Ionization spectrum transformation and compression of powerful femtosecond laser pulses in experiments on the propagation in gas-filled dielectric capillaries

The propagation of an intense (I≤10⁶ W/cm²) femtosecond laser radiation with a duration of ～100 fs through gas-filled dielectric capillaries was studied. The radiation with a power up to 0.2 TW propagates along the paths up to 20 cm with a transmission efficiency of ～45%. The beam transverse structure at the output is close to the capillary fundamental mode under gas-ionization conditions. The transformation of pulse spectrum was studied as a function of input intensity. It is demonstrated experimentally that the pulse is compressed to a duration of ～30 fs due to the compensation of ionization-induced self-phase modulation in a linear dispersive element at the capillary output.     

Results of Experiments on Modification of the Sporadic E-Layer by Powerful HF Radio Waves

We present the results of experiments on modification of the sporadic E-layer of the Earth's ionosphere by a powerful vertical beam of HF radio waves. The experiments were performed at the Sura heating facility (Radiophysical Research Institute, Nizhny Novgorod) from May 31 to June 4, 2001. The main results were obtained in May 31 from 17:30 to 20:00 MSK, when the E_s layer was fairly stable and the pump frequency was in the range where this layer was transparent. The diagnostics of the ionosphere was performed using X-mode probe waves in the frequency range f_pr=4.3-7.8 MHz. The ionosphere plasma heating was accompanied by an increase in the intensity of probe waves, reflected from the E_s layer, with frequencies close to or slightly higher than the critical frequency of the layer. Artificial fluctuations of the reflected signal appeared in the entire frequency range of sounding. We consider the dynamical characteristics of the observed phenomena and their dependence on the frequency f_pr and the parameters of the E_s layer.     

Influence of small-scale irregularities on the features of the overshoot effect in the temporal evolution of stimulated electromagnetic emission. Part I. Development stage

We present the results of experimental studies of the evolution of stimulated electromagnetic emission of the ionosphere (SEE) under the F-layer modification by powerful HF radio waves in a broad range of the pump wave frequencies. We compare the parameters of the overshoot effect in SEE evolution to the observations of anomalous attenuation and field-aligned scattering of radio waves. We show the overshoot effect to result from the anomalous attenuation under scattering at artificial small-scale ionospheric irregularities. We found the characteristic time scales of the overshoot effect development to decrease and its value to increase as the pump wave frequency decreases from 6 to 4 MHz; this is attributed to the observed increase of the irregularities amplitude at scales m. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 42, No. 7, pp. 619–634, July 1999.     

Heating of electrons in superconductor in the resistive state due to electromagnetic radiation

The effect of heating electrons with respect to phonons in a thin superconducting film driven into the resistive state by the current and the external magnetic field has been observed and investigated. This effect caused by the electromagnetic radiation is manifested in the increased resistance of the film and is not selective over the frequency range from 10¹⁰ to 10¹⁵ Hz. That the effect is frequency independent under the conditions of strong electron scattering caused by static defects is explained by the decisive role of electron -electron collisions in forming the distribution function. The characteristic time of resistance change, obtained experimentally, corresponds to the relaxation time of the order parameter $\text{τ}{}_{\mathrm{\Delta }}$ near the superconducting transition and to the relaxation time of the nonelastic electron-phonon interaction $\text{τ}{}_{\mathrm{<mtext>eph</mtext>}}$ at lower temperatures and in lower magnetic fields.     

Hyperfine interactions of Rn and Ra implanted into Fe,Al, Cd and Bi metals

The interactions on radon and radium nuclei implanted into metals have been studied by means of α-γ angular correlations. The influence of source preparation and of surface oxide layers have been studied in sources prepared by evaporation and isotope separator implantation. The interactions in iron are found to have contributions from static magnetic fields as well as static quadrupole interactions. The magnetic fields for radon and radium in iron were determined to be +920±85 kOe and −380±35 kOe, respectively. The theoretical models used to describe the perturbations due to mixed interactions and their importance in internal field determinations are discussed.     

Possibilities for controlling attosecond x-ray pulse generation during molecular ionization by femtosecond laser radiation

We discuss the role of different factors (molecular sizes and configuration, orientation of the molecular axis with respect to the electric field of a laser pulse, the type of the molecular orbital, etc.), which characterize molecules and their state, in the formation of the nonlinear response of a molecule ionized by a high-power femtosecond laser pulse. In numerical experiments within the framework of a two-dimensional model for the H ₂ ⁺ molecular ion, we study possibilities for controlling the process of nonlinear frequency conversion of femtosecond optical radiation into X-ray radiation of attosecond duration by means of preliminary vibrational or electronic excitation of molecules. We demonstrate the possibilities of using the attosecond pulse generation as a diagnostic tool for probing vibration-rotational dynamics of molecules.