Initiation of nuclear reactions under laser irradiation of metal nanoparticles in the presence of thorium aqua ions

Laser irradiation of tungsten and gold nanoparticles in aqueous solutions of Th(NO₃)₄ was experimentally studied. Picosecond Nd:YAG lasers with a wavelength of 1.06 μm and a peak power from 10¹¹ to 10¹³ W cm⁻² were used. The composition of colloidal solutions before and after laser irradiation was analyzed using atomic absorption and gamma spectrometry. It was found that laser irradiation initiates nuclear reactions involving thorium nuclei, occurring via two different channels. Radioactive decay of thorium nuclei within its radioactive series is enhanced under laser irradiation in D₂O; one of the fission fragments is ¹³⁷Cs. Possible mechanisms of the process are discussed.     

Near IR laser light visualizators using nonlinear GaSe and other layered crystallites

Two methods of preparation of the devices for visualization of pulsed and continuous near-IR (near infrared) are described and the results of conversion of pulsed and continuous IR (800–1360 nm) laser radiation into the visible range of spectra (400–680 nm) by using a transparent substrate covered with the particles (including nanoparticles) of effective nonlinear materials of GaSe _x S_{1 − x} (0.2 ≤ x ≤ 0.8) are presented. Converted light can be detected in transmission or reflection geometry as a visible spot corresponding to the real size of the incident laser beam. Developed device structures can be used for checking if the laser is working or not, for optical adjustment, for visualization of distribution of laser radiation over the cross of the beam and for investigation of the content of the laser radiation. Low energy (power density) limit for visualization of the IR laser pulses with 2–3 ps duration for these device structures are: between 4.6–2.1 μJ (3 × 10⁻⁴−1 × 10⁻⁴ W/cm²) at 1200 nm; between 8.4–2.6 μJ (4.7 × 10⁻⁴−1.5 × 10⁻⁴ W/cm²) at 1300 nm; between 14.4–8.1 μJ (8.2 × 10⁻⁴–4.6 × 10⁻⁴ W/cm²) at 1360 nm. Threshold damage density is more than 10 MW/cm² at λ = 1060 nm, pulse duration τ = 35 ps. The results are compared with commercially existing laser light visualizators.     

Picosecond laser ablation of SiO2 layers on silicon substrates

In this work, we report on laser ablation of thermally grown SiO₂ layers from silicon wafer substrates, employing an 8–9 ps laser, at 1064 (IR), 532 (VIS) and 355 nm (UV) wavelengths. High-intensity short-pulse laser radiation allows direct absorption in materials with bandgaps higher than the photon energy. However, our experiments show that in the intensity range of our laser pulses (peak intensities of <2×10¹² W/cm²) the removal of the SiO₂ layer from silicon wafers does not occur by direct absorption in the SiO₂ layer. Instead, we find that the layer is removed by a “lift off” mechanism, actuated by the melting and vaporisation of the absorbing silicon substrate. Furthermore, we find that exceeding the Si melting threshold is not sufficient to remove the SiO₂ layer. A second threshold exists for breaking of the layer caused by sufficient vapour pressure. For SiO₂ layer ablation, we determine layer thickness dependent minimum fluences of 0.7–1.2 J/cm² for IR, 0.1–0.35 J/cm² for VIS and 0.2–0.4 J/cm² for UV wavelength. After correcting the fluences by the reflected laser power, we show that, in contrast to the melting threshold, the threshold for breaking the layer depends on the SiO₂ thickness.     

Nonlinear optical properties of nanometer-size silver composite azobenzene containing polydiacetylene film

Diacetylene monomer containing p-nitrophenyl azobenzene moiety (NADA) was synthesized. Silver nanoparticles with different concentrations were adulterated in the above polymerized NADA (PNADA) films and the third-order nonlinear optical properties were investigated in detail. UV–vis spectra and transmission electron microscopy were used to confirm the formation of PNADA/Ag nanocomposite films. The silver nanoparticles (average size of 10 nm) were well dispersed in the polymer films. The value of the nonlinear refractive index n ₂ for PNADA films (8.48×10⁻¹⁵ cm²/W) was much higher than that of pure polydiacetylene films. Further, the introduction of silver nanoparticles into the PNADA polymer films led to the further enhancement of nonlinear optical properties. The maximum value of n ₂ for PNADA/Ag nanocomposite films could be 11.6×10⁻¹⁵ cm²/W. This enhancement should be ascribed to the surface plasmon resonance of silver nanoparticles.     

Nonlinear transformation of a spectrum of femtosecond laser pulses in a gas-filled microcapillary

Features of light pulse propagation and nonlinear optical transformation of the spectrum generated by titanium-sapphire laser pulses (τ_0.5 = 27 fs, λ₀ = 790 nm) have been studied experimentally in a 50-cm cylindrical hollow waveguide (microcapillary with 280-μm diameter core) filled with gaseous molecular nitrogen and helium. Stable guided propagation of light pulses with an intensity of ~1.5⋅10¹⁴ W/cm² in the fundamental EH₁₁ mode of the gas-filled capillary has been demonstrated. Exact focusing of the laser light made it possible to obtain rather high relative (≥95%) and absolute (~60%) energy transmission efficiencies for the pulses at gas pressures equal to or lower than 760 Torr. A method to determine the nonlinear phase shift of the pulses has been proposed. Values of the nonlinear refractive index n₂ ≈ 4.5⋅10^–23 cm²/(W⋅Torr) (N₂) and n₂ ≈ 2.8⋅10^–23 cm²/(W⋅Torr) (He) have been found. A short-wavelength shift in addition to the Kerr nonlinearity has been shown to be contributed by the generated electron plasma at high pulse intensities (≥10¹⁴ W/cm²).     

Measurements of the temperature and water vapor concentration in a hot zone by tunable diode laser absorption spectrometry

A tunable diode laser absorption spectroscopy (TDLAS) technique and appropriate instrumentation was developed for the measurement of temperature and water vapor concentrations in heated gases. The technique is based on the detection of the spectra of H₂O absorption lines with different energies of low levels. The following absorption lines of H₂O were used: 7189.344 cm⁻¹ (E″=142 cm⁻¹), 7189.541 cm⁻¹ (E″=1255 cm⁻¹), 7189.715 cm⁻¹ (E″=2005 cm⁻¹). Spectra were recorded using fast frequency scanning of a single distributed feedback (DFB) laser. A unique differential scheme for the recording of the absorption spectra was developed. An optimal technique for fitting the experimental spectra was developed.     

Femtosecond ionization and fragmentation of free silicon nanoparticles

Femtosecond-laser spectroscopy is used to study the photoionization and photofragmentation of large neutral silicon clusters in a beam. Silicon clusters Si_n with sizes up to n≈6000, corresponding to nanoparticles with diameters up to 6 nm, are generated in a laser vaporization source. Nanosecond- and femtosecond-laser ionization are employed to characterize the free silicon nanoparticles. Excitation with intense femtosecond-laser pulses leads to prompt formation of doubly and triply charged Si_n clusters. Additionally, strong fragmentation of charged clusters occurs by Coulomb explosion, resulting in high released kinetic energies. Multiply charged atoms up to Si⁴⁺ with initial kinetic energies in the range of 500 eV are observed for laser intensities of about 10¹³ W/cm². Pump–probe spectroscopy yields decay times of the intermediate resonances of the nanoparticles. Received: 22 January 2000 / Published online: 7 August 2000     

Laser induced breakdown spectroscopy of germane plasma induced by IR CO2 pulsed laser

Laser-induced breakdown spectroscopy (LIBS) in germane (GeH₄), initially at room temperature and pressures ranging from 2 to 10 kPa, was studied using a high-power transverse excitation atmospheric (TEA) CO₂ laser (λ=10.653 μm, τ _FWHM=64 ns and power densities ranging from 0.28 to 5.52 GW cm⁻²). The strong emission spectrum of the generated plasma is mainly due to electronic relaxation of excited Ge, H and ionic fragments Ge⁺, Ge²⁺ and Ge³⁺. The weak emission is due to molecular bands of H₂. Excitation temperatures of 8100±300 K and 23,500±2500 K were estimated by Ge atomic and Ge⁺ singly ionized lines, respectively. Electron number densities of the order of (0.7–6.2)×10¹⁷ cm⁻³ were deduced from the Stark broadening of several atomic Ge lines. The characteristics of the spectral emission intensities from different species have been investigated as functions of the germane pressure and laser irradiance. Optical breakdown threshold intensities in germane at 10.653 μm have been determined. The mechanism of initiation of the laser-induced plasma in germane has been analyzed.     

Line strength and collisional broadening studies of hydrogen sulphide in the 1.58 μm region using diode laser spectroscopy

High resolution diode laser spectroscopy has been applied to the detection of hydrogen sulphide at ppm levels utilizing different transitions within the region of the ν ₁+ν ₂+ν ₃ and 2ν ₁+ν ₂ combination bands around 1.58 μm. Suitable lines in this spectral region have been identified, and absolute absorption cross sections have been determined through single-pass absorption spectroscopy and confirmed in the Doppler linewidth regime using cavity enhanced absorption spectroscopy (CEAS). The desire for a sensitive system potentially applicable to H₂S sensing at atmospheric pressure has led to an investigation on suitable transitions using wavelength modulation spectroscopy (WMS). The set-up sensitivity has been calculated as 1.73×10⁻⁸ cm⁻¹ s^1/2, and probing the strongest line at 1576.29 nm a minimum detectable concentration of 700 ppb under atmospheric conditions has been achieved. Furthermore, pressure broadening coefficients for a variety of buffer gasses have been measured and correlated to the intermolecular potentials governing the collision process; the H₂S–H₂S dimer well depth is estimated to be 7.06±0.09 kJ mol⁻¹.     

Metal-on-oxide nanoparticles produced using laser ablation of microparticle aerosols

A continuous aerosol process has been studied for producing nanoparticles of oxides that were decorated with smaller metallic nanoparticles and are free of organic stabilizers. To produce the oxide carrier nanoparticles, an aerosol of 3–6 μm oxide particles was ablated using a pulsed excimer laser. The resulting oxide nanoparticle aerosol was then mixed with 1.5–2.0 μm metallic particles and this mixed aerosol was exposed to the laser for a second time. The metallic micron-sized particles were ablated during this second exposure, and the resulting nanoparticles deposited on the surface of the oxide nanoparticles producing an aerosol of 10–60 nm oxide nanoparticles that were decorated with smaller 1–5 nm metallic nanoparticles. The metal and oxide nanoparticle sizes were varied by changing the laser fluence and gas type in the aerosol. The flexibility of this approach was demonstrated by producing metal-decorated oxide nanoparticles using two oxides, SiO₂ and TiO₂, and two metals, Au and Ag.     

Non-steady-state photo-EMF in nanostructured GaN and polypyrrole within porous matrices

We report an experimental investigation of the non-steady-state photoelectromotive force in nanostructured GaN within porous glass and polypyrrole within chrysotile asbestos. The samples are illuminated by an oscillating interference pattern created by two coherent light beams and the alternating current is detected as a response of the material. Dependences of the signal amplitude versus temporal and spatial frequencies, light intensity, and temperature are studied for two wavelengths λ=442 and 532 nm. The conductivity of the GaN composite is measured: σ=(1.1–1.6)×10⁻¹⁰ Ω⁻¹ cm⁻¹ (λ=442 nm, I ₀=0.045–0.19 W/cm², T=293 K) and σ=(3.5–4.6)×10⁻¹⁰ Ω⁻¹ cm⁻¹ (λ=532 nm, I ₀=2.3 W/cm², T=249–388 K). The diffusion length of photocarriers in polypyrrole nanowires is also estimated: L _D=0.18 μm.     

Detection of the structure of the wing of the Rayleigh line in ice, water, and heavy water using four-photon polarization spectroscopy

The spectral structure of the wing of the Rayleigh line in ice, ordinary water (H₂O), and heavy water (D₂O) is recorded in the frequency range 0–50 cm⁻¹ by means of four-photon polarization spectroscopy. It is shown that this structure can be explained by the collective rotational motion of molecules in cells determined by the structure of hexagonal ice. Pis’ma Zh. éksp. Teor. Fiz. 69, No. 1, 12–14 (10 January 1999)     

Wavelength modulation and cavity enhanced absorption spectroscopy using ～1.9?μm radiation produced by difference frequency generation with a MgO doped PPLN crystal

We have demonstrated the production of ∼1.9 μm near-infrared radiation by using difference frequency generation within a 5% MgO doped PPLN crystal by coupling ∼735 nm radiation from a tunable external cavity diode laser with relatively high powered 532 nm radiation from both Nd:YVO₃ and Nd:YAG lasers. The radiation produced is of low power, ∼15 μW, and was used in conjunction with the sensitivity enhancing techniques of wavelength modulation spectroscopy (WMS) and cavity enhanced absorption spectroscopy (CEAS). Experiments were carried out on rotationally resolved transitions in the combination bands of NH₃ and CO₂ in the 1.9 μm region. An α _min value of 3.6×10⁻⁶ cm⁻¹ Hz^−1/2 was achieved for WMS measurements on CO₂. A comparable α _min value of 2.2×10⁻⁶ cm⁻¹ Hz^−1/2 was achieved for NH₃ using CEAS. The low NIR power indicates that despite the level of MgO doping quoted for the crystal, under prolonged exposure photorefractive damage has occurred.     

Resonance Scattering Spectral Detection of Catalase Activity Using Au@Ag Nanoparticle as Probe and Coupling Catalase Catalytic Reaction with Fenton Reaction

The Au_coreAg_shell (Au@Ag) nanoparticles in size of 30 nm were prepared using 10 nm gold nanoparticles as seeds at 90°C, and were purified by high-speed centrifugation to remove the excess trisodium citrate to obtain Au@Ag nanoprobe. In the medium of pH 4.0 acetate buffer solution—7.2 μmol/L H₂O₂–67 μmol/L Fe(II), Au@Ag nanoparticles exhibited a resonance scattering (RS) peak at 538 nm. Upon addition of Catalase (Ct), the system produced hydroxyl radical that oxidized the Au@Ag nanoprobe to form the AuAg nanoparticles with partly bare nanogold. Those AuAg nanoparticles aggregated to large nanoclusters that led to the RS peak wavelength red-shift and its RS peak intensity enhanced. The catalase activity (C) is linear to the enhanced RS intensity (ΔI) in the range of 6 to 2,800 U/L, with regression equation of ΔI = 0.168 C-0.2, the correlation coefficient of 0.9952, and detection limit of 2.8 U/L. This method was applied to the detection of serum samples, and the results were agreement with that of the spectrophotometry. A new catalytic mechanism of catalase was proposed with oxywater principle that was agreement with the results of resonance scattering spectroscopy, absorption spectrophotometry, transmission electron microscopy and laser scattering.     

Absorption spectrum of HDO in the 1.06 μm region at a temperature of 1000 K

In the 9387–9450 cm^–1 region at temperatures of 300–1000 K, we have used an intracavity laser spectrometer based on a neodymium laser with threshold sensitivity to absorption 10^–8 cm^–1 and spectral resolution 0.035 cm^–1 to study the absorption spectrum of D₂¹⁶O, H₂¹⁶O, and HD¹⁶O vapor. The high-temperature spectrum contains more than 450 absorption lines, 240 of which are assigned to the HDO isotopomer. The absorption lines of HDO were identified and belong to nine vibrational transitions: 3ν₂ +ν₃, 2ν₁ + 3ν₂, 2ν₁ + ν₃, 4ν₂ + ν₃, 7ν₂, ν₁ + 2ν₂ + ν₃, ν₁ + 5ν₂, ν₁ + 2ν₂, and 3ν₃ – ν₂.     

Near-infrared diode laser absorption sensor for rapid measurements of temperature and water vapor in a shock tube

A fast-response (100 kHz) tunable diode laser absorption sensor is developed for measurements of temperature and H₂O concentration in shock tubes, e.g. for studies of combustion chemistry. Gas temperature is determined from the ratio of fixed-wavelength laser absorption of two H₂O transitions near 7185.60 cm^-1 and 7154.35 cm^-1, which are selected using design rules for the target temperature range of 1000–2000 K and pressure range of 1–2 atm. Wavelength modulation spectroscopy is employed with second-harmonic detection (WMS-2f) to improve the sensor sensitivity and accuracy. Normalization of the second-harmonic signal by the first-harmonic signal is used to remove the need for calibration and minimize interference from emission, scattering, beam steering, and window fouling. The laser modulation depth for each H₂O transition is optimized to maximize the WMS-2f signal for the target test conditions. The WMS-2f sensor is first validated in mixtures of H₂O and Ar in a heated cell for the temperature range of 500–1200 K (P=1 atm), yielding an accuracy of 1.9% for temperature and 1.4% for H₂O concentration measurements. Shock wave tests with non-reactive H₂O–Ar mixtures are then conducted to demonstrate the sensor accuracy (1.5% for temperature and 1.4% for H₂O concentration) and response time at higher temperatures (1200–1700 K, P=1.3–1.6 atm). PACS 42.62.Fi; 42.55.Px; 42.60.Fc; 07.35.+k     

Sheath and electron density dynamics in the normal and self-pulsing regime of a micro hollow cathode discharge in argon gas

A microplasma is generated in the microhole (400 μm diameter) of a molybdenum-alumina-molybdenum sandwich (MHCD type) at medium pressure (30–200 Torr) in pure argon. Imaging and emission spectroscopy have been used to study the sheath and electron density dynamics during the stationary normal regime and the self-pulsing regime. Firstly, the evolution of the microdischarge structure is studied by recording the emission intensity of the Ar (5p[3/2]₁–4s[3/2]₁)_{1}) line at 427.217 nm, and Ar⁺ (4p′ ²P_3/2–4s′ ²D_5/2)_{5/2}) line at 427.752 nm. The maximum of the Ar⁺ line is located in the vicinity of the sheath-plasma edge. In both regimes, the experimental observations are consistent with the position of the sheath edge calculated with an ionizing sheath model. Secondly, the electron density is recorded by monitoring the Stark broadening of the H_b_\beta-line. In the self-pulsing regime at 150 Torr, the electron density reaches its maximum value of 4 × 10¹⁵ cm^-3, a few tens of ns later than the discharge current maximum. The electron density then decays with a characteristic decay time of about 2 μs, while the discharge current vanishes twice faster. The electron density in the steady-state regime is two orders of magnitude lower, at about 6–8 × 10¹³ cm^-3.     

Dynamics of femtosecond-laser-ablated liquid-aluminum nanoparticles probed by means of spatiotemporally resolved X-ray absorption spectroscopy

We investigated spatiotemporal evolution of expanding ablation plume of aluminum created by a 100-fs, 10¹⁴–10¹⁵-W/cm² laser pulse. For diagnosing dynamic behavior of ablation plume, we employed the spatiotemporally resolved X-ray absorption spectroscopy (XAS) system that consists of a femtosecond-laser-plasma soft X-ray source and a Kirkpatrick–Baez (K–B) microscope. We successfully assigned the ejected particles by analyzing structure of absorption spectra near the L _II,III absorption edge of Al, and we clarified the spatial distribution of Al⁺ ions, Al atoms, and liquid droplets of Al in the plume. We found that the ejected particles strongly depend the irradiated laser intensity. The spatial distribution of atomic density and the expansion velocity of each type of particle were estimated from the spatiotemporal evolution of ablation particles. We also investigated a temperature of the aluminum fine particles in liquid phase during the plume expansion by analyzing the slope of the L _II,III absorption edge in case of 10¹⁴-W/cm² laser irradiation where the nanoparticles are most efficiently produced. The result suggests that the ejected particles travel in a vacuum as a liquid phase with a temperature of about 2500 to 4200 K in the early stage of plume expansion.     

Europium-sensitized Chemiluminescence of System Tetracycline–H<Subscript>2</Subscript>O<Subscript>2</Subscript>–Fe(II)/(III) and Its Application to the Determination of Tetracycline

Chemiluminescence (CL) of the reaction system tetracycline–H₂O₂–Fe(II)/(III)–Eu(III) was used for the determination of tetracycline hydrochloride in water, pharmaceutical preparations, and honey. The CL spectrum registered for this system shows emission bands typical of Eu(III) ions, with a maximum at λ ∼ 600 nm, corresponding to the electronic transitions of ⁵D₀ → ⁷F₁ and ⁵D₀ → ⁷F₂. A strong chemiluminescence intensity characteristic of europium(III) ions in the system tetracycline–H₂O₂–Fe(II)/(III)–Eu(III), as contrasted to the emission of the system tetracycline–H₂O₂–Fe(II)/(III) without Eu(III), proves that the Eu(III) ion plays the role of a chemiluminescence sensitizer, accompanying tetracycline oxidation in the Fenton system (H₂O₂–Fe(II)/(III)). A linear dependence was observed for the integrated CL light intensity on the tetracycline concentration in the range of 2 × 10⁻⁷ to 3 × 10⁻⁵ mol l⁻¹ with the detection limit of 5 × 10⁻⁸ mol l⁻¹ in aqueous solution.     

Particular features of the transmission of laser radiation with wavelength 0.438 µm and intensity (3–7)·1014 W/cm2 through an undercritical plasma from polymer aerogels

The velocities of energy transport in an undercritical plasma of polymer aerogel with and without copper nanoparticles were measured. Transmission of the laser light through targets of different thicknesses such as submicron three-dimensional polymer networks with densities below the critical value (0.13–0.52 N _cr) for a wavelength of 0.438 μm and intensity of (3–7)·10¹⁴ W/cm² at a half-height pulse duration of 0.32 ns was studied. The transfer of a heating laser radiation was registered on the rear side of the target. It ranged from a level of ∼0.5% for the thickness of a low-density layer of 400 μm and density of 9 mg/cm³ (mass per unit square of 0.36 mg/cm²) up to 50–60% for a thickness of 100 μm and density of 2.25 mg/cm³ (mass per unit square of 0.02 mg/cm²). The time dependences of the optical emission from the rear side of the targets were measured. They appear to be indicative of the plasma dynamics in two-layer targets (polymer foam on Al foil) and enable the estimation of the absorption depth for the laser light in an undercritical plasma. __________ Translated from Preprint No. 8 of the P. N. Lebedev Physical Institute, Moscow (2007).