Photophysical processes stimulated in nanoporous silicon by high-power laser radiation

Photoprocesses initiated on the surface of porous silicon irradiated with laser radiation with wavelengths (λ = 266, 337, and 532 nm) in a wide range of intensities (up to 2 × 10⁷W/cm²) were investigated. Laser-induced luminescence and laser mass-spectrometry were used as experimental procedures. X-ray reflection was used to determine the parameters of the porous silicon films. The photoluminescence spectra obtained at different wavelengths and low intensities were analyzed. This analysis showed that for an optically thin layer of porous silicon the luminescence spectrum does not depend on the wavelength of the exciting radiation. This indicates the existence of a separate system of levels in porous silicon that are responsible for the luminescence. The behavior of the photoluminescence spectra as a function of the intensity q of the exciting radiation was investigated. It was shown that the luminescence intensity is a nonlinear function of q. At high intensities of the exciting radiation, the luminescence intensity saturates and a short-wavelength shift of the spectra is observed; this is due to the high concentrations of photoexcited carriers. This increases the probability of the experimentally observed nonequilibrium photodesorption of H₂ and Si from the surface of porous silicon.     

Phonon fluorescence of metal films and semiconductors stimulated by intense laser radiation

The kinetic equations have been obtained to describe both electron-impurities and electron-phonon interaction when the system is subjected to the intense electromagnetic radiation. Frequency distribution of the generated acoustic phonons is calculated for the case of the degenerate electron system.     

Decay of high-spin isomers in Os nuclei by barrier penetration

A decay scheme for the 130±20 ns high-spin isomer in ¹⁸²Os has been established. The excitation energy of the isomer is 7049±1 keV and it has I^π = 25⁽⁺⁾. A 2.4% decay directly to the yrast 24⁺ level at 5988 keV is observed. In ¹⁸⁴Os a 20±5 ns isomer is observed at 2366±1 keV excitation energy with I^π = 10⁺. Again, direct transitions into the yrast 8⁺ and 10⁺ levels are observed. Contrary to previous speculations, there is no compelling evidence for stable triaxial shapes in the structure of the levels through which the isomers decay. The abnormally short half-lives observed, as well as the unusual decay patterns, are best understood in terms of a γ-soft nuclear potential. Motion in the γ-direction allows the isomer to decay via barrier penetration from an axially symmetric prolate shape with the angular momentum along the nuclear symmetry axis (deformation aligned state) via oblate shapes to another prolate shape with the angular momentum perpendicular to the nuclear symmetry axis (rotation-aligned state).     

Second harmonic stimulated Raman scattering of laser radiation in aplasma

A high-power linearly polarized laser propagating through a plasma produces oscillatory electron velocity at the second harmonic due to-the ν&oarr;×B&oarr; force, This velocity couples a Langmuir wave (ω, k&oarr;) and an electromagnetic wave (ω₁, k&oarr;₁), where ω₁=ω-2ω₀ , k-2k&oarr;₀ and ω₀, k&oarr;₀ are frequency and wavenumber of the laser pump, causing second harmonic Raman scattering. The growth rate is maximum for side scattering. This process can occur above the quarter critical density, unlike the first harmonic stimulated Raman scattering which occurs below the quarter critical density     

Polarization characteristics of stimulated emission of organic molecules when excited by intense XeCl laser radiation

We study the influence of the molecular structure, medium, and exciting-radiation parameters on the polarization, energy, and spectral characteristics of the emission of laser-active media based on the dicarboxylic derivative of p-terphenyl, rhodamine 101, and phenalemine 512. We found that the polarization of the stimulated emission of molecules when strongly excited under the conditions studied significantly depends on their structure, the pumping mode, the intensity and polarization of the exciting radiation, the shape and dimensions of the excited volume, and the phase state of the medium.     

Triggering of nuclear isomers by X-ray laser

We studied the decay of nuclear isomeric states in the field of the X-ray laser. The laser pulses are described by the Gaussian wave packet of linearly polarized electromagnetic waves. At first stage the laser short pulse generates nuclear transition in the intermediate excited state, which afterwards decays into the final state with emission of γ quantum. Simple formulas are derived for the induced transition probability, which well correlate with known results, obtained previously for the incoherent X-ray radiation.     

Decay studies of K isomers in 254No

A detailed $ \gamma$ spectroscopic decay study of two K isomers in ²⁵⁴No was performed. In addition to the previously reported $ \gamma$ lines two new transitions of E = 778 , 856keV could be attributed to the decay pattern of ^254m1No ( T _1/2 = 275±7 ms). The population of an excited band built up on this isomer ( $\ensuremath K^{\pi} =8^{-}$ by the decay of ^254m2No ( T _1/2 = 198±13 μs) could be proven by measuring delayed $ \gamma$ - $ \gamma$ coincidences between transitions stemming from the decay of both isomeric states. The energies of the band members could be established up to $\ensuremath I^{\pi} = 15^{-}$ . A spontaneous fission branch of (2.0±1.2)×10^-4 was measured for ^254m1No , an upper limit of $ \le$ 1.2×10^-4 was estimated for ^254m2No . These values demonstrate the high stability of multi-quasiparticle configurations against spontaneous fission. Evidence for an $ \alpha$ decay branch of ^254m1No in the order of 1×10^-4 was found.     

Atom-molecule laser fed by stimulated three-body recombination

Using three-body recombination as the underlying process, we propose a method of coherently driving an atomic Bose-Einstein condensate (BEC) into a molecular BEC. Superradiantlike stimulation favors atom-to-molecule transitions when two atomic BECs collide at a resonant kinetic energy, the result being two molecular BEC clouds moving with well-defined velocities. Potential applications include the construction of a molecule laser.     

Decay of the 55 min and 12 min Te isomers

Gamma rays emitted in the decay of the ¹³³Te isomers, 55.4 min ^133mTe and 12.5 min ^133gTe, have been observed with Ge(Li) detectors. Sources were prepared by separating Sb from fission products, allowing several minutes for decay of 2.7 min ¹³³Sb and then separating the Te daughter activities. A total of 29 γ-rays between 312 and 2541 keV were attributed to decay of ^133gTe and 30 from 74 to 2050 keV to that of ^133mTe. Gamma-gamma coincidences in the decay of ^133gTe were observed with a NaI(Tl)---Ge(Li) set-up. The decay scheme of ^133gTe was constructed, involving placement of 25 of the γ-rays emitted. The decay of ^133gTe populates levels in ¹³³I at 0, 312, 720, 787, 1313, 1333, 1374, 1565, 1718, 2137, 2194 and 2541 keV. The decay of ^133mTe is very complicated, and we have not attempted construction of a complete decay scheme for it; however, levels at 913 and 1561 keV in ¹³³I are definitely populated in ^133mTe decay. The value for Q_β of ¹³³Te was measured as 3.52±0.10 MeV. The low-lying levels of the odd-mass iodine isotopes vary smoothly through the series ¹²⁵I through ¹³³I. The levels of ¹³³I are fitted rather well by predictions based on quasi-particle-phonon coupling.     

Effect of diffraction on stimulated Raman scattering of laser radiation in the middle atmosphere

The theoretical aspects of propagation of laser radiation along slanted paths in the atmosphere at high altitudes under conditions of stimulated Raman scattering (SRS) by molecular nitrogen are considered. The SRS power conversion coefficient, the transverse intensity distribution, and the effective size of laser beams with different initial spatial profiles are numerically simulated along the propagation path. The behavior of these parameters is studied for different ratios between the optical path length and the length of free diffraction of the initial laser beam. It is shown that, at a given level of the increment of the SRS amplification, the diffraction can both increase and decrease the efficiency of the Raman interaction of waves owing to the intensity redistribution in the transverse beam profile. This effect is the most pronounced for beams with non-Gaussian initial spatial intensity profiles.     

Thermal source of stimulated radiation

The action of a source of stimulated radiation is investigated under conditions when the energy is supplied from a thermal reservoir. For the three-level system a calculation based on thermodynamic considerations and kinetic equations is carried out. Relations expressing the dependence of the minimum temperature of the hot reservoir, which is necessary for inversion, on the parameters of the system and on the maximum intensity of exciting radiation are found.The expression for the efficiency of conversion of thermal energy into stimulated radiation is _max<(1–T ₀/T)(A ₃₂–A ₂₁)/(A ₃₂+A ₃₁).This efficiency is always lower than the Carnot efficiency and depends also on the value of the Einstein coefficients for spontaneous emission between different levels.The author heartily thanks M. Trlifaj, K. Pátek and A. Fousková for stimulating discussions.     

Cooling of gases by laser radiation

It is shown that a low-density gas can be cooled by illuminating it with intense, quasi-monochromatic light confined to the lower-frequency half of a resonance line's Doppler width. Translational kinetic energy can be transferred from the gas to the scattered light, until the atomic velocity is reduced by the ratio of the Doppler width to the natural line width.     

Stimulation of beta-decay by laser radiation

This paper deals with the study of new beta-decay acceleration mechanisms. Estimations have proved that there is a real possibility for stimulated beta-decay detection. The required laser intensity has been achieved by now. The mechanism under discussion may be characterized as the opposite process of K-electron capture. This paper makes an essential distinction between the above-mentioned mechanism and other beta-decay acceleration mechanisms. This revised version was published online in August 2006 with corrections to the Cover Date.     

Cooling of semiconductors by laser radiation

The optimum intensity of laser radiation and the lowest attainable temperature of the sample to be cooled are estimated on the basis of a model that takes into account radiative and nonradiative recombination of charge carriers, absorption of radiation due to band-to-band transitions and by free charge carriers, reabsorption of spontaneously emitted photons in the bulk of the sample, and saturation of the absorption coefficient.     

Particle acceleration by stimulated emission of radiation in cylindrical waveguide

In particle acceleration by stimulated emission of radiation(PASER), efficient interaction occurs when a train of micro-bunches has periodicity identical to the resonance frequency of the medium. Previous theoretical calculations based on the simplified model have only considered the energy exchange in the boundless condition.Under experimental conditions, however, the gas active medium must be guided by the metal waveguide. In this paper, we have developed a model of the energy exchange between a train of micro-bunches and a gas mixture active medium in a waveguide boundary for the first time, based on the theory of electromagnetic fields, and made detailed analysis and calculations with Math CAD. The results show that energy density can be optimized to a certain value to get the maximum energy exchange.     

Reaction of Cu and Cl2 stimulated by synchrotron radiation

Light-induced reactions of poly-Cu with Cl₂ and the formation of CuCl _x films were studied in the spectral range of 105–400 nm by using synchrotron radiation. The efficiencies of the reactions with Cl₂ pressures between 10^–5 and 10^–2 mbar were determined from the height distributions of spatial structures in the CuCl _x films. The heights typically range from 10 to 10⁴ nm. An efficiency of about 10⁷ CuCl _x molecules per dissociated Cl₂ molecule is observed at high Cl₂ exposures. At low Cl₂ pressures CuCl _x formes anisotropically in the irradiated area. The efficiency is determined by light-induced surface processes supporting the build up of CuCl _x in the long wavelength range and competing processes at short wavelengths which reduce the efficiency.     

KrF laser picosecond pulse source by stimulated scattering processes

A new short-pulse KrF laser source was developed by combining stimulated scattering processes, namely, Raman, Brillouin, and four-wave-mixing (FWM). Short pulses of 1.1 ps were obtained from 4 ns oscillator output. Only two commercial discharge KrF lasers are required for this system. This method is very simple and could be extended to an ArF laser wavelength in principle.