Application of snow nanograin targets for the generation of fast ions in femtosecond laser plasma

We have measured the energy of the directed motion of multiply charged ions produced when solid targets are exposed to low-contrast (10^?3–10^?2) femtosecond laser pulses with intensities 10¹⁵–10¹⁶ W cm^?2. The measurements are based on the recording of spatially resolved X-ray spectra for H-and He-like oxygen ions in the target plane. Analysis of the He_β and Ly_α line profiles has revealed fractions of accelerated ions in plasma with energies from several to several tens of kiloelectronvolts. We show that using a layer of frozen nanometer-size water droplets as the targets leads to an effective absorption of laser pulses and a twofold rise in the energy (to 0.1 MeV) of He-like oxygen ions compared to the use of solid targets.     

Field-induced ionization and Coulomb explosion of nitrogen

Femtosecond-laser field-induced ionization and Coulomb explosion of diatomic nitrogen were systematically investigated using time-of-flight mass and photoelectron spectrometry. Both linearly and circularly polarized femtosecond laser pulses were used at intensities varying from 5×10¹³ to 2×10¹⁵ W/cm². Strong N₂ ⁺, N₂ ²⁺, N⁺, N²⁺ and N³⁺ ion signals were observed for horizontally polarized pulses. Moreover, signals from the atomic ions exhibited a double-peak structure. Suppression of ionization was observed for circularly polarized pulses, while for vertically polarized pulses, only N₂ ⁺ and N₂ ²⁺ ions were observed. The angular distributions of the ions were measured under zero-field conditions in the ionization zone. The atomic ions N⁺, N²⁺ and N³⁺ exhibited highly anisotropic distributions, with maxima along the laser polarization vector and zeroes normal to the laser polarization vector. In contrast to the atomic ions, N₂ ⁺ exhibited a strong isotropic angular distribution. These observations indicate that dynamic alignment is responsible for the observed anisotropic angular distribution of the atomic ions. The kinetic energy spectrum of the photoelectrons is featureless and broad, extending above the ponderomotive potential of the laser pulse. The angular distribution is markedly anisotropic, with a maximum along the laser polarization vector. These observations further support the notion that the field-ionization mechanism is dominant under our experimental conditions. Received: 29 January 2002 / Revised version: 15 March 2002 / Published online: 12 July 2002     

On the role of prepulses during solid target heating by picosecond laser pulses

X-ray emission spectra of the plasma created at the surface of magnesium, aluminum, copper, and zinc targets heated by 1-ps laser pulses with a peak power density of up to 10¹⁶ W/cm² were measured. The effect of a picosecond prepulse on the spectra was studied for various power densities and intensity contrasts of the main laser pulse. It is established that the emission spectra of laser plasmas are weakly affected by a change from 10⁵ to 10⁷ in the main pulse contrast relative to the first prepulse. Variations in the parameters of emission from aluminum and magnesium plasmas were calculated using relative intensities and widths of the resonance lines of H-and He-like ions and their two-electron satellite peaks.     

Characterization of compact bright soft X-ray source based on picosecond laser plasma

Radiation emission of silicon and aluminum plasmas produced by 40-ps laser pulses with peak intensity above 10¹⁴ W/cm² was studied. High-resolution soft X-rayspectra of H-like and He-like ions were analyzed to determine plasma parameters. We compared the line shape of resonance transitions and their intensity ratios to corresponding dielectronic satellites and the intensities of the intercombination lines of He-like ions with the results of model calculations. Such comparisons gave average values of the electron number density N_e=(1-1.9)×10²¹ cm^-3 and the electron temperature T_e=460–560 eV for Si plasmas and about 560 eV for Al plasmas produced by the first and the second laser harmonics. The plasma size is about 100 μm. According to our estimations, more than 10¹² photons were produced within the resonance line spectral width and in the solid angle 2π during the total decay period. PACS 41.50.+h; 52.25.Os; 52.50.Jm     

Generation and characterization of a bright X-ray source using picosecond laser

High-resolution soft X-ray spectra of H-like and He-like ions were produced from laser irradiated silicon and aluminum targets. Plasma size was about 100 μm. X-ray spectra were analyzed to determine plasma parameters. We compared the line shape of resonance transitions and their intensity ratios to corresponding dielectronic satellites and the intensities of the inter combination lines of He-like ions, with the results of model calculations. Such comparison gave average values of the electron density N _e=(1?1.9)×10²¹ cm^?3 and the electron temperature T _e=460–560 eV for Si plasmas and about 560 eV for Al plasmas produced by the first and the second laser harmonics. According to our estimations, more than 10¹² photons were produced within the resonance line spectral width and in the solid angle 2π steredian during the total decay period.     

Spectral analysis of K-shell X-ray emission of magnesium plasma produced by ultrashort high-intensity laser pulse irradiation

Spectral analysis of K-shell X-ray emission of magnesium plasma, produced by laser pulses of 45 fs duration, focussed up to an intensity of ～10¹⁸ W cm^?2, is carried out. The plasma conditions prevalent during the emission of X-ray spectrum were identified by comparing the experimental spectra with the synthetic spectra generated using the spectroscopic code PrismSPECT. It is observed that He-like resonance line emission occurs from the plasma region having sub-critical density, whereas K-α emission arises from the bulk solid heated to a temperature of 10 eV by the impact of hot electrons. K-α line from Be-like ions was used to estimate the hot electron temperature. A power law fit to the electron temperature showed a scaling of I ^0.47 with laser intensity.     

Studies of the state of iron contained in α- and β-silicon nitride by Mössbauer and Raman effects

The Mössbauer spectra of β- and α-Si₃N₄ containing Fe have been measured at 300 and 93 K. The former consists of a singlet and a doublet, whereas the latter exhibits paramagnetic hyperfine patterns and a doublet. The single line and hyperfine patterns arise from isolated high-spin Fe³⁺ ions which are bonded equivalently with 4 nitrogen neighbors. Both doublets arise from high-spin Fe³⁺ ion-pairs each of which is strongly bonded by a kind of superexchange force. In β-Si₃N₄ the Fe³⁺ ions occupy the same sites regardless of whether they are doped into the crystal lattice before or after its preparation, while in α-Si₃N₄ the Fe³⁺ ions are located at different sites depending on the doping procedure. From the Raman spectra, the difference in the Mössbauer spectra for the two compounds is attributable to different relaxation by lattice phonons of the host crystals.The Mössbauer spectral lines for β-Si₃N₄ alone have been found to decrease in intensity with tune even at room temperature, such an ageing being observed to be entirely suppressed by γ-irradiation. From different behaviors of the three lines in the ageing and quenching from 1150°C down to room temperature, the ageing is interpretable based on a model that the Fe³⁺ ions jump from substitutional sites into interestitial sites and take place thermal vibration with a large amplitude there.     

Femtosecond ionization of magnesium clusters grown in ultracold helium droplets

Magnesium clusters grown in helium droplets and ionized with femtosecond laser pulses have been studied by high resolution mass spectrometry. For moderate laser intensities the abundance spectra show characteristic features indicating electronic shell effects. Compared to clusters of s¹-electron metals additional shell closures appear resulting from an electron rearrangement. Irradiation with higher laser intensities leads to a decomposition of the magnesium clusters into atomic ions. Due to charge exchange with the surrounding helium matrix mainly singly and doubly charged magnesium ions remain. In addition, the occurrence of MgHe_N ⁺-complexes is observed. Their abundance depends on the shape of the laser field, i.e. the laser width and the optical delay when applying the pump-probe technique. Received 2 January 2001     

Investigation of X-ray Emission from Plasmas Created by NIXE 6-nsec Nd: Glass Laser System

This paper reports details on the design of the Nd: glass laser facility NIXE at the Central Institute of Optics and Spectroscopy, including the target chamber and the x-ray diagnostics, spatial and temporal behaviour of the 1.06 μm radiation generated are given. Plane, massive targets of aluminium and silicon have been irradiated with 6-nsec FWHM laser pulses. Investigations of laser produced Al- and Si-plasmas have been performed with spectral and spatial high resolution diagnostics, consisting of a 3 channel crystal-spectrometer, 2 crystal-microscopes, and 2 pinhole cameras and the results are presented. Line spectra have been interpreted within the hypothesis of a steady state coronal equilibrium. He-like lines of the Al-plasma indicate temperatures up to 400 eV and densities up to 2 · 10²⁰ cm^?3. Total x-ray photon flux in the He-like resonance line has been calculated giving 10¹³ phot/ster.     

Observation of ions with energies above 100 keV produced by the interaction of a 60-fs laser pulse with clusters

The x-ray spectra of a plasma generated by heating CO₂ and Ar clusters with high-intensity femtosecond laser pulses with q _las≃10¹⁸ W/cm² are investigated. Spatially resolved x-ray spectra of a cluster plasma are obtained for the first time. Photoionization absorption is observed to influence the spectral line profiles. The recorded features of the x-ray emission spectra definitely indicate the existence of a large relative number of excited ions (≃10⁻²–10⁻³) with energies of 0.1–1 MeV in such a plasma. Possible mechanisms underlying the acceleration of ions to high energies are discussed. It is shown that the experimental results can be attributed to the influence of ponderomotive forces in standing waves generated by the reflection of laser radiation from the clusters. Zh. éksp. Teor. Fiz. 115, 2051–2066 (June 1999)     

A comparison of selected organic tracers for quantitative scalar imaging in the gas phase via laser-induced fluorescence

Single crystal of La₃Ga_5.5Ta_0.5O₁₄ (LGT) containing intentionally 0.5 % of Ho³⁺ and 1 % of Yb³⁺ was grown by the Czochralski method. Examination of chemical composition of the grown crystal revealed that luminescent holmium and ytterbium ions are preferably retained in the melt and their actual concentrations are 0.12 and 0.24 %, respectively. Spectroscopic investigation performed encompassed IR host absorption spectra and Raman spectra at room temperature, optical absorption and luminescence spectra of Ho³⁺ and Yb³⁺ at room temperature and at 5 K, and luminescence decay curves at room temperature. It was found that all spectral bands recorded show important inhomogeneous line broadening. This feature was attributed to structural disorder inherent to the crystal lattice in which pentavalent Ta⁵⁺ ions occupy octahedral Ga(1) sites together with trivalent Ga³⁺ ions. Despite small concentrations of luminescent ions, the occurrence of nonradiative interaction that feeds the ⁵I₆ and ⁵I₇ levels of Ho³⁺ ions by transfer of an excitation from the ²F_5/2 level of Yb³⁺ ions was evidenced. Based on examination of spectroscopic parameters evaluated, it was concluded that LGT:Ho, Yb may be considered as a potential intermediate-gain laser active material able to emit infrared radiation from the ⁵I₇ → ⁵I₈ transition of Ho³⁺ around 2 micrometres upon laser-diode pumping into Yb³⁺ absorption band.     

Study of ESR spectra of Ce<Superscript>3+</Superscript> ions in polycrystalline Sr<Subscript>2</Subscript>B<Subscript>5</Subscript>O<Subscript>9</Subscript>Br

ESR spectra of Ce³⁺ ions in polycrystalline Sr₂B₅O₉Br were studied, and the two crystallographic positions of the Ce³⁺ ion in this compound were identified on the basis of the data obtained. The ESR spectrum of Ce³⁺ ions with local charge compensation contains a broad line indicating the existence of several types of charge compensation. ESR spectra of Ce³⁺ ions in samples activated additionally by K⁺ ions are similar to those of the regular Ce³⁺ centers, which indicates that the effect of the univalent cation on Ce³⁺ is negligible.     

Electron spin resonance investigation of the substitution of Fe3^＋ for Ti4^＋ ions in rutile TiO2 single crystal

A Fe doped rutile TiO 2 single crystal is grown in an O 2 atmosphere by the floating zone technique.Electron spin resonance (ESR) spectra clearly demonstrate that Fe 3+ ions are substituted for the Ti 4+ ions in the rutile TiO 2 matrix.Magnetization measurements reveal that the Fe:TiO 2 crystal shows paramagnetic behaviour in a temperature range from 5 K to 350 K.The Fe 3+ ions possess weak magnetic anisotropy with an easy axis along the c axis.The annealed Fe:TiO 2 crystal shows spin-glass-like behaviours due to the aggregation of the ferromagnetic clusters.     

Characteristic features of the x-ray spectra of a plasma produced by heating CO2 clusters by intense femtosecond laser pulses with λ=0.8 and 0.4 μm

The x-ray spectra of a plasma produced by heating CO₂ clusters with intense femtosecond laser pulses with λ=0.8 μm and λ=0.4 μm are investigated. Spatially resolved x-ray spectra of the cluster plasma are obtained. The observed characteristic features of the x-ray emission spectra show unequivocally that such a plasma contains quite a large relative number of ions (≃10⁻²–10⁻³) with energies of 0.1–1 MeV. The contour of the OVIII Ly_α line is found to have characteristic features that are especially conspicuous when the clusters are heated with second-harmonic pulses. These features cannot be explained by any mechanisms known to the authors. Pis’ma Zh. éksp. Teor. Fiz. 68, No. 6, 454–459 (25 September 1998)     

Temperature investigations of Raman spectra of stoichiometric and congruent lithium niobate crystals

In the temperature range 100–450 K, we have investigated Raman spectra of congruent and stoichiometric LiNbO₃ crystals. We have found that, in this temperature range, frequencies and widths of all the spectral lines depend linearly on temperature. However, the width of the line that corresponds to vibrations of the A₁(TO) symmetry of Li⁺ ions depends on temperature much more weakly than the width of the line that corresponds to vibrations of the A₁(TO) symmetry of Nb⁵⁺ ions. This fact indicates that the anharmonicity of vibrations of Nb⁵⁺ ions along the polar axis is much stronger compared to vibrations of Li⁺ ions. It is likely that this anharmonicity is noticeably contributed by O^2? ions, which are characterized by an anharmonic potential, vibrations of which, according to calculations from first principles, are mixed with vibrations of Nb⁵⁺ ions. The anharmonicity of vibrations of O^2? ions is evidenced by a strong temperature dependence of the width of the line that corresponds to vibrations of the A₁(TO) symmetry of O^2? ions perpendicularly to the polar axis. We have found that the temperature dependence of the intensity of lines that correspond to fundamental vibrations is nonmonotonic. At the same time, the temperature dependence of the intensity of “superfluous lines” is strictly linear. It is likely that this behavior of the intensities of lines of fundamental vibrations is related to the occurrence of clusters and microstructures in the crystal structure.     

Study of carbon material surface layers modified by nitrogen and argon ion irradiation

We present the results of the study of the elemental composition and defects of the electronic structure of the surface layer modified by high-dose irradiation (10¹⁸–10¹⁹ ion/cm²) of highly oriented pyrolytic graphite (UPV-1T) by 30-keV N ₂ ⁺ and Ar⁺ ions in the temperature range from 180 to 400°C. The EPR spectra observed during irradiation with argon ions at high temperatures and with nitrogen ions at temperatures near the liquid-nitrogen temperature T = 77 K exhibit anomalously narrow lines which probably result from the exchange interaction inside paramagnetic clusters of displaced carbon atoms. During nitrogen ion irradiation at room and higher temperatures, paramagnetic defects typical of many carbon materials (single EPR lines with g = 2.0027–2.0029) and belonging to carbon atoms bound to one or three nitrogen atoms were detected.     

ESR experiments with egg shells of chickens

The electron spin resonance spectra fromGallus domesticus egg shells exhibit a strong free radical singlet, a spectrum due to Mn²⁺ ions in a calcite site, a low field line line atg≈30/7 due to Mn²⁺ in a rhombic site, and spectral lines arising from Fe³⁺ ions. Details of the ESR spectral lines did not correlate with fertile, non-fertile or color type egg-shell characteristics.     

Photoluminescence properties of rare earth doped α-Si3N4

The photoluminescence properties of Eu²⁺, Ce³⁺ and Tb³⁺ doped α-Si₃N₄ have been studied and a possible structural model has been proposed on the basis of the Rietveld refinement of X-ray powder diffraction data. Nearly single phase rare earth doped α-Si₃N₄ was synthesized by a solid state reaction at 1600 °C in N₂-H₂ atmosphere starting from amorphous Si₃N₄ and rare earth oxides or nitrides. Because of small crystal field splitting of the 5d levels, the excitation and emission bands of Eu²⁺ and Ce³⁺ are positioned at higher energies as isolated ions in comparison with that in Ca-α-Sialon. Both Eu²⁺- and Ce³⁺-doped α-Si₃N₄ show blue band emission peaking at about 470 and 450 nm, respectively, under UV excitation. α-Si₃N₄:Tb³⁺ exhibits dominant green line emission mainly arising from ⁵D₄→⁷F_J (J=6-3) with weak ⁵D₃→⁷F_J (J=6-3) transitions of Tb³⁺ when excited by UV light. The thermal stability of α-Si₃N₄:Eu²⁺ is comparable with that of Ca-α-Sialon:Eu²⁺ and is much better than that of α-Si₃N₄:Ce³⁺.     

Optical and EPR study of BaY2F8 single crystals doped with Yb

Optical and electron paramagnetic resonance study have been carried out on BaY₂F₈ single crystals doped with Yb ions at 0.5 and 10 mol%. The crystals have been obtained using the Czochralski method modified for fluoride crystal growth. Optical transmission measurements in the range of 190-3200 nm and photoluminescence measurements were carried out at room temperature. Absorption spectra of BaY₂F₈ single crystals doped with Yb due to the ²F_7/2→²F_5/2 transitions have been observed in the 930-980 nm range. To analyze the possible presence of Yb²⁺ ions in the investigated crystals, irradiation with γ-quanta with a dose of 10⁵ Gy have been performed. The observed photoluminescence bands show usual emission in IR and other one in VIS, being an effect of cooperative emission of Yb³⁺ ions and energy up-conversion transitions of photons from IR to UV-vis(visible) due to hoping process between energy levels of paired Yb³⁺ and Er³⁺, where Er³⁺ ions are unintentional dopants. The EPR spectra of BaY₂F₈:Yb 10 mol% consist of many overlapping lines. They have been analyzed in terms of spin monomers, pairs, and clusters. The angular dependence of the resonance lines positions have been studied also to find the location of coupled ytterbium ions in the crystal structure.     

Enhanced photoluminescence of Sm/Bi co-doped Gd2O3 phosphors by combustion synthesis

Gd₂O₃:Sm³⁺ and Gd₂O₃:Sm³⁺,Bi³⁺ powders were prepared by a combustion method. Their structures were determined using X-ray diffraction. UV-visible absorption and photoluminescence spectra were investigated for Gd₂O₃:Sm³⁺ and Gd₂O₃:Sm³⁺,Bi³⁺ at different annealing temperatures and different doping concentrations. The emission spectra of all samples presented the characteristic emission narrow lines arising from the ⁴G_5/2→⁶H_J transitions (J=5/2, 7/2, and 9/2) of Sm³⁺ ions upon excitation with UV irradiation. The emission intensity of Sm³⁺ ions was largely enhanced with introducing Bi³⁺ ions into Gd₂O₃:Sm³⁺ and the maximum occurred at a Bi³⁺ concentration of 0.5 mol%. The relevant mechanisms were discussed with the sensitization theory by Dexter and the aggregation behavior of Bi³⁺ ions.