Parametric excitation of spin waves in ferromagnets by longitudinal pumping localized in space

The equations of motion for the slowly varying complex amplitudes of spin waves parametrically excited by a localized pumping magnetic field have been derived. A solution of these equations satisfying given boundary and initial conditions has been obtained. The energy dissipated by spin waves decreases with the pumping intensity beyond a certain pumping power, which can be termed the regeneration threshold. The losses vanish and change sign at the instability threshold. Both thresholds depend heavily on the linear dimension L of the pumping zone, increasing with decreasing L. Owing to the regeneration process, the dissipation length of spin waves increases without bound as the pumping power approaches the instability threshold. Consequently, perturbations of a uniform state due to the boundary penetrate throughout the pumping zone, regardless of the dimension L. As a result, the full pattern of parametric instability is strongly affected by the zone boundary: 1) the spatial distribution of wave amplitudes becomes nonuniform everywhere inside the zone; 2) the amplitude growth rate in the unstable regime decreases at all points when perturbations due to the boundary reach these points; 3) the instability threshold is independent of the spin-wave frequency offset from the parametric resonance frequency. The calculated minimum instability threshold as a function of the bias magnetic field (the “butterfly” curve) changes shape with L, in agreement with the available experimental data. Zh. éksp. Teor. Fiz. 111, 199–219 (January 1997)     

Dynamic registration of D2 16O absorption spectrum in silica aerogel

Absorption spectra of the gas phase and adsorbed D₂О in the silica aerogel with nanoscale pores were investigated in 3700–5400 cm^?1 range using dynamic registration with Fourier Transform spectrometer IFS-125M. Two types of sample with pores of 60 nm wide – the nitrogen gas-treated and untreated aerogels – were examined. The surface treatment of the sample changes noticeably the broadband absorption of adsorbed water. Spectrum of D₂O in the pores differs from the spectrum of bulk water as for bandwidth so for band maximum. It was found that treatment of the pores by dry nitrogen leads to increasing hydrophilic properties of the material and to change water band contour. The D₂О line widths in both the aerogels exceed those of free monomer in 1.1–3 times at the same pressure. Calculations of self-broadening coefficients of the D₂O lines were performed using semi-empirical method based on the impact theory of broadening and includes the correction factors. The calculated results well agree with experimental data. Greater differences were found for the shift of the line centre. The D₂O line shifts in the treated pores significantly exceed line shifts in the untreated pores. For some lines, these shifts have the opposite sign indicating complex nature of the molecule–wall interaction.     

Anomalous Broadening of CF4 Molecule Lines. Observation of Carbon Tetrafluoride Hydrates?

Spectroscopic studies of pure carbon tetrafluoride and carbon tetrafluoride in the presence of water vapor have been carried out. Studies have revealed changes in the absorption spectrum of the 1280 cm^‒1 band of CF₄, indicating the formation of new molecules, CF₄–H₂O hydrates. The bond between CF₄ and H₂O is not chemical in nature. The formation of these molecules can accelerate the removal of carbon tetrafluoride from the atmosphere with precipitation in the form of rain or snow.

     

Study of the Vibrational Spectrum of Hydrogen Molecules in an Aerogel

The pressure dependence of the absorption spectrum of hydrogen molecules inside pores of an aerogel has been found. Two components with linear and quadratic pressure dependences have been revealed. It has been assumed that the first component can be attributed to the absorption of an individual molecule near the surface of pores, whereas the second component is due to intermolecular interaction. A method has been proposed to estimate in the first approximation the size of pores from the absorption spectrum.

     

Luminescence of metal surface due to nonequilibrium excitation of electrons by laser radiation

Laser-induced nonequilibrium electromagnetic radiation (NEMR) from the surface of metal target has been observed. NEMR has a high temperature and can't be explained by means of usual heating of surface by laser radiation. This phenomenon is connected with the “hot” electron emission and can be described in terms of physical kinetics.