Routes to control of H2 Coulomb explosion in few-cycle laser pulses

We have measured coincident ion pairs produced in the Coulomb explosion of H2 by 8-30 fs laser pulses at different laser intensities. We show how the Coulomb explosion of H2 can be experimentally controlled by tuning the appropriate pulse duration and laser intensity. For laser pulses less than 15 fs, we found that the rescattering-induced Coulomb explosion is dominated by first-return recollisions, while for longer pulses and at the proper laser intensity, the third return can be made to be the major one. Additionally, by choosing suitable pulse duration and laser intensity, we show H2 Coulomb explosion proceeding through three distinct processes that are simultaneously observable, each exhibiting different characteristics and revealing distinctive time information about the H2 evolution in the laser pulse.     

Modeling of fast phase transitions dynamics in metal target irradiated by pico- and femtosecond pulsed laser

We investigate laser pulse influence on aluminum target in irradiance range 10⁹ to 10¹⁶ W/cm², pulse duration between 10⁻⁸ and 10⁻¹⁵ s, Gaussian time profile with wavelength of 0.8 μm. For all computations energy density was 10 J/cm². Plasma in the evaporated material is generated at the energy density above 10 J/cm²as the modeling showed.Long and short laser pulses distinguish by the mechanisms of energy transformation. For short laser pulses there is volumetric energy absorption, together with rapid phase transitions it lead to overheating in solid and liquid states, overheated solid temperature rises up to (6-8)T_m. Under influence of the energy saved in overheated solid, duration of the phase transitions becomes nanosecond, which is several orders of magnitude longer than laser pulse.     

A new approach to the problem of motion in general relativity

The dynamical meaning of the equationsT ^/j/ij =0 is derived as a consequence of the mathematical structure of Einstein's equations. A generalization of Lichnerowicz's analysis of the gravitational equations is proposed.Lavoro eseguito nel centro di Matematica e Fisica Teorica del C.N.R. presso l'Università di Genova.     

A non-Boltzmann balance equation approach to electronic transport in a type-I superlattice

Force and energy balance equations are derived for steady state hot electron transport in a type-I superlattice composed of periodically arranged finite-width quantum wells. The scatterings by remote and background impurities and by acoustic and polar optical phonons are taken into account, together with Coulomb interactions between intralayer electrons and interlayer electrons. The carrier mobility and electron temperature are calculated as functions of drift velocity ν up to ν=3×10⁷ cm / s, showing significant nonlinearity and dependence on geometrical superlattice parameters (quantum well width and separation, etc.)     

A renormalization approach to describe charge transport in quasiperiodic dangling backbone ladder (DBL)-DNA molecules

We study the charge transport properties of a dangling backbone ladder (DBL)-DNA molecule focusing on a quasiperiodic arrangement of its constituent nucleotides forming a Rudin-Shapiro (RS) and Fibonacci (FB) Poly (CG) sequences, as well as a natural DNA sequence (Ch22) for the sake of comparison. Making use of a one-step renormalization process, the DBL-DNA molecule is modeled in terms of a one-dimensional tight-binding Hamiltonian to investigate its transmissivity and current-voltage (I-V) profiles. Beyond the semiconductor I-V characteristics, a striking similarity between the electronic transport properties of the RS quasiperiodic structure and the natural DNA sequence was found.     

Femtosecond laser-induced dynamic alignment in Coulomb explosion of CO: Roles of laser wavelength, intensity and pulse duration

Dependences of dynamic alignment of CO molecules induced by intense femtosecond laser fields on laser wavelength, intensity and pulse duration are investigated by numerical simulations. A counting approach and a fourth-order Runge-Kutta algorithm are used to calculate the angular distribution and the time evolution of molecules. A two-step Coulomb explosion model of diatomic molecules in intense laser fields is used to determine the instant that CO molecular dynamic alignment is over. Our calculating results show that the linear polarizability and the damping force play an important role in the angular rotation of CO molecule in conditions of 800 nm laser wavelength and 10¹⁵ W/cm² laser intensity. The contributions of the second-order field-induced dipole moment and the higher-order correction term to molecular rotation acceleration comparing to the linear polarizability and damping force are negligible. The extent of dynamic alignment of CO molecules reduces with the increasing of laser intensity. The dynamic alignment time of CO molecules is tightly connected to the laser pulse duration. The angular distributions of CO molecules as the laser pulse length varied from 50 to 250 fs at laser intensity of 3×10¹⁴ W/cm² are shown and discussed.     

The continuum orbital problem in the calculation of auger decay rates: Application to Ne

Summary Described here are new computational procedures which have been developed for obtaining the continuum orbital in Auger problems. A Lippmann-Schwinger (LS) equation is solved for the inner, atomic or molecular region and in the asymptotic region the LS orbital is matched with the eigenfunctions of the long-range Hamiltonian. A detailed analysis of the form of the continuum orbital for various decay states of the KLL Auger spectrum of the neon atom is presented. Decay rates are given for the Ne and compared with previous results and with experiments.

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Riassunto Si presenta una nuova tecnica per ottenere l'orbitale del continuo in problemi Auger. Essa consiste nella soluzione di un'equazione alla Lippmann-Schwinger (LS) per la regione atomica o molecolare e nel raccordo dell'orbitale cosí ottenuto con le autofunzioni dell'hamiltoniana contenente la parte asintotica del potenziale. Si effettua inoltre un'analisi dettagliata della forma dell'orbitale del continuo per vari possibili stati di decadimento Auger dell'atomo di Ne ionizzato nel guscio 1s e si interpreta lo spettro KLL di tale sistema in termini di energie di risonanza e velocitá di decadimento.

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Резюме В работе описываются новые вычислительные процедуры, которые были развиты для получения непрерывной орбитали в Оже-проблемах. Решается уравнение Липмана-Мвнгера для внутренней атомной или молекулярной области. В асимптотической области орбиталь Липмана-Швингера согласуется с собственными функциями Гамильтониана. В работе проводится подробный анализ формы непрерывной орбитали для различных возможных состояниЙ Оже-распада атома Ne. Приводятся интенсивности распада для Ne и сравниваются с предыдущими результатами и данными экспериментов.

     

Generation,transport, and focusing of fast electrons in nanofilaments of a target irradiated by a short laser pulse with ultrarelativistic intensity

Absorption of an intense short laser pulse by targets consisting of a bungle of carbon nanofilaments either parallel or converging to the vertex of a cone has been numerically simulated. Such targets efficiently generate a flux of relativistic electrons propagating along filaments and following their bends. The focusing of fluxes from several filaments to one flux makes it possible to reduce the transverse size of an electron bunch as compared to the diameter of the laser beam. The energy flux density of electrons in such a bunch propagating along one nanofilament is several times higher than the energy flux density of the laser pulse.     

A self consistent approach to the transport equation for moving atoms in interaction with laser fields

The transport equations for moving atoms in interaction with laser fields, are derived using the hamiltonian in terms of the quantum dynamical variables for the center of mass motion and by introducing suitably defined local density matrices. Use is also made of the phase space variables associated with the quantum dynamical variables of center of mass and the projection operator techniques.     

A general isomorphism approach to thermodynamic and transport properties of binary fluid mixtures near critical points

A general isomorphism approach to critical phenomena in binary fluid mixtures that may exhibit complex critical-line behavior is developed by relating the two relevant scaling fields to linear combinations of three physical field variables. These physical field variables are related to the temperature and chemical potentials of the two components. The proposed approach includes crossover from vapor-liquid critical behavior to liquid-liquid critical behavior and incorporates also the critical behavior near other special points on critical loci. It is shown that the key factor which determines the apparent behavior of the thermodynamic and transport properties of near-critical mixtures is the shape of the critical locus. The number of system-dependent coefficients that determine the asymptotic critical behavior is elucidated. The choice of zero-points of entropy and energy in binary mixtures is also discussed. The approach provides a powerful tool for predicting thermodynamic and transport properties of fluid mixtures in the critical region.     

Hot electrons in indium nitride: a new numerical approach to solving the electron transport problem

The electron transport in indium nitride in a heating electric field has been calculated. The results obtained are in good agreement with the known experimental data. The Boltzmann transport equation in the isotropic spatially homogeneous case has been solved using a new numerical approach based on the approximation of the distribution function from the points of a relatively thin grid in momentum space and the iterative search for a stationary solution.     

The three-body bound-state problem in atomic physics: A field-theoretic approach

The three-body bound-state problem is formulated in terms of fieldtheoretic (FT) amplitudes. The two-particle effective potentials of this theory are expressed in terms of the random-phase approximation and ladder schemes. These are cast into convenient single-particle equations by means of FT renormalization.     

A two-density approach to the general many-body problem and a proof of principle for small atoms and molecules

An extended electron model fully recovers many of the experimental results of quantum mechanics while it avoids many of the pitfalls and remains generally free of paradoxes. The formulation of the manybody electronic problem here resembles the Kohn–Sham formulation of standard density functional theory. However, rather than referring electronic properties to a large set of single electron orbitals, the extended electron model uses only mass density and field components, leading to a substantial increase in computational efficiency. To date, the Hohenberg–Kohn theorems have not been proved for a model of this type, nor has a universal energy functional been presented. In this paper, we address these problems and show that the Hohenberg–Kohn theorems do also hold for a density model of this type. We then present a proof-of-concept practical implementation of this method and show that it reproduces the accuracy of more widely used methods on a test-set of small atomic systems, thus paving the way for the development of fast, efficient and accurate codes on this basis.     

An alternative approach to determine the fractional heat load in solid state laser materials: application to diode-pumped Nd:YVO4 laser

A simple approach is described and used for on-line measurement of the fractional heat-load parameter ξ in an operating diode-pumped Nd:YVO₄ laser at 1.06 μm wavelength for different doping concentrations. The method is based on the fact that if the thermo-optical properties of the sample are known then the ξ-parameter can be estimated from the measured effective focal length induced by the pump beam and any other lensing effect due to mechanical mounting of the sample for a given pumping configuration. The value of the fractional heat-load parameter estimated by our technique was in excellent agreement with the earlier reported values.     

The influence of the dynamics of ionic multiplets onto electronic transport properties of heavy-fermion systems: a semi-phenomenological approach

We present calculations of the electronic transport properties of heavy-fermion systems within a semi-phenomenological approach to the dynamical mean field theory. In this approach the dynamics of the Hund's rules 4f (5f )-ionic multiplet split in a crystalline environment is taken into account. Within the scope of this calculation we use the linear response theory to reproduce qualitative features of the temperature-dependent resistivity and hall conductivity, the magneto-resistivity and the thermoelectric power typical for heavy-fermion systems. The model calculations are directly compared with experimental results on CeCu ₂ Si ₂. Received 30 June 2000 and Received in final form 15 December 2000     

The electronic structure of arbitrarily shaped atomic clusters: A formal MSW approach

In this paper a formal MSW approach is established to calculate the energy levels of atomic clusters and molecules. It is shown that the standard secular equation is formally preserved, though the muffin-tin approximation is replaced by the only assumption that the total potential is a sum of (possibly overlapping) atomic contributions. The transition matrix still contains all information about the scattering properties of the atoms, but is no longer diagonal in the angular momentum indices.     

The application of Raman and anti-stokes Raman spectroscopy for in situ monitoring of structural changes in laser irradiated titanium dioxide materials

The use of Raman and anti-stokes Raman spectroscopy to investigate the effect of exposure to high power laser radiation on the crystalline phases of TiO₂ has been investigated. Measurement of the changes, over several time integrals, in the Raman and anti-stokes Raman of TiO₂ spectra with exposure to laser radiation is reported. Raman and anti-stokes Raman provide detail on both the structure and the kinetic process of changes in crystalline phases in the titania material. The effect of laser exposure resulted in the generation of increasing amounts of the rutile crystalline phase from the anatase crystalline phase during exposure. The Raman spectra displayed bands at 144 cm⁻¹ (A1g), 197 cm⁻¹ (Eg), 398 cm⁻¹ (B1g), 515 cm⁻¹ (A1g), and 640 cm⁻¹ (Eg) assigned to anatase which were replaced by bands at 143 cm⁻¹ (B1g), 235 cm⁻¹ (2 phonon process), 448 cm⁻¹ (Eg) and 612 cm⁻¹ (A1g) which were assigned to rutile. This indicated that laser irradiation of TiO₂ changes the crystalline phase from anatase to rutile. Raman and anti-stokes Raman are highly sensitive to the crystalline forms of TiO₂ and allow characterisation of the effect of laser irradiation upon TiO₂. This technique would also be applicable as an in situ method for monitoring changes during the laser irradiation process.     

A non-perturbative approach to the infrared problem in QED: Construction of charged states

A non-perturbative treatment of the infrared problem in quantum electrodynamics is presented. Starting from the local and covariant correlation functions of the Gupta-Bleuler formulation an explicit construction of the physical charged states is given. They are shown to satisfy a non-(Lorentz) covariant infrared condition, of Bloch-Nordsieck type, compatible with the Gupta-Bleuler condition (resolution of Zwanziger's paradox). The breaking of the Lorentz group in the charged sectors and its physical meaning is explained. The infrared structure of the S-matrix elements between physical charged states is shown to be that of the classical Bloch-Nordsieck factors, (proof of the Bloch-Nordsieck ansatz), and to be simply related to that calculated by using local covariant states.