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Baldea I.,University of Heidelberg | Baldea I.,National Institute for Laser, Plasma and Radiation Physics
Journal of the American Chemical Society | Year: 2012

The first simultaneous measurements of transition voltage (V t) spectroscopy (TVS) and conductance (G) histograms (Guo et al., J. Am. Chem. Soc.2011, 133, 19189) form a great case for studying stochastic effects, which are ubiquitous in molecular junctions. Here an interpretation of those data is proposed that emphasizes the different physical content of V t and G and reveals that fluctuations in the molecular orbital alignment have a significantly larger impact on G than initially claimed. The present study demonstrates the usefulness of corroborating statistical information on different transport properties and gives support to TVS as a valuable investigative tool. © 2012 American Chemical Society.

Popa A.,National Institute for Laser, Plasma and Radiation Physics
Laser and Particle Beams | Year: 2012

The interaction between laser and relativistic electron beams is a promising source of very energetic X rays. We present an accurate model for the collisions between very intense linearly polarized laser beams, corresponding to relativistic parameters of the order of unity or greater, and electrons having energies up to 100 MeV. Our approach uses only one approximation, namely it neglects the radiative corrections. We consider the two cases in which the laser field polarization is either perpendicular or parallel to the plane defined by the directions of propagation of the laser beam and electron beam, and calculate accurately the properties of the σ and π polarized scattered beams. The angle between the directions of the laser and electron beams, denoted by θ L, is allowed to have arbitrary values, so that the widely analyzed 180° and 90° geometries, in which the two beams collide, respectively, head on and perpendicularly, are particular cases. We prove that the polarization properties of the scattered beam depend on the angle θ L. By varying this angle, the polarization of the scattered beam can be varied between the two limit configurations in which the electromagnetic field of the scattered beam is σ or π polarized with respect to the scattering plane. Our theoretical results are in good agreement with experimental results published in literature. Our model shows that current technologies can be used to produce hard harmonics of the scattered radiations. These harmonics can have relatively high intensities comparable to the intensities of the first harmonics, and energies higher than 1 MeV. Our results lead to the possibility to realize an adjustable photon source with both the energy and polarization of the scattered radiations accurately controlled by the value of the θ L angle. Copyright © 2012 Cambridge University Press.

Dabu R.,National Institute for Laser, Plasma and Radiation Physics
Optics Express | Year: 2010

Gain spectra were calculated at critical wavelength degeneracy (CWD) in a collinear phase-matching geometry optical parametric amplification (OPA) process. The frequency bandwidth available through CWD-OPA is broader compared to the gain bandwidth obtained by the noncollinear OPA geometry. A solution for very broad bandwidth chirped pulse amplification based on partially deuterated DKDP (P-DKDP) crystals, pumped by pulsed green lasers, is proposed. 1.38 × 1014 Hz frequency bandwidth and peak intensity gain G ≈62 were calculated in a 5-mm long 58% deuterated DKDP crystal, pumped by 527-nm wavelength at 64-GW/cm2 intensity. Parametric amplification at CWD in few-mm thin PDKDP crystals, pumped by picosecond pulses of nearly 100-GW/cm2 intensity, possesses a true potential for generating high energy laser pulses compressible to one-cycle duration. © 2010 Optical Society of America.

Budriga O.,National Institute for Laser, Plasma and Radiation Physics
European Physical Journal D | Year: 2011

We study the influence of the incoherent pump field on the transient response of a three-level V-type system with two near-degenerate excited states. In this system due to interaction of the field with the vacuum an interference term appears, the spontaneously generated coherence term. We find that due to the incoherent pump the transient proprieties of the atomic system can be significantly modified. The enhancement of the transient gain can be obtained for proper values of the rate of the incoherent pump field and relative phase between the two fields. © 2011 EDP Sciences, SIF, Springer-Verlag Berlin Heidelberg.

Popa A.,National Institute for Laser, Plasma and Radiation Physics
Physical Review A - Atomic, Molecular, and Optical Physics | Year: 2011

We prove that the analytical expression of the intensity of the relativistic Thomson scattered field for a system composed of an electron interacting with a plane electromagnetic field can be written in the form of a composite periodic function of only one variable, that is, the phase of the incident field. This property is proved without using any approximation in the most general case in which the field is elliptically polarized, the initial phase of the incident field and the initial velocity of the electron are taken into consideration, and the direction in which the radiation is scattered is arbitrary. This property leads to an exact method for calculating the angular and spectral distributions of the scattered field, which reveals a series of physical details of these distributions, such as their dependence on the components of the initial electron velocity. Since the phase of the field is a relativistic invariant, it follows that the periodicity property is also valid when the analysis is made in the inertial system in which the initial velocity of the electron is zero in the case of interactions between very intense electromagnetic fields and relativistic electrons. Consequently, the calculation method can be used for the evaluation of properties of backscattered hard radiations generated by this type of interaction. The theoretical evaluations presented in this paper are in good agreement with the experimental data from literature. © 2011 American Physical Society.

Pavel N.,National Institute for Laser, Plasma and Radiation Physics
Laser Physics | Year: 2010

Continuous-wave (CW) simultaneous laser emission on the 0.9-μm 4F3/2 → 4I9/2 transition and the 4F3/2 → 4I11/2 transition at 1.06 μm is obtained in Nd-based laser crystals of thin-disk geometry and using a multi-pass pumping scheme. A Nd:Y3Al5O 12 (Nd:YAG) thin disk emitted simultaneous laser radiation at 946 and 1064 nm with 5.1 W output power, and Nd:YVO4 and Nd:GdVO4 thin-disk lasers with more than 3 W output power at 0.91 and 1.06 μm were realized. The ratio between the output power at one of the wavelengths and the total output power could be varied by the laser resonator design. An intracavity frequency-doubled Nd:YVO4 thin-disk laser with alternate green at 532 nm and "deep-blue" at 457 nm generation of high average output powers is demonstrated. © 2010 Pleiades Publishing, Ltd.

Stancalie V.,National Institute for Laser, Plasma and Radiation Physics
European Physical Journal D | Year: 2013

We report results from detailed state selective photo-recombination study along with the doubly excited autoionizing resonances in Be-like C2+ and Al9+ ions. In the present investigation, the primary focus is on detailed energy profiles of the individual photo-recombination cross sections. The calculation was carried out for the excited Rydberg states of type 1s 22sns(1Se) which interact with the odd-parity continua up to the C3+ and Al10+ 2p threshold limit. The numerical evaluation has been performed at a fine energy mesh across all the autoionizing Rydberg series of resonances 1s22pns(1P 0) converging to Li-like ion 2p threshold. The method of calculation keeps the essential ingredients of the Feshbach projection operator approximation. The photo-ionization cross sections have been evaluated with and without relativistic effects included into the R-matrix numerical procedures, while the allowance for both quantum interference between dielectronic and radiative recombination, and overlapping resonances has been done utilizing results from the earlier R-matrix Floquet calculation. We discuss all these results with respect to the effect of quantum interference term on the energy dependence profile of photo-recombination cross section for studied transitions. © EDP Sciences, Società Italiana di Fisica, Springer-Verlag 2013.

Vlad M.,National Institute for Laser, Plasma and Radiation Physics
Physical Review E - Statistical, Nonlinear, and Soft Matter Physics | Year: 2013

Test modes on turbulent magnetized plasmas are studied taking into account the stochastic ion trapping or eddying that characterizes the E×B drift in the background turbulence. It is shown that ion trapping provides an important physical mechanism for the complex nonlinear processes in drift turbulence evolution: generation of large-scale correlations and of zonal flow modes. © 2013 American Physical Society.

Budriga O.,National Institute for Laser, Plasma and Radiation Physics
European Physical Journal D | Year: 2012

We propose a closed three-level V-type system with an incoherent pumping which achieves a high index of refraction without absorption via spontaneously generated coherence. The system has two near-degenerate excited levels. We derive full analytical formulas for the index of refraction and the gain (absorption) coefficient. We found that the proper choice of the incoherent pumping, the relative phase of the two coherent fields, the Rabi frequencies, the probe detuning and the angle between the two transitions leads to the enhancement of the refractive index without absorption. © The Author(s) 2012.

Rotaru A.,National Institute for Laser, Plasma and Radiation Physics
Journal of Thermal Analysis and Calorimetry | Year: 2012

Solid fuels represent one of the most used sources of energy in many countries. In terms of ranking for the coal deposits, Romania occupies the 26th place in the world, and the 11th place in Europe, with reserves of 22 million tones of bituminous coal (BC) and 472 million tones of lignite. The National Bituminous Coal Company extracts the most significant amount of BC from the Jiu Valley area, a Subcharpatian basin in the Parâng Mountains. In the present article, the BC extracted from the Livezeni depth mine next to Petroşani city is investigated from the microstructural, thermal, and kinetic point of view, in comparison with a sample from Ural Mountains in Russia. Scanning electron microscopy, FTIR spectroscopy, and thermal analysis (TG/DSC/DTA in air and inert atmosphere) measurements were performed. The KAS isoconversional kinetic method was applied for the in-depth understanding of thermal decompositions and burning processes that occur. Even if the thermal behavior of the two samples is generally similar, the non-isothermal kinetic study revealed important differences in the pathways of the oxidative decomposition of volatiles and formation of coke. Also, the kinetics of coke burning depends only on the amount of fix carbon, regardless of the provenience of BC. © Akadémiai Kiadó, Budapest, Hungary 2011.

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