[en] The results of band structure calculation of the Gd₂Ni₂Al₁₅ intermetallic compound by the augmented plane wave method are presented. It was found that the positions of nickel 3d bands and gadolinium 4f bands are the same which explains the magnetic moment value of the nickel atom in this compound about 1.6 μ/sub B/

[en] A quantum mechanical model has been developed for the transport of very light and small gas molecules through polymers. The experimental results on isotope effects in diffusion, permeation, and solvation of H₂ and D₂ in polymer membranes made of polyethylene terephthalate, polyethylene, and polyvinylchloride can be interpreted by the model. Approximate information about the geometrical dimensions of the normal and activated energy state of polymers can be gained. (author)

[en] The multiple-photon absorption (MPA) of CO₂ laser radiation in gaseous Si₂F₆ was measured. The number of photons absorbed per molecule as a function of laser frequency i.e. the MPA spectra are reported in the CO₂ laser 10 μm R branch for three different laser fluences. The spectral structures observed, as well as other absorption characteristics are discussed in connection with the isotopically selective multiple-photon dissociation process. (author)

[en] The effective Hamiltonian formalism has been used in the development of a model for infrared multiple-photon absorption (IRMPA) process in symmetric top molecules. A computer program has been written and used for the calculation of the number of photons absorbed by Si₂F₆ as a function of the exciting laser frequency and for different laser fluences. The theoretical results are compared with the experimental ones and discussed in an attempt to explain the origin of the structures observed in the Si₂F₆ IRMPA spectra. (author)

[en] The laser frequency dependence of the enrichment factors in the separation of silicon isotopes by infrared multi-photon dissociation (MPD) were calculated. Based on the knowledge of the isotopic shifts, a simple model was developed to estimate the dependence of the MPD yield on the laser frequency. The results agree well with the experimental results reported elsewhere. This new method allows us to determine the relative contribution to the MPD yield of the 6 Si₂F₆ isotopic species and to explain the behavior of the enrichment factors. (author)

[en] Generation of high-order harmonics in Xe was studied in a static cell at high pump energies, 5 m focal length, and up to 14 cm interaction lengths. Self-guided propagation of the pulse was observed experimentally and confirmed by a three-dimensional model. Phase-matched generation was demonstrated in the self-guided beam, and the high energy and low divergence of the harmonic radiation were explained. Harmonic field calculations, in good agreement with experimental results, allow for the explanation of the higher-order harmonic generation dynamics in the self-guided region

[en] We investigate numerically the generation of water window x-rays under phase-matched conditions at very low ionization level. We show that at high-gas pressure and short-wavelength infrared driving pulse, phase matching condition is obtained only if one accounts for the effects ionization has on both driving and harmonic fields. Recent experimental results obtained in these conditions by Takahashi et al.[Phys. Rev. Lett. 101, 253901 (2008)] are also discussed.

[en] For the analysis of high-order harmonics generated in a long gas jet by intense chirped femtosecond laser pulses, calculations of the laser propagation and harmonic generation were performed in terms of a nonadiabatic three-dimensional model. The self-guided propagation, observed at the conditions of bright harmonic generation, was confirmed by the model calculations. When using negatively chirped pulses, the calculated distribution of the harmonic field is spatially and spectrally confined, being generated on axis with a narrow spectral profile. The positively chirped pulses generate broad spectral distribution on axis, and narrow off axis, but in the latter case with a large emission angle. The estimation of harmonic beam divergence agreed well with experimentally measured data, showing the lowest divergence at the conditions of the brightest harmonic generation. Spectral, temporal, and spatial modifications of the propagated laser pulse are found to influence decisively the single-atom response and ultimately the harmonic field, providing a coherent picture of harmonic generation

[en] The effective Hamiltonian formalism has been used to develop a model for infrared multiple-photon absorption (IRMPA) process in asymmetric top molecules. Assuming a collisionless regime, the interaction between the molecule and laser field can be described by the time-dependent Schroedinger equation. By using the rotating wave approximation and Laplace transformation, the time-dependent problem reduces to a time-independent eigen problem for an effective Hamiltonian which can be solved only numerically for a real vibrational-rotational structure of polyatomic molecule. The vibrational-rotational structure is assumed to be an anharmonic oscillator coupled to an asymmetric rigid rotor. The main assumptions taken into account for this model are the following: (1) the excitation is coherent, i.e. the collision (if present during the laser pulse) does not influence the excitation; (2) the excitation starts from the ground state and is near resonant to a normal mode, thus, the rotating wave approximation can be applied; (3) after absorbing N photons the vibrational energy of the excited mode leak into a quasicontinuum; (4) the thermal population of the ground state is given by the Maxwell-Boltzmann distribution law. The energy levels of the asymmetric top molecules cannot be represented by an explicit formula analogous to that for the symmetric top, according to quantum mechanics, but we can consider it a deviation from the prolate or oblate case of the symmetric top, and we can find in the same manner the selection rules of the asymmetric case using the selection rules for the symmetric case. The infrared bands of asymmetric top molecules are not resolved, but if the dispersion used is not too small, so that the envelopes of the bands can be distinguished from simple maxima, it is possible to draw conclusions as to the type of the bands. In this case, the simulation of the absorption spectra can give us some important information about the types of these bands. In particular, for CF₂HCl (with the asymmetric constant τ = -0.58) we present the basic algorithm used for selecting the vibration-rotation states involved in the excitation process. A computer program has been written and used for the calculation of the number of photons absorbed by the CF₂HCl as a function of the temperature, exciting laser frequency and laser fluences. (authors)

[en] The nuclear spin lattice relaxation time is calculated for ^235U situated at the center of an octahedral ^235UF₆ molecule. Vibrational distortion, collisional deformation, and electric fields induced by neighbouring hexadecapole moments are considered as possible mechanisms that could account for the observed ^235U quadrupolar relaxation rate. The efficiency of these mechanisms in causing relaxation is discussed by comparing the calculated value with the experimentally determined one. (Author) 20 refs., 2 tabs