Composition of higher fullerenes obtained by laser ablation of carboniferous materials

The compositions of higher fullerenes produced by ablation of various carboniferous materials are experimentally studied. The yield of fullerenes versus the type of carboniferous materials and laser radiation parameters is found.     

Multiple photon laser photochemistry

Ｍｕｌｔｉｐｌｅｐｈｏｔｏｎｌａｓｅｒｐｈｏｔｏｃｈｅｍｉｓｔｒｙ￥Ｖ．Ｓ．Ｌｅｔｏｋｈｏｖ（ＩｎｓｔｉｔｕｔｅｏｆＳｐｅｃｔｒｏｓｃｏｐｙ，ＵＳＳＲＡｃａｄｅｍｙｏｆＳｃｉｅｎｃｅｓ，Ｔｒｏｉｔｚｋ，ＭｏｓｃｏｗＲｅｇｉｏｎ，１４２０９２，Ｒｕｓｓ...     

Phosphorus cluster production by laser ablation

Neutral and charged phosphorus clusters of a wide size range have been produced by pulsed laser ablation (PLA) in vacuum at 532, 337, and 193 nm ablating wavelengths and investigated by time-of-flight mass spectrometry. The neutral P_n clusters are even-numbered with local abundance maxima at n=10 and 14, while the cationic and anionic clusters are preferentially odd-numbered with P₇⁺, P₂₁⁺, and P₁₇^- being the most abundant ions. The dominance of the magic clusters is more pronounced at 337-nm ablation that is explained by efficient direct ejection of their building blocks under these conditions. Nanocrystalline phosphorus films have been produced by PLA in ambient helium gas. PACS 52.38.MF; 61.46.+w; 79.20.Ds; 81.07.B; 81.16.Mk     

Study of the X-ray emission from Ta plasma obtained by ns laser ablation

In this work, the continuum spectrum of X-rays originated from the interaction of a moderate intensity ns Nd:YAG laser (1064 nm, 9 ns, 30 Hz, 900 mJ, 10¹¹ W/cm²) with Ta target producing plasma is investigated. Plasma expands unisotropically with a velocity, depending on the pressure of the residual gas in the vacuum chamber. The X-ray intensity is a function of the laser energy and of the gas pressure inside the chamber. The X-ray energy is measured with an X-ray filter positioned in front of the Si(Li) solid-state detector. A temperature of about ～1–2 keV of the hot electrons, responsible for the continuum spectrum emission from the plasma, is calculated from the fit of the X-ray spectrum, applying a Maxwellian distribution.     

UV laser photochemistry of acetylene at 193 nm

ArF laser photolysis of acetylene at 193 nm yields diacetylene, ethylene and hydrogen (besides a polymer). No benzene or vinylacetylene is formed. Photolysis with deuterium gives no deuterated compounds. These observations are taken as evidence for a pure excited-molecule mechanism based upon C₂H ₂ ^* (¹ A _u).     

Study on thermal diffusivity of materials by laser photothermal reflection technique

A method of measuring thermal diffusivity of materials at room temperature by photothermal reflection technique is described. An intensity-modulated Ar+ laser beam is used as incident light. The beam is focused to about 1 mm diameter spot and illuminates the sample surface. HgCdTe infrared detector is used to receive photothermal signal. Using this technique, the photothermal signals are experimentally measured as the function of different frequencies. The thermal diffusivities can be obtained by fitting the experimental data. On the other hand, the thermal diffusivities of one-way composite and orthogonal symmetric arranged composites Al2O3/Al are measured in transverse, longitudinal and arbitrary directions. The results show that the diffusivity of one-way material decreases with the increase of the measurement angle; the diffusivity of orthogonally arranged material almost keeps the same when measurement angle changes.     

Chemical analysis of products obtained by nanosecond laser ablation

The chemical analysis of a laser jet was performed with a combination of laser back mass transfer and X-ray photoelectron spectroscopy (XPS). It was revealed that, as compared to the source material, films deposited from the jet under laser mass transfer in air show increased oxidizability and changed chemical composition.     

Structural state of second-generation HTSCs obtained by laser ablation

The structure of buffer layers and deposited YBa₂Cu₃O _x (Y123) films with a high critical current density (～10⁶ A/cm²) has been investigated by different methods. These superconducting films and buffer layers are found to have a fine-grained structure, which, along with the high texture of Y123 films of the (001) type, is believed to be responsible for the high critical current density. An unusual texture is revealed in buffer layers, which differs from that of substrates and Y123 films. The superconducting films deposited on buffer CeO₂ layers exhibit a system of orthogonal lines; in the case under consideration, this is a manifestation of a domain structure with Y123 particles at boundaries.     

Ceramic components manufacturing by selective laser sintering

In the present paper, technology of selective laser sintering/melting is applied to manufacture net shaped objects from pure yttria-zirconia powders. Experiments are carried out on Phenix Systems PM100 machine with 50 W fibre laser. Powder is spread by a roller over the surface of 100 mm diameter alumina cylinder. Design of experiments is applied to identify influent process parameters (powder characteristics, powder layering and laser manufacturing strategy) to obtain high-quality ceramic components (density and micro-structure).The influence of the yttria-zirconia particle size and morphology onto powder layering process is analysed. The influence of the powder layer thickness on laser sintering/melting is studied for different laser beam velocity V (V = 1250-2000 mm/s), defocalisation (−6 to 12 mm), distance between two neighbour melted lines (so-called “vectors”) (20-40 μm), vector length and temperature in the furnace. The powder bed density before laser sintering/melting also has significant influence on the manufactured samples density.Different manufacturing strategies are applied and compared: (a) different laser beam scanning paths to fill the sliced surfaces of the manufactured object, (b) variation of vector length (c) different strategies of powder layering, (d) temperature in the furnace and (e) post heat treatment in conventional furnace. Performance and limitations of different strategies are analysed applying the following criteria: geometrical accuracy of the manufactured samples, porosity. The process stability is proved by fabrication of 1 cm³ volume cube.     

Surface photochemistry: from hot reactions to hot materials

The field of surface photochemistry has been rather fruitful for two major reasons: its contribution to the understanding of surface dynamics and its importance in the fabrication of surface materials. On the fundamental side, instead of addressing electronic transitions, it is my attempt here to illustrate the importance of surface photochemistry in understanding dynamics of the ground electronic states, those which are generally associated with thermal reactions. In the world of materials fabrication, I would like to point out the unique advantages of surface photochemistry over gas-phase photochemistry or thermal chemistry in the stringent control of ultra-thin films and ultra-small dimensions, i.e. nanotechnology. Both aspects of surface photochemistry are demonstrated using selected examples from my laboratory.     

Preparation of solder pads by selective laser scanning

We propose a new laser preparation technique to solder Sn-Ag3.5-Cu0.7 on a copper clad laminate （CCL）. The experiment is conducted by selective laser heating and melting the thin solder layer and then preprinting it on CCL in order to form the matrix with solder pads. Through the analysis of macro morphology of the matrix with solder pads and microstructure of single pads, this technique is proved to be suitable for preparing solder pads and that the solder pads are of good mechanical properties. The results also reveal that high frequency laser pulse is beneficial to the formation of better solder pad, and that the 12-W fiber laser with a beam diameter of 0.030 mm can solder Sn-Ag3.5-Cu0.7 successfully on CCL at 500-kHz pulse frequency. The optimized parameters of laser soldering on CCL are as follows： the laser power is 12 W, the scanning speed is 1.0 mm/s, the beam diameter is 0.030 ram, the lead-free solder is Sn-Ag3.5-Cu0.7, and the laser pulse frequency is 500 kHz.     

Study of the protoporphyrin luminescence under selective laser excitation

The structural luminescence spectra of protoporphyrin IX solid solutions in ethanol and hydrochloric acid were obtained under selective laser excitation in the region of the inhomogeneouly broadened pure electronic and vibronic bands at T = 3.8 K. The dependence of the excitation selectivity on the excitation frequency was investigated. The vibrational frequencies of protoporphyrin IX were obtained in the ground and excited electronic states when the excitation frequency has been scanned in the region of pure electronic and vibronic bands, respectively. A universal apparatus is described for the investigation of the absorption and luminescence spectra of polyatomic molecules in the temperature range from 3.5 to 300 K under the dye laser excitation tunable in the spectral region 265–365 nm, 435–730 nm, detection of spectra in the region 300–900 nm with a polychromator and silicon intensified image detector and processing of the spectral information with an optical spectrum analyser controlled with a microcomputer.     

Cleaning with water decomposed products obtained by laser irradiation

ArF excimer laser irradiation can decompose water, and decomposed products contain highly reactive substrates, such as radicals. We propose cleaning using pure water with the aid of water decomposed products obtained by ArF excimer laser irradiation. In this study, the oxidation potential of decomposed products was estimated in metal etching. Then, cleaning of cutting oil was examined. The focal point of the lens used was set at the water surface. Specimens were aligned parallel to the laser beam, so that only decomposed products affected contaminants. As a result, decomposed products could not etch nickel and copper plates, but only zinc plates. Cutting oil was cleaned after 18 000 irradiation pulses. The range of the region cleaned was 5 mm around the focal point.     

Lead-free ferroelectric thin films obtained by pulsed laser deposition

Na_0.5Bi_0.5TiO₃-BaTiO₃ (NBT-BT) thin films grown by pulsed laser deposition have been investigated by X-ray diffraction, scanning electron microscopy, and dielectric spectroscopy in order to clarify the role of substrate temperature on crystalline structure, grain morphology, and dielectric properties. We have shown that the structural and dielectric properties of NBT-BT thin films with composition at morphotropic phase boundary (6% BT) critically depend on the substrate temperature: small variations of this parameter induce structural changes, shifting the morphotropic phase boundary toward tetragonal or rhombohedral side. Higher deposition temperature (1000 K) favor the formation of rhombohedral phase, films deposited at 923 K and 973 K have tetragonal symmetry at room temperature. Grains morphology depends also on the deposition temperature. Atomic force micrographs show grains with square or rectangular shape in a compact structure for films grown at lower temperatures, while grains with triangular shape in a porous structure are observed for films grown at 1000 K. Dielectric spectroscopy measurements evidenced the phase transition between ferroelectric and antiferroelectric phase at 370 K. Films grown at 1000 K shown low electrical resistivity due to their porous structure. High dielectric constant values (about 800 at room temperature and 2700 at 570 K) have been obtained for films grown at temperatures up to 973 K.     

UV excimer laser photochemistry of hybrid organometallic compounds of gallium

The gas phase ultraviolet (UV) excimer laser induced photolysis of the gallium-alkyls Ga(t-C₄H₉) _n ^– (CH₃)_3–n (n=0, 1, 2, 3) was studied, using photolysis wavelengths of 308, 248, and 193 nm. The photofragments Ga, GaH, and GaCH₃ were detected by laser ionization time-of-flight mass spectroscopy, while the hydrocarbon products CH₄, C₂H₆, HC(CH₃)₃ and H₂C=C(CH₃)₂ were identified using Fourier transform infrared (FTIR) spectroscopy. The formation of the GaH photofragment, and a high olefin-to-alkane product ratio, for Ga(t-C₄H₉)₂(CH₃) and Ga(t-C₄H₉)₃ are interpreted to indicate a -hydrogen elimination process. However, -hydrogen elimination only occurs after fission of the weakest Ga-C bond, thus no -hydride elimination is observed for Ga(t-C₄H₉)(CH₃)₂. Detection of C₂H₆ for Ga(CH₃)₃ and Ga(t-C₄H₉)(CH₃)₂, but not for Ga(t-C₄H₉)₂(CH₃), shows that under our experimental conditions the formation of ethane is as a result of the reductive elimination of the methyl groups, and is not due to the recombination of two free methyl radicals.     

Characterization of Copper nanoparticles obtained by laser ablation in liquids

We prepared copper nanoparticles by ns laser ablation in pure water and in aqueous solutions of 1,10-phenanthroline at 1064 nm and 532 nm wavelengths. Although not fully impairing progressive oxidation, ligand molecules prevent the colloids to collapse. UV–vis absorption spectroscopy showed that particle production is more efficient at 1064 nm, while transmission electron microscopy gave evidence that 532-nm pulses cause photofragmentation of the structures, resulting in reduced particle size. Furthermore, from Raman and fluorescence tests we found that colloids obtained at 1064 nm show better Surface Enhanced Raman activity, while colloids obtained at 532 nm exhibit a more intense fluorescence emission.