionizing or dissociating.

Our recent analysis [J. Phys. B 33, R141 (2000)] of the problem of ${\mathrm{H}}^{\ensuremath{-}}$ in both laser and strong static electric fields, which treated effects that are linear in the laser intensity I (e.g., the dynamic polarizability and the single-photon detachment cross section), is extended here to treat effects of higher order $(\ensuremath{\sim}{I}^{2})$ in the laser intensity (e.g., the dynamic hyperpolarizability, the two-photon detachment cross section, the linear in I corrections to the single-photon detachment rate, etc). We introduce the concept of the dynamic hyperpolarizability of an atom in the presence of a strong electric field based on the complex quasienergy approach with properly normalized, quasistationary quasienergy wave functions. Our analysis of the general structure of the dynamic hyperpolarizability tensor of an atom in a nondegenerate $(S$-) state is performed for arbitrary field geometries and laser polarizations. The connection of the hyperpolarizability $\ensuremath{\gamma}(\mathcal{F};\ensuremath{\omega})$ to the complex quasienergy and to atomic ionization rates is established. Analytic results (in terms of Airy functions) are obtained for five irreducible components of a hyperpolarizability tensor (that are independent of the laser polarization and the field geometry) for the case of a weakly bound electron in a three-dimensional, zero-range potential. These results are used for the analysis of the frequency, field geometry, and laser polarization dependence of the two-photon detachment rate as well as of the linear in laser intensity corrections to the single-photon detachment rate. It is shown that the oscillatory behavior of the frequency dependence of both the real and the imaginary parts of $\ensuremath{\gamma}(\mathcal{F};\ensuremath{\omega})$ exists for both the coplanar and orthogonal field geometries, and that this behavior is qualitatively different for frequencies below and above the single-photon detachment threshold. The threshold behavior of the two-photon detachment rate is analyzed in detail and the static electric-field-induced modification of Wigner's threshold law for a short-range potential is discussed.

The results of a linear analysis for the stability of a homogeneous dielectric liquid with respect to density perturbations in a uniform electric field are presented. The electric field increases the instability increment for the stratification along the field and decreases this increment in the transverse direction. Thus, a strong electric field can induce anisotropic decay into liquid and vapor phases for a fluid that is initially both in a labile state and in a metastable or stable state. Theoretical calculations are confirmed by the results of numerical modeling of the fluid dynamics. The new regions of a low-density phase have the form of narrow cylindrical channels oriented along the field. This new mechanism of gas phase formation in strong local electric fields probably plays a key role in the inception and ultra-fast propagation of streamers during the breakdown in dielectric liquids in a nanosecond range.

This chapter contains sections titled: Introduction Previous Treatments of the Nonlinear Dielectric and Birefringence Relaxation Orientational Relaxation Processes in the Presence of Strong Electric Fields: The Rotational Diffusion Model Nonlinear Transients in Dielectric Relaxation and Dynamic Kerr Effect Arising from Sudden Changes of a Strong dc Electric Field Steady-State Response Arising fom a Weak ac Field Superimposed on a Strong dc Bias field Nonlinear stationary Respones in Strong ac and dc Electric Fields Effects Due to Hyperpolarizabilities of Molecules on Kerr Effect Relaction Intertial Effects in Dielectric and Birefringence Relaction in a Strong sc ElectricField: Linear response Application to Related Relaxation Phenomena Conclusion

We give the general results for spontaneous and photon-induced pair production in strong, constant parallel electric and magnetic fields. For photons propagating exactly parallel to the constant fields there is no effect. For photons propagating at right angles to the constant fields the vacuum behaves in a birefringent manner. These results generalize earlier work in the field by others who did not include a constant electric field. Some numerical estimates are given for both the spontaneous and photon-induced pair production rates using numbers currently thought to obtain near the surface of a pulsar-which is believed to possess not only a strong magnetic field but also a strong electric field.

In this paper we obtain a simple analytic formula for the photodetachment cross section of H 2 in parallel electric and magnetic fields. The three-dimensional semiclassical approximation predicts oscillations in the spectrum and correlates these oscillations with closed classical orbits. The cylindrical symmetry of the Hamiltonian produces some interesting effects. In particular, at boundary energies the semiclassical approximation fails as a focused cusp approaches the origin. @S1050-2947~97!03306-4# PACS number~s!: 32.80.Gc I. INTRODUCTION Experimental measurements of the photodetachment cross section of H 2 in strong static electric fields were made by Bryant et al. @1#. For energies above the threshold energy the resulting cross section was found to be a smooth background upon which was superposed sinusoidal oscillations. Theoretical discussions of the measurements have been given by a number of authors @2,3#. The oscillations arise as an interference effect because the outgoing electron can move against the electric force and then return to the atom. It was shown in Ref. @3# that the cross section could be expressed in the following way: s~ E!5s 0~ E!1C~ E!sinF~ E!, where s 0 is the cross section in the absence of any external fields andC(E) and F(E) are called the recurrence strength and recurrence phase associated with the returning orbit. Theoretical calculations @4,5# show that analogous oscillations occur for photodetachment in crossed electric and magnetic fields, and that the oscillations are again associated with closed orbits. Quantum calculations have shown that very strong oscillations are present in parallel fields @6#, but the relationship to closed orbits was not made clear. Our original motivation in the present paper was to complete this subject by calculating the closed orbits and the resulting spectral oscillations for photodetachment of an electron from H 2 in parallel electric and magnetic fields. This is a system in which the recurrences are simple, strong, relatively easy to observe, and easy to understand. We found results which go far beyond this topic, and which are connected with many other problems of current interest. We know from study of nonlinear dynamics that even when long-time motion is chaotic, short-time motion remains relatively simple and predictable. Periodic orbits play a central role: in Poincare ´ ’s words, they offer ‘‘the only opening through which we might try to penetrate the fortress ~chaos! which has the reputation of being impregnable.’’ This classical statement holds also in quantum mechanics, wherein the periodic-orbit theory of Gutzwiller, Balian and Bloch, and Berry and Tabor @7# provides a general theoretical framework for studying quantum manifestations of classical chaos. Besides producing oscillations in absorption spectra @8#, periodic orbits produce scars in wave functions @9#, oscillations in the density of states @10#, and real-time recurrences that have been observed in many atoms and molecules @11#. @Similar phenomena are observed in microwaves in cavities @12~a!#, in microjunctions @12~b!#, and they are calculated to be consequences of certain models in nuclear physics @12~c!#.# Bifurcations of periodic orbits are of particular interest. A

This work focuses on the numerical study of electrohydrodynamic multiphase fluid flow problems. Liquid bodies experience deformations caused by strong external electric fields. These deformations can be used in industrial applications to accurately control the motion of single droplets. In the limiting case, strong electric fields can force atomisation of fine microdroplets from much larger liquid bulks. Due to differences in the electrical properties of the different fluid phases, the flow affects in turn the electric field distribution. The analysis of such coupled flows requires numerical modelling of the interdependent hydrodynamic and electrodynamic problems. Since most liquids exhibit some conductivity due to intrinsic ionic species and dissolved impurities, the electrodynamic problem must be modelled by an electroquasistatic model taking into account capacitive, resistive and convective electrical currents. This electroquasistatic problem is coupled to an incompressible fluid flow problem described by the Navier-Stokes equations. Both problems are solved on the same computational grid, using the finite volume method. The fluid-fluid interface is modelled using the volume of fluid method. The resulting diffuse interface captures the motion of the phase boundaries while efficiently handling topology changes. The motion of contact lines is furthermore modelled using a dynamic contact angle model including hysteresis effects. Pinned contact lines and stick-slip contact line motion in transient problems can therefore be represented. The developed solver is readily applicable to a large range of electrohydrodynamic flow problems. This work investigates electrohydrodynamic flows occurring in three technical applications. First, the dynamics of sessile droplets subjected to an AC electric field on the surface of an insulator are considered. The dynamics of the oscillating droplets are compared with experimental data. Partial discharge inception fields are then estimated for similar configurations. Secondly, the detachment dynamics of two liquids in an on-demand droplet generator, where droplet detachment is enforced by electric fields, are considered and compared to experimental data. The different conductivities of the liquids are shown to lead to substantially different detachment dynamics. Relevant parameters in the detachment dynamics are extracted from the simulations. Finally, transient electrosprays in the cone-jet mode are characterised for a number of liquids with different electromechanical properties. The charge-radius correlations of the first ejections are found to obey the scaling laws reported in the literature. Moreover, additional scaling laws are found for the subsequent ejections.

Three director profiles of nematic liquid crystal (NLC) cells are presented to elucidate the mechanism of light leakage induced by some defects hidden in NLC cells. In accordance with Landau-de Gennes theory, we simulated the dynamics of these defect profiles using the 2-D finite-difference iterative method and calculated their transmittance using the Jones calculus. Results show that light leakage induced by these defects is caused by spontaneous azimuthal twisting of the director when no electric field is applied. Spontaneous distortion occurs only in the director field around defects and depends on larger nematic anisotropies. The light leakage intensity in cells I and II differs because of difference in the defect structures of the cells. Regardless of the cell type, light leakage is exacerbated when the surface anchoring strength is nonuniform. Light leakage disappears at a critical value of electric field $E_{c}$ . However, a very strong static electric field $E_{c}^{\ast}$ is necessary to force the light leakage to disappear when the flexoelectric effect is considered.

A non perturbative approach is used to solve the problem of a rigid linear molecule with both a permanent dipole moment and a static dipole polarizability, in a static electric field. Eigenenergies are obtained and compared to perturbative low field and high field approximations. Analytical expressions for the orientation parameters and for the gradient of the energy are given. This non perturbative approach is applied to the simulation of beam deviation experiments in strong electric field. Results of simulations are given for inhomogeneous alkali dimers. For LiNa, the simulations are compared to experimental data. For LiK, deviation profiles have been simulated in order to prepare future experiments on this molecule.

The photochemical conversion of CO 2 into valuable fuels faces challenges of low efficiency and poor selectivity. Hydrated electrons (e aq − ), with their extremely negative reduction potential, are promising CO 2 -reducing agents, yet their short lifetime limits selectivity and high-energy-density product formation. Herein, we show that microdroplet interfaces with strong electric fields (10 9 volts per meter) substantially extend the lifespan of e aq − generated from industrial sulfite pollutants (SO 3 2− ), lowering energy barriers in the CO 2 reduction reaction and enabling targeted product formation. The machine learning strategy identified microdroplet size as the key parameter controlling electric field strength, product yield, and selectivity. In our lab-based scaled-up system, microdroplets <10 micrometers improved performance by four to seven orders of magnitude over bulk-phase systems, achieving ~99% methanol selectivity. Strong interfacial electric fields stabilize intermediates and modulate carbon-oxygen bond lengths, directing pathways to high-value products. This approach enables sustainable CO 2 utilization via microdroplets, potentially producing fuels from waste.

In complex electromagnetic environments, the stability of energetic materials (EMs) encounters substantial challenges. Revealing the evolution of charge structures in EMs under electromagnetic fields is crucial for enhancing their safety. EMs are generally non-magnetic, so the effect of magnetic fields in the system can usually be negligible, with the focus instead on the process of electric fields acting on EMs. To systematically investigate the response characteristics of EMs under electric field, this study employed density functional theory (DFT) to conduct quantitative calculations on the charge characteristics and chemical bond orders of RDX, TNT and their composites doped with single-walled carbon nanotubes (SWCNTs) of different sizes under different electric field conditions (strength and direction). A method based on the amount of charge transfer is proposed to assess the microwave-sensitivity of each system and estimate the threshold breakdown field strength. These predictions were subsequently validated through microwave resonant cavity ignition experiments conducted on RDX and TNT compositions. Under electric fields, the N-N bonds in the RDX molecule are weakened. The introduction of SWCNT molecules further weakens the C-N bonds, which may facilitate the formation of new reaction pathways. The breakdown field strength of each system can be ascertained based on the amount of charge displacement under an electric field. The threshold breakdown field strengths for RDX and TNT molecules respectively under a strong electric field are on the order of 1×10 -3 V/Å. After being doped with carbon nanotubes, TNT shows stronger microwave susceptibility than RDX. This enhanced effect, as well as the predictions about electromagnetic sensitivity, has been verified through experiments.

We develop methods for computing the dynamics of fermion pair production by strong color electric fields using the semi-classical Boltzmann-Vlasov equation. We present numerical results for a model with SU(2) symmetry in (1+1) dimension.