نشریه پژوهش سیستم های بس ذره ای
سال چهارم شماره 1 (پیاپی 7، تابستان 1393)

Three (or many) -level atoms interacting with two (or more) classical electromagnetic fields are often studied in the steady state and weak field probe approximation. This approximation is generally not suitable for quantized field، where all modes of electromagnetic fields contain a few photons or especially where the coupling mode is in vacuum. In this article، upper-coupling and lower-coupling، master equations of a three-level cascade atom، interacting with a couple of quantized electromagnetic field are exactly solved in the steady state and zero detuning coupling field. The atomic population of levels and its absorption and dispersion are investigated. A comparison between the exact and the corresponding weak field approximation methods is presented.

In this paper، by using the Hubbard local field correction and also the form factor، we study the short-range interaction and thickness effects on two-dimensional electron gas system and its effect on dielectric function. It is well known that، at low electron densities، it is necessary to include short-range effects through Hubbard approximation. We show that while the dielectric function within RPA and Hubbard approximation behaves qualitatively similarly، the effect of different approximations and also thickness is considerable.

In this research، Ni0. 8Fe0. 2 alloy powder is fabricated by mechanical alloying method using a planetary mill. The crystal structure and morphology of the powders are studied by X-ray diffraction (XRD) and scanning electron microscope (SEM)، respectively. Also، the effect of cobalt doping on structural properties of Ni0. 8Fe0. 2 alloy with the chemical component of Ni0. 8-xCoxFe0. 2 with x=0، 0. 2، 0. 4، 0. 6 and 0. 8 has been studied. By increasing the amount of Co doping، the diffraction peaks of Ni0. 8Fe0. 2 alloy changed gradually and the peaks of iron-cobalt alloy and finally Co0. 8Fe0. 2 appeared. In other words، by increasing the amount of cobalt doping، because of the ionic radius of Co being smaller than the Ni، the peaks shift to bigger angles. Therefore، the crystal structure of cobalt solid liquid (hcp) appears.

Among the anti-particles of cosmic rays، anti-protons are of particular importance. In cosmic ray spectrum، the major part of anti-protons is outcome of collision of cosmic rays with interstellar medium gas. Considering the non-homogenous or fractal interstellar medium، proposed by Lagutinet. al.، excess flux of secondary anti-protons is calculated. This excess flux is the probable component of extra galactic anti-protons. In addition، the ratio of extra galactic and galactic anti protons to cosmic rays protons flux is calculated as a reason for matter and anti-matter asymmetry and also energy density of extra galactic anti protons.

One of the difficulties in excitation channel with first order approximation and statements related to electronic and nuclear interactions at intermediate and high impact energies range is that considering a plane-wave as the projectile wave function case، the nuclear term portion disappears in calculation of differential and total cross section due to ourtogonality of initial and final wave functions related to bonding subsystem. To solve this problem we have used coulombic wave function as a distorted wave in calculations and investigated dynamics of three-body impact with this wave function. This study shows that momentum transfer from projectile to target is not simple as carried out with plane-wave، and weight function redistributes momentum transfer and affects the differential and total cross sections. Because there are no experimental data reported in the literature for discussing energy range our results have been compared with the available theoretical data.

In this study، the efficiency of a self-doubler NYAB laser، which is simultaneously an active and a nonlinear crystal، in a double-pass cavity is investigated. To this end، first، a model based on the coupling of five differential equations describing the interaction of pump beam of 807 nm، laser beam of 1060 nm، and second harmonic beam of 530 nm is presented. In this model، the conversion of pump beam to laser beam through the fluorescence process is described by the laser rate-equations. The generation of second harmonic beam is performed through the nonlinear processes. The model consists the linear and nonlinear interactions of light as well as the optical absorption. The results give the laser and second harmonic generation efficiencies against the pump powers.