Optical emission spectroscopy and modeling of plasma produced by laser ablation of titanium oxides

In the present study, the time evolution of electron number density, of electron, atom and ion temperatures, of plasma produced by KrF excimer laser ablation of titanium dioxide and monoxide targets, are investigated by temporally and spatially resolved optical emission spectroscopy over a wide range of laser fluence from 1.7 to 6 J cm⁻², oxygen pressures of 10⁻²–10⁻¹ torr and in a vacuum. A state-to-state collisional radiative model is proposed for the first time to interpret the experimental results at a distance of 0.6 mm from the target surface, in vacuum and for a time delay from 100 to 300 ns from the beginning of the laser pulse. In particular, we concentrate our attention on problems concerning the existence of the local thermodynamic conditions in the laser-induced plasma and deviation from them, as observed in our experiment. The numerical model proposed for calculating the electron number density and the population densities of atoms and ions in excited states give good quantitative agreement with the experimental results of the optical emission spectroscopy measurements.     

La distribution on the crater surface of W-1%La2O3 produced by a single laser pulse

The W-1%La₂O₃ alloy has been irradiated by a single laser pulse (λ = 1064 nm) to simulate transient thermal loads of high energy occurring in a tokamak under operative conditions. A zone with a diameter of ~2 mm, namely, much larger than the focal spot, results to be affected by the pulse, and a crater of about 300 μm is observed in its center. La₂O₃ particles are not present inside the crater. The change of surface morphology is accompanied by elemental redistribution. Multipoint XPS analysis evidenced that the concentration of La is very low in the crater and increases moving toward the border of the affected zone while that of W shows an opposite trend. The composition changes involve only the outmost 5 nm of the sample: through depth profiling, no differences of chemical composition were detected deeper in the alloy between the center and external border of the affected area.     

The influence of laser wavelength and fluence on palladium nanoparticles produced by pulsed laser ablation in deionized water

Homogeneous spherical palladium (Pd) nanoparticles were synthesized by pulsed laser ablation of a solid Pd foil target submerged in deionized water, without the addition of any external chemical surfactant. The influence of laser wavelength (355, 532, and 1064 nm) and fluence (8.92, 12.74, and 19.90 J/cm²) on nucleation, growth, and aggregation of Pd nanoparticles were systematically studied. Microstructural and optical properties of the obtained nanoparticles were studied by field emission transmission electron microscopy (FETEM), energy dispersive X-ray spectroscopy, and UV–vis spectroscopy. FETEM micrographs indicate that the average nanocrystallite sizes are relatively low (3–6 nm) and homogeneous for the particles synthesized at the laser wavelengths of 355 and 532 nm. However, at a laser wavelength of 1064 nm, the average nanocrystallite size is relatively large and inhomogeneous in nature. Moreover, we observe that the mean diameter and production rate of particles increases with an increase in laser fluence. The selected area electron diffraction patterns obtained from isolated Pd nanoparticles show the characteristic diffused electron diffraction rings of polycrystalline materials with a face-centered cubic structure. Absorbance spectrum of the synthesized nanoparticle solution shows a broad absorption band, which corresponds to a typical inter-band transition of a metallic system, indicating the production of pure palladium nanoparticles. The present work provides new insights into the effect of laser wavelength and fluence on the control of size and aggregation of palladium nanoparticles in the liquid medium.     

Gold nanoparticles fabricated by pulsed laser ablation in supercritical CO2

Nanosecond pulsed laser ablation (PLA) of gold plate with an excitation wavelength of 532?nm was carried out in supercritical CO₂ (scCO₂) to fabricate gold nanoparticles. Surface morphology of the gold plate after irradiation and the crater depth after PLA were observed by scanning electron microscopy and laser scanning microscopy, while extinction spectra of gold nanoparticles collected in the glass slide was measured by UV?CVis spectrophotometer. The gold plate was ablated at various scCO₂ densities and irradiation times at constant temperature of 40??C. The ablation was also conducted at atmospheric condition with air to evaluate the environmental dependence of ablation. Both surface morphology of the irradiated gold plate and crater depth formation were significantly affected by the changes in scCO₂ density, the surrounding environment, and irradiation time. As expected, the increasing scCO₂ density resulted in a deeper ablation crater, however, the deepest crater was obtained at a density of 0.63?g/cm³ or pressure of 10?MPa. Gold nanoparticles generated by PLA in scCO₂ have been confirmed at the spectra band near 530?nm.     

Nickel nanoparticles generated by pulsed laser ablation in liquid CO2

Research on Chemical Intermediates - Nickel nanoparticles with various structures were synthesized by a pulsed laser ablation process in liquid CO2 at 17 °C and 5.2 MPa. A...     

Artificial construction of the layered Ruddlesden-Popper manganite La2Sr2Mn3O10 by reflection high energy electron diffraction monitored pulsed laser deposition

Pulsed laser deposition has been used to artificially construct the n = 3 Ruddlesden-Popper structure La(2)Sr(2)Mn(3)O(10) in epitaxial thin film form by sequentially layering La(1-x)Sr(x)MnO(3) and SrO unit cells aided by in situ reflection high energy electron diffraction monitoring. The interval deposition technique was used to promote two-dimensional SrO growth. X-ray diffraction and cross-sectional transmission electron microscopy indicated that the trilayer structure had been formed. A site ordering was found to differ from that expected thermodynamically, with the smaller Sr(2+) predominantly on the R site due to kinetic trapping of the deposited cation sequence. A dependence of the out-of-plane lattice parameter on growth pressure was interpreted as changing the oxygen content of the films. Magnetic and transport measurements on fully oxygenated films indicated a frustrated magnetic ground state characterized as a spin glass-like magnetic phase with the glass temperature T(g) ≈ 34 K. The magnetic frustration has a clear in-plane (ab) magnetic anisotropy, which is maintained up to temperatures of 150 K. Density functional theory calculations suggest competing antiferromagnetic and ferromagnetic long-range orders, which are proposed as the origin of the low-temperature glassy state.     

Wet-type Fe2O3 solar cells based on Fe2O3 films prepared by laser ablation: Drastic temperature effect

A wet-type solar cell based on Fe₂O₃ films prepared by the laser ablation method was investigated. The performance was increased drastically by the thermal treatment of the Fe₂O₃/FTO films. This enhancement effect is caused by the improvement of the binding interface between FTO and Fe₂O₃.     

Generation of Ag2O micro-/nanostructures by pulsed excimer laser ablation of Ag in aqueous solutions of polysorbate 80

A new route to synthesis of Ag(2)O micro-/nanostructures, including a mixture of cubes, pyramids, triangular plates, pentagonal rods, and bars, has been developed by pulsed excimer laser ablation of bulk silver in water using polysorbate 80 as surfactant. The polysorbate 80 played an important role in the formation of the Ag(2)O structures, and similar structures could be obtained in polysorbates 20 and 40 aqueous solutions. We have proposed a mechanism to explain the formation of Ag(2)O structures. This laser ablation method provides a unique approach to discover and fabricate new Ag(2)O morphologies.     

Metal particles produced by laser ablation for ICP-MS measurements

Pulsed laser ablation (266 nm) was used to generate metal particles of Zn and Al alloys using femtosecond (150 fs) and nanosecond (4 ns) laser pulses with identical fluences of 50 J cm⁻². Characterization of particles and correlation with inductively coupled plasma mass spectrometer (ICP-MS) performance was investigated. Particles produced by nanosecond laser ablation were mainly primary particles with irregular shape and hard agglomerates (without internal voids). Particles produced by femtosecond laser ablation consisted of spherical primary particles and soft agglomerates formed from numerous small particles. Examination of the craters by white light interferometric microscopy showed that there is a rim of material surrounding the craters formed after nanosecond laser ablation. The determination of the crater volume by white light interferometric microscopy, considering the rim of material surrounding ablation craters, revealed that the volume ratio (fs/ns) of the craters on the selected samples was approximately 9 (Zn), 7 (NIST627 alloy) and 5 (NIST1711 alloy) times more ablated mass with femtosecond pulsed ablation compared to nanosecond pulsed ablation. In addition, an increase of Al concentration from 0 to 5% in Zn base alloys caused a large increase in the diameter of the particles, up to 65% while using nanosecond laser pulses. When the ablated particles were carried in argon into an ICP-MS, the Zn and Al signals intensities were greater by factors of ∼50 and ∼12 for fs versus ns ablation. Femtosecond pulsed ablation also reduced temporal fluctuations in the ⁶⁶Zn transient signal by a factor of 10 compared to nanosecond laser pulses.     

Glass particles produced by laser ablation for ICP-MS measurements

Pulsed laser ablation (266 nm) was used to generate glass particles from two sets of standard reference materials using femtosecond (150 fs) and nanosecond (4 ns) laser pulses with identical fluences of 50 J cm⁻². Scanning electron microscopy (SEM) images of the collected particles revealed that there are more and larger agglomerations of particles produced by nanosecond laser ablation.In contrast to the earlier findings for metal alloy samples, no correlation between the concentration of major elements and the median particle size was found. When the current data on glass were compared with the metal alloy data, there were clear differences in terms of particle size, crater depth, heat affected zone, and ICP-MS response. For example, glass particles were larger than metal alloy particles, the craters in glass were less deep than craters in metal alloys, and damage to the sample was less pronounced in glass compared to metal alloy samples. The femtosecond laser generated more intense ICP-MS signals compared to nanosecond laser ablation for both types of samples, although glass sample behavior was more similar between ns- and fs-laser ablation than for metal alloys.     

Preparation of La2O3-TiO2-SiO2 Inorganic-Organic Hybrid Materials and Their Optical Properties

Inorganic-organic hybrid materials in La₂O₃-TiO₂-SiO₂ system were prepared from La(NO₃)₃, titanium tetraisopropoxide, and silicon alkoxides. Ternary transparent materials prepared from glycidyloxypropyltrimethoxysilane (GPTS) were obtained in a wide composition region (La₂O₃ 50 mol%, TiO₂ 40 mol%). Hybrid materials prepared using methyltrimethoxysilane (MTMS) at MTMS/TMOS = 1 were transparent in the composition of La₂O₃ 30 mol% and TiO₂ 25 mol%. La(NO₃)₃ crystallines or TiO₂ small particles were precipitated in translucent materials. La₂O₃ content strongly affected the bulk density compared with TiO₂ content. Absorption edge of the hybrid materials was shifted to longer wavelength as TiO₂ and La₂O₃ contents increased. Partial coefficient of refractive index for the three metal oxides increased in the order SiO₂ < TiO₂ < La₂O₃ for both hybrid materials.     

Diagnostics of silicon plasmas produced by visible nanosecond laser ablation

The second harmonic of a pulsed Nd:YAG laser (532 nm) has been used for the ablation of silicon samples in air at atmospheric pressure. In order to study the interaction for silicon targets, the laser-induced plasma characteristics were examined in detail with the use of a space- and time-resolved technique. Electron temperatures, ionic temperatures and electron number densities were determined. A discussion of thermodynamic equilibrium status of the silicon-microplasma is presented. Electron number densities are deduced from the Stark broadening of the line profiles of atomic silicon. Plasma ionization and excitation temperatures were determined from the Boltzmann plot and the Saha–Boltzmann equation, respectively. A limited number of suitable silicon lines for the studies of temperatures were found and the effect of these lines on the temperature measurements is discussed. Electron temperatures in the range of 6000–9000 K and ionic temperatures of 12 000–17 000 K with electron number densities of the order of 10¹⁸ cm⁻³ were observed. The breakdown threshold fluence has been also measured. Silicon plasmas were also characterized in terms of their morphology (shape and size) as a function of laser energy and delay time.     

Direct observation of a 1MLCT state by ultrafast transient absorption spectroscopy in Mo2(O2C-9-anthracene)4

We have observed the excited-state dynamics of Mo2(O2C-9-anthracene)4 in THF using ultrafast transient UV/vis absorption spectroscopy. Following excitation at 514.5 nm into the metal-to-ligand charge-transfer (MLCT) state, transient absorption bands of the 1MLCT state appear at 410 and 610 nm. We assign these features to the 1MLCT state, which has a lifetime of approximately 10 ps. The decay of 1MLCT is accompanied by the formation of the triplet 3MLCT state, with an absorption band peaking at 410 nm. Excitation at 347 nm populates directly the ligand-centered (LC) state. From the rise of the 1MLCT band, the lifetime of the 1LC state is estimated to be less than 1 ps.     

Femtosecond time-resolved transient absorption spectroscopy of xanthophylls

Xanthophylls are a major class of photosynthetic pigments that participate in an adaptation mechanism by which higher plants protect themselves from high light stress. In the present work, an ultrafast time-resolved spectroscopic investigation of all the major xanthophyll pigments from spinach has been performed. The molecules are zeaxanthin, lutein, violaxanthin, and neoxanthin. beta-Carotene was also studied. The experimental data reveal the inherent spectral properties and ultrafast dynamics including the S(1) state lifetimes of each of the pigments. In conjunction with quantum mechanical computations the results address the molecular features of xanthophylls that control the formation and decay of the S* state in solution. The findings provide compelling evidence that S* is an excited state with a conformational geometry twisted relative to the ground state. The data indicate that S* is formed via a branched pathway from higher excited singlet states and that its yield depends critically on the presence of beta-ionylidene rings in the polyene system of pi-electron conjugated double bonds. The data are expected to be beneficial to researchers employing ultrafast time-resolved spectroscopic methods to investigate the mechanisms of both energy transfer and nonphotochemical quenching in higher plant preparations.     

Optical properties and valence change of europium ions in a sol-gel Al2O3-B2O3-SiO2 glass by femtosecond laser pulses

The Al2O3-B2O3-SiO2 glass containing europium ions was prepared by a sol-gel method. Fluorescence line-narrowing spectra (FLN) indicate two different environments of the Eu 3+ ions. The calculated second crystal-field parameters exhibit the opposite behaviors of the two different environments. The FLN excitation and emission spectra before and after irradiation show that the change of the emission mainly comes from the Eu 3+ ions at site I, revealing that the concentration ratio of the Eu 3+ ions at site I to site II was decreased. The emission spectra confirmed that some Eu 3+ ions were reduced into Eu 2+ ions. The excitation spectra indicate that the Eu 3+ ions at the sites with higher covalence degree can be easily reduced, implying that the Eu 3+ ions are more easily reduced at site I than at site II. The absorption spectra before and after irradiation exhibit that the absorption of Eu 2+ ions increases and that the positive hole centers appear. These results suggest a mechanism of the formation of the Eu 2+ ions by femtosecond laser irradiation.     

Photodissociation of thioglycolic acid studied by femtosecond time-resolved transient absorption spectroscopy

Steady-state and time-resolved spectroscopies were employed to study the photodissociation of both the neutral (HS-CH(2)-COOH) and doubly deprotonated ((-)S-CH(2)-COO(-)) forms of thioglycolic acid (TGA), a common surface-passivating ligand used in the aqueous synthesis and organization of semiconducting nanostructures. Room temperature UV-Vis absorption spectroscopy indicated strong absorption by the S(1) and S(2) excited states at 250 nm and 185 nm, respectively. The spectrum also contained a weaker absorption band that extended to approximately 550 nm, which was assigned to the π(CO) (*)←n(O) transition. Femtosecond time-resolved transient absorption spectroscopy was performed on TGA using 400 nm excitation and a white-light continuum probe to provide the temporally and spectrally resolved data. Both forms of TGA underwent a photoinduced dissociation from the excited state to form an α-thiol-substituted acyl radical (α-TAR, S-CH(2)-CO(●)). For the acidic form of TGA, radical formation occurred with an apparent time constant of 60 ± 5 fs; subsequent unimolecular decay took 400 ± 60 fs. Similar kinetics were observed for the deprotonated form of TGA (70 ± 10 fs radical formation; 420 ± 40 fs decay). The production of the α-TAR was corroborated by the observation of its characteristic optical absorption. Time-resolved data indicated that the photoinduced dissociation of TGA via cleavage of the C-OH bond occurred rapidly (≤100 fs). The prevalence of TGA in aqueous semiconducting nanoparticles makes its absorption in the visible spectral region and subsequent dissociation key to understanding the behavior of nanoscale systems.     

Detection of transient laser magnetic resonance signals in pulsed CO2 laser irradiation of SF6/H2O and SF6/NO mixtures

A technique for detecting transient far-infrared laser magnetic resonance (LMR) signals, induced by pulsed CO₂ laser photolysis, has been developed. The method is illustrated with preliminary data from photolysis of SF₆ containing H₂O or NO. In the former mixture. OH(X ²Π) formation and decay is observed, while in the latter, a transient decrease in NO(X ²Π) concentration followed by a return to the original value is observed. Possible physical and chemical mechanisms are discussed.     

A facile method of producing femtoliter metal cups by pulsed laser ablation

Cuplike structures of Au, Ag, Cu, Zn, Nb, Cd, Al, In, and Sn in the size range of 300 nm to a few micrometers with an internal volume of a few femtoliters have been produced by the laser ablation of metal targets in a vacuum, by optimizing, in each case, the laser fluence and the substrate temperature. The metal droplets impinging on the substrate seem to undergo a hydraulic jump driven by the surface tension forces before solidifying into cups. The cups are robust and can be functionalized with biomarkers, filled with nanoparticle sols, oxidized to crucibles, or detached from the substrate without causing any deformation. We envisage their potential applications as femtoliter metal containers.     

Methane Decomposition over Ni/α-Al2O3 Promoted by La2O3 and CeO2

The decomposition of methane on Ni/a-Al2O3 modified by La2O3 and CeO2 with differ-ent contents has been investigated and the ralationship between methane decomposition and removal of carbon by CO2 over these catalyst has also been studied by pulse-chromatography. The catalysts were characterized by TPR and XRD. It was shown that Ni/a-Al2O3 could be promoted by adding La2O3, and the carbon species produced over this catalyst was activated and eliminated by CO2. But CeO2 would suppress the decomposition of methane over Ni crystallite. Both La2O3 and CeO2 can inhibit aggregation of the Ni particles. Decomposition of methane over the Ni-based catalysts is structure sensitive to a certain extent.     

Methane Decomposition over Ni/α-Al_2O_3 Promoted by La_2O_3 and CeO_2

The decomposition of methane on Ni/a-Al2O3 modified by La2O3 and CeO2 with different contents has been investigated and the ralationship between methane decomposition and removal of carbon by CO2 over these catalyst has also been studied by pulse-chromatography. The catalysts were characterized by TPR and XRD. It was shown that Ni/a-Al2O3 could be promoted by adding La2O3, and the carbon species produced over this catalyst was activated and eliminated by CO2. But CeO2 would suppress the decomposition of methane over Ni crystallite. Both La2O3 and CeO2 can inhibit aggregation of the Ni particles. Decomposition of methane over the Ni-based catalysts is structure sensitive to a certain extent.