XeCl laser ablation of polyetheretherketone

Ablation of polyetheretherketone (PEEK), a high temperature thermoplastic, by XeCl laser radiation occurs at fluences in excess of 0.07±0.01 J cm^–2. The volatile products of ablation are CO and C₂H₂ with smaller quantities of CH₄, C₄H₂, C₆H₆ and other C₃ and C₄ hydrocarbons. At fluences close to the threshold ablation produces involatile material of relatively high molecular weight but at high fluences extensive disruption of the PEEK structure occurs with conversion of all of the oxygen in the polymer to carbon monoxide.     

Silicon cluster formation in the laser ablation of SiO at 308 nm

Neutral silicon cluster formation in the laser (308 nm) ablation of silicon monoxide was investigated through the analysis of composition and dynamics of the ablation plume under different laser fluence conditions. The neutral species were ionized by a second laser (193 nm) and the positionized species detected by TOF-MS (time-of-flight mass spectrometry). At low laser fluences, plume composition is dominated by SiO; above 0.6 J/cm² Si, SiO and Si₂ have comparable intensity and Si_n (n≤7) clusters are observed. Flow velocities and temperatures of the ejected species are nearly mass-independent, indicating that the plume dynamics are close to the strong expansion limit, implying a collisional regime. Through the relation between the estimated values of terminal flow velocity and surface temperature, u_T²∝T_S, it is found that, at low laser fluences, the surface temperature increases linearly with laser fluence, whereas, at the laser fluence at which Si_n clusters are observed, the increase of temperature is below the linear dependence. The population distribution of the ejected Si_n provides some indication of a formation mechanism based on condensation. Analogies between the ablation behavior of silicon monoxide and silicon targets are considered. PACS 82.30.Nr; 81.05.Gc; 78.70.-g     

Interaction of 157-nm excimer laser radiation with fluorocarbon polymers

Two important fluoropolymers, polytetrafluoroethylene [PTFE—(C₂F₄)_N] and polyvinylidene fluoride [PVDF—(C₂H₂F₂)_N], respond to 157-nm laser radiation in dramatically different ways. At fluences sufficient to produce rapid etching, the volatile emissions from PTFE are dominated by (CF₂)_N fragments. The velocities of the fastest (CF₂)_N molecules at each mass are consistent with kinetic energies on the order of an electron volt—and change little with fluence. This fluence independence suggests that the velocities are not affected by collisions after emission. To account for the high kinetic energies and the unusual, half-monomer mass distribution, we propose that these fragments are produced by photochemical scission of the polymer backbone, and that a fraction of the excitation energy is delivered to each fragment as kinetic energy. In contrast, the principle neutral species from PVDF is HF. HF is produced by the scission of C-F bonds, followed by chemical reactions with nearby hydrogen. This process is accompanied by the conjugation of backbone C-C bonds. The photochemical cleavage of C-C bonds in PTFE and C-F bonds in PVDF is consistent with the lower C-C bond energy of PTFE.     

Picosecond laser ablation of nickel-based superalloy C263

Picosecond laser (10.4 ps, 1064 nm) ablation of the nickel-based superalloy C263 is investigated at different pulse repetition rates (5, 10, 20, and 50 kHz). The two ablation regimes corresponding to ablation dominated by the optical penetration depth at low fluences and of the electron thermal diffusion length at high fluences are clearly identified from the change of the surface morphology of single pulse ablated craters (dimples) with fluence. The two corresponding thresholds were measured as F _th(D1)¹=0.68±0.02 J/cm² and F _th(D2)¹=2.64±0.27 J/cm² from data of the crater diameters D _1,2 versus peak fluence. The surface morphology of macroscopic areas processed with a scanning laser beam at different fluences is characterised by ripples at low fluences. As the fluence increases, randomly distributed areas among the ripples are formed which appear featureless due to melting and joining of the ripples while at high fluences the whole irradiated surface becomes grainy due to melting, splashing of the melt and subsequent resolidification. The throughput of ablation becomes maximal when machining at high pulse repetition rates and with a relatively low fluence, while at the same time the surface roughness is kept low.     

MeV carbon ion irradiation-induced changes in the electrical conductivity of silver nanowire networks

MeV carbon ion irradiation-induced changes in the electrical conductivity of Silver nanowire (Ag-NW) networks is demonstrated systematically at different C⁺ ion fluences ranging from 1 × 10¹² to 1 × 10¹⁶ ions/cm² at room temperature. At low C⁺ ion fluences, the electrical conductivity of Ag-NWs decreases and subsequently increases with increase fluence. Finally, at high C⁺ ion fluences, conductivity again decreases. The variation in the electrical conductivity of Ag NW network is discussed after analysis using scanning electron microscopy (SEM) and X-ray diffraction (XRD) techniques. The observed increase in electrical conductivity is thought to be due to ion induced coalescence of Ag-NWs at contact position, which causes reduction of wire–wire contact resistance, while the decrease in electrical conductivity may be due to defect production by C⁺ ions into Ag-NWs. Ion beam technology is therefore a very promising technology that is capable of fabricating highly conductive Ag-NW networks for transparent electrodes. Moreover, a method for thinning, slicing and cutting of Ag-NWs using ion beam technology is also reported.     

Processing of C<Subscript>60</Subscript> thin films by Matrix-Assisted Pulsed Laser Evaporation (MAPLE)

Thin films of fullerenes (C₆₀) were deposited onto silicon using matrix-assisted pulsed laser evaporation (MAPLE). The deposition was carried out from a frozen homogeneous dilute solution of C₆₀ in anisole (0.67 wt%), and over a broad range of laser fluences, from 0.15 J/cm² up to 3.9 J/cm². MAPLE has been applied for deposition of fullerenes for the first time and we have studied the growth of thin films of solid C₆₀. The fragmentation of C₆₀ fullerene molecules induced by ns ablation in vacuum of a frozen anisole target with C₆₀ was investigated by matrix-assisted laser desorption/ionization (MALDI). Our findings show that intact fullerene films can be produced with laser fluences ranging from 0.15 J/cm² up to 1.5 J/cm².     

μSR studies of the metallic alkali fullerides

We report ZF μSR measurements in Cs₁C₆₀ at low temperature and LF μSR in the superconductor Rb₃C₆₀. In Cs₁C₆₀, the internal magnetic field distribution is broad and static, and the relaxation at 1.9 K resembles that of a spin density wave. In Rb₃C₆₀, we observe a strong field dependence to the coherence peak in the LF relaxation rate of endohedral muonium ( Mu@C₆₀) at anomalously low field and a residual low temperature relaxation. This revised version was published online in August 2006 with corrections to the Cover Date.     

Laser mass spectrometry study of the properties of fullerene structures

Fullerene films grown by various methods are studied using mass spectrometry. The mass spectra of the films formed onto an aluminum foil using thermal deposition (TD) or supersonic molecular beam (SMB) exhibit a small change in the mass peak distribution in the C₁₂₀ dimer range as compared to the initial fullerene powder during desorption by laser radiation irrespective of the radiation wavelength (λ = 259, 518 nm). Under the action of laser radiation with wavelength λ = 259 nm, fullerene films grown on a silicon substrate with an SMB also exhibit a small change in the mass peak distribution in the C₁₂₀ dimer range. At λ = 518 nm, the mass peak distribution in the dimer range shifts significantly toward small masses, so that the intensity maximum corresponds to M ≅ C₁₀₂. This fact is assumed to be related to the polymerization of an SMB fullerene film caused by heating due to the absorption of laser radiation with a wavelength λ = 518 nm.     

Influence of Nitrogen Supply and Temperature on Stable Carbon Isotope Ratios in Plants of Different Photosynthetic Pathways (C3, C4, CAM)

Abstract

One representative species of each of the three photosynthetic pathways (C₃, C₄, CAM) were cultivated in growth chambers with high and low nitrogen nutrition level respectively once at 20°C day/13°C night temperature, once at 30°C day/13°C night. Leaf conductances and δ¹³C values of the leaves of each plant were determined. At 20°C day temperature the C₃ species showed higher leaf conductance with low nitrogen nutrition level than with high nitrogen level, which is also reflected in a more negative δ¹³C value, whereas both C₄ and CAM plants did not respond in this manner to nitrogen supply. An increase of day temperature to 30°C diminished the significant response of the C₃ plant, while the response of C₄ and CAM representatives to nitrogen nutrition did not change markedly.     

Investigation of radiation resistance of fullerenes under irradiation with fast neutrons

Fullerenes C₆₀ and C₇₀ synthesized by the electric arc method and fractionated (purity grades of 99.99 and 99.90 wt %, respectively) were irradiated in a solid phase in the WWR-M reactor (Konstantinov Petersburg Nuclear Physics Institute, National Research Centre “Kurchatov Institute,” Gatchina, Russia) with the aim of determining the survivability in the range of fast neutron fluences Φ = 4 × 10¹⁵?3 × 10¹⁷ n/cm². The irradiated samples were dissolved in carbon disulfide, and intact fullerenes were extracted. With an increase in the fluence, their weight fraction in the samples S(Φ), a measure of radiation resistance of molecules, decreased, to a first approximation, exponentially: S(Φ) = exp(?Φ/Φ _D ). The estimated characteristic fluences were Φ _D = 2.4 × 10¹⁷ and 4.0 × 10¹⁷ n/cm² for C₆₀ and C₇₀, respectively.     

Temperature variation of phonon frequency distribution in the fast ion conductor lead fluoride

A weighted phonon frequency distribution has been measured in PbF₂ at temperatures 10, 302, 660 and 910 K, using a neutron scattering technique. At 10 K good agreement is found between the measured distribution and the phonon density-of-states calculated from the low temperature dispersion relation of PbF₂. At the higher temperatures, near the ionic conductivity transition temperature, T_c ～ 700 K, the optic modes are observed to broaden into a high energy tail consistent with strong anharmonicity or extensive disorder. A low energy peak arising from transverse acoustic modes remains well defined even at temperatures above T_c.     

Raman scattering of neutron irradiated 6H-SiC

Confocal micro-Raman spectroscopy was preformed to investigate the structural damage of SiC neutron irradiated with the fluences of 1.72×10¹⁹ and 1.67×10²⁰ n/cm². In addition to characteristic peaks, several additional signals related to Si–Si, Si–C, and C–C vibration modes were monitored. The vibration mode associated with C_SiV _C complexes was identified to be the unambiguous peak at 575 cm^?1 which appears initially in the sample post-irradiation annealed at 800 °C. The defect-induced phonon confinement effect results in an asymmetric broadening with a low-frequency tail of the optical phonon peaks. The sigmoidal thermal recovery behavior of the optical phonon frequencies indicates that the reduction of FLO_0/6–FTO_2/6 splitting originates mainly from the isolated vacancies and interstitials.     

Adsorption of acetylene and ethylene on a clean Ir(111) surface

The adsorption of C₂H₂ and C₂H₄ on Ir(111) is studied within the temperature range 180–500 K by the HREELS and XPS methods. The absolute concentration of hydrocarbon coverage is estimated by XPS. Data are obtained on the kinetics of adsorption of the two gases at different temperatures. It is established by HREELS studies that at 180 K C₂H₄ forms ethylidyne (CCH₃ whereas C₂H₂ is adsorbed as CCH and ethylidyne species. At 300 K both kinds of species are found on the Ir(111) surface after C₂H₂ or C₂H₄ exposures. The ethylidyne decomposes completely to CCH at 500 K, which can be accompanied by polymerization of adsorbed hydrocarbon species.     

Electroluminescence from thin SiO2 layers after Si- and C-coimplantation

In this paper we explore the electroluminescence (EL) properties of thermally grown 350 nm thick SiO₂ layers co-implanted with Si⁺ and C⁺ ions. The implanted fluences were chosen in such a way that peak concentrations of excess Si and C of 5–10 at% were achieved. The devices show a broad photoluminescence (PL) between 2.0 and 3.2 eV with a main peak around 2.7 eV. The broad EL spectra show additional peaks around 3.3 eV and between 2.1 and 2.5 eV which are decreased with increasing Si/C concentration. The shape of the EL spectra does not change with increasing injection currents which implies that various types of defects occur for the different concentrations. The device stability is improved in comparison to Ge or Sn implanted oxide layers.     

Small size TiO2 nanoparticles prepared by laser ablation in water

Titanium dioxide nanoparticles in distilled H₂O solvent were prepared by laser ablation. The experiments were performed irradiating a Ti target with a second harmonic (532 nm) output of a Nd:YAG laser varying the operative fluence between 1 and 10 J cm⁻² and for an ablation time ranging from 10 to 30 min. Electron microscopy measurements have evidenced the predominant presence of nanoparticles with diameter smaller than 10 nm together with agglomerations of 100-200 nm whose content increases with the laser fluence. At low laser fluence the particles’ size distribution shows that more than 85% of the nanoparticles have a size smaller than 5 nm while at mid and high fluences the presence of 5-7 nm nanoparticles is predominant. XPS analysis has revealed the presence of different titanium suboxide phases with the prevalence of Ti-O bonds from TiO₂ species. The optical bandgap values, determined by UV-vis absorption measurements, are compatible with the anatase phase.     

Matrix effect of bombardment-induced Gibbsian segregation on Cu depletion at a Cu x Ni100−x subsurface

A dynamic Monte-Carlo program, including an improved BIGS (Bombardment-Induced Gibbsian Segregation) model was employed to study the matrix effect of BIGS and its influence on Cu depletion at the subsurface under 1 keV Ar-ion bombardment of a Cu–Ni alloy. The calculation results show that not only at high fluences (the steady state) but also at low fluences, the Cu subsurface depletion depends on its bulk composition. This is because the atomic jump rateW ₂₁ of Cu from the second layer to the first layer is correlated to Cu bulk composition at any fluence. We also found that the product of the non-segregating species concentration at the first layer with the segregating species concentration at the second layer plays a more important role than other parameters in determiningW ₂₁ under low current conditions.     

Strain rate and fuel composition dependence of chemiluminescent species profiles in non-premixed counterflow flames: comparison with model results

A detailed comparison has been conducted between chemiluminescence (CL) species profiles of OH^?, CH^?, and C₂ ^?, obtained experimentally and from detailed flame kinetics modeling, respectively, of atmospheric pressure non-premixed flames formed in the forward stagnation region of a fuel flow ejected from a porous cylinder and an air counterflow. Both pure methane and mixtures of methane with hydrogen (between 10 and 30 % by volume) were used as fuels. By varying the air-flow velocities methane flames were operated at strain rates between 100 and 350 s^?1, while for methane/hydrogen flames the strain rate was fixed at 200 s^?1. Spatial profiles perpendicular to the flame front were extracted from spectrograms recorded with a spectrometer/CCD camera system and evaluating each spectral band individually. Flame kinetics modeling was accomplished with an in-house chemical mechanism including C₁–C₄ chemistry, as well as elementary steps for the formation, removal, and electronic quenching of all measured active species. In the CH₄/air flames, experiments and model results agree with respect to trends in profile peak intensity and position. For the CH₄/H₂/air flames, with increasing H₂ content in the fuel the experimental CL peak intensities decrease slightly and their peak positions shift towards the fuel side, while for the model the drop in mole fraction is much stronger and the peak positions move closer to the fuel side. For both fuel compositions the modeled profiles peak closer to the fuel side than in the experiments. The discrepancies can only partly be attributed to the limited attainable spatial resolution but may also necessitate revised reaction mechanisms for predicting CL species in this type of flame.     

A comparative study of ion-induced damages in C60 and C70 fullerenes

The stability of fullerenes (C₆₀ and C₇₀) under swift heavy ion irradiation is investigated. C₆₀ and C₇₀ thin films were irradiated with 120 MeV Ag ions at fluences from 1×10¹² to 3×10¹³ ions/cm². The damage cross-section and radius of damaged cylindrical zone were found to be higher for C₆₀ than C₇₀ as evaluated by Raman spectroscopy, which shows that the C₇₀ molecule is more stable under energetic ion impact. The higher damage cross-section of the C₆₀ molecule compared with that of the C₇₀ molecule is explained on the basis of thermal conductivity in the framework of the thermal spike model. The surface morphology of pristine C₆₀ and C₇₀ films is studied by atomic force microscopy. UV-visible absorption studies revealed that band gap for C₆₀ and C₇₀ fullerenes thin films decreases with increasing ion fluence. Resistivity of C₆₀ and C₇₀ thin films decreases with increasing ion fluence but the decrease is faster for C₆₀ than C₇₀, indicating higher damage in C₆₀. Irradiation at a fluence of 3×10¹³ ions/cm² results in complete damage of fullerenes (C₆₀ and C₇₀) into amorphous carbon.     

Carbon-13 Isotopic Species of H2C3, H2C4, and H2C5: High-Resolution Rotational Spectra

The microwave rotational spectra of the carbon-13 isotopic species of H₂C₃, H₂C₄, and H₂C₅ have been observed in a pulsed supersonic molecular beam by Fourier transform microwave spectroscopy. At high resolution all of the rotational lines exhibit hyperfine structure produced by the magnetic interaction between the nuclear spin of ¹³C and the overall rotation of the molecule. The component of the nuclear spin-rotation tensor along the a-inertial axis is large for most isotopic species, especially at the carbene carbon; at this position C_aa is two to three times larger than at other substituted positions along the chain. In contrast to both H₂C₃ and H₂C₃, in H₂C₄C_aa exhibits a pronounced alternation along the carbon chain backbone. Following detection of the five carbon-13 isotopic species and D₂C₅, an experimental structure (r₀) has been determined to high accuracy for H₂C₅.     

Scaling analysis of phase transition in Hg-based superconductor

With the vibrating-reed technique, the internal friction (IF) Q⁻¹ is measured for sing-phase (Hg_0.66Pb_0.34)Ba₂Ca₂Cu₃O_8+x superconductor as a function of temperature at low applied magnetic field up to 0.5 T and as a function of frequency at normal state temperatures. An IF peak associated with flux motion can be found below T_C. The IF peak becomes higher and shifts towards lower temperature with increasing magnetic field. In addition an IF peak is found near 200 K. By scaling analysis we have demonstrated that the internal friction around the peak temperature can be collapsed into a single curve, indicating that the IF peak below T_C is originated from a phase transition associated with a vortex glass transition and a structural phase transition occurs at around 200 K in the superconductor.