Diagnostic study of laser ablated YBa2Cu3Oy plumes

Post-ablation ionisation in conjunction with a reflectron time-of-flight mass spectrometer has been used to investigate a number of species in the ablation plume from a YBa₂Cu₃O_y target. The experiments were carried out using a Q-switched Nd:YAG laser with typical intensities of ≈ 10⁸ W cm⁻² characteristic of the fluences (1 J cm⁻²) required for the pulsed laser deposition of thin superconducting films. By varying the delay between the ablation and the ionisation laser, the velocity distributions of several of the species from the target have been measured simultaneously. It has been observed that, although some of the atoms and molecules (i.e. Cu, Ba and BaO) have similar velocities, the atoms and oxides of Y (Y and YO) have very different velocities. The yttrium atoms and oxides were observed to be slower than the barium atoms and oxides at both ablation wavelengths examined (355 and 532 nm) and at two different distances from the target surface (2 and 3 mm). It is suggested that Ba, Cu and their oxides are ablated directly from the surface as neutrals, whereas Y and YO form clusters in the ablation plume. These clusters are then fragmented by the post-ionisation laser to produce Y and YO ions.     

Ground state C2 density measurement in carbon plume using laser-induced fluorescence spectroscopy

The temporal evolution and spatial distribution of C₂ molecules produced by laser ablation of a graphite target is studied using optical emission spectroscopy, dynamic imaging and laser-induced fluorescence (LIF) investigations. We observe peculiar bifurcation of carbon plume into two parts; stationary component close to the target surface and a component moving away from the target surface which splits further in two parts as the plume expands. The two distinct plumes are attributed to recombination of carbon species and formation of nanoparticles. The molecular carbon C₂ moves with a faster velocity and dies out at ~ 800 ns whereas the clusters of nanoparticle move with a slower velocity due to their higher mass and can be observed even after 1600 ns. C₂ molecules in the d³Π_g state were probed for laser-induced fluorescence during ablation of graphite using the Swan (0,0) band at 516.5 nm. The fluorescence spectrum and images of fluorescence d³Π_g − a³Π_u(0,1)(λ = 563.5 nm) are recorded using a spectrograph attached to the ICCD camera. To get absolute ground state C₂ density from fluorescence images, the images are calibrated using complimentary absorption experiment. This study qualitatively helps to get optimum conditions for nanoparticle formation using the laser ablation of graphite target and hence deducing optimum conditions for thin film deposition.     

Reaction Dynamics of Electronically Excited Alkali Atoms with Simple Molecules

The reactions of electronically excited sodium atoms with simple molecules have been studied in crossed molecular beams experiments. Electronically excited Na(3²P_3/2,4²D_5/2, and 5²S_1/2) were produced by optical pumping using single frequency dye lasers. The effects of the symmetry, and the orientation and alignment of the excited orbital on the chemical reactivity, and detailed information on the reaction dynamics were derived from measurements of the product angular and velocity distributions.     

Plasma diagnostics of barrier-torch discharge in argon flow in a capillary by cross-correlation spectroscopy

The radiation kinetics of the plasma of barrier-torch disrcharge in argon flow in a capillary has been studied by cross-correlation spectroscopy. It was established that the discharge emission spectrum consists of peaks of electronically excited states of argon, bands of hydroxyl radicals, and a second positive system of nitrogen. An analysis of the spatio-temporal distributions of emission intensity for the selected spectral indicators showed that the causes of the torch are ionization waves that extend through the capillary from the electrode system with a speed of 10⁵ m/s and project up to 3–4 mm. It was established that the formation of electronically excited molecules of nitrogen N₂(C ³Π _u ) in the torch of discharge occurs mainly on the reaction between metastable electronically excited atoms of argon and molecules of nitrogen in the electronic ground state.     

Resonantly laser induced plasmas in gases: The role of energy pooling and exothermic collisions in plasma breakdown and heating

The present work is a systematic experimental study of the plasma formation in cesium vapor induced by a continuous laser tuned to the resonance transition 6S_1/2–6P_3/2. Taking into account the measured absolute population densities of Cs ground and excited state atoms as well as the electron densities derived from Stark broadening of the Cs lines, complete local thermodynamic equilibrium in the laser-produced plasma was found for laser power densities ≈ 10 Wcm^− 2 at cesium ground state number densities of about 10¹⁷ cm^− 3. Direct conversion of the excitation energy or parts of the excitation energy in exothermic collisions of laser-excited atoms is concluded to be the major process for atomic vapor heating and subsequent formation of LTE plasmas.     

A study of the collisional quenching of O(21D2) by the noble gases employing time-resolved attenuation of atomic resonance radiation in the vacuum ultraviolet

Electronically excited oxygen atoms O(2¹D₂) have been generated by the pulsed irradiation of ozone in the Hartley-band continuum and monitored photoelectrically in absorption by time-resolved attenuation of atomic resonance radiation at λ = 115.2 nm [O(3¹D₂°) → O(2¹D₂)]. Collisional quenching of the excited atom has been investigated for all the noble gases, and the first absolute values for the second-order deactivation rate constants are reported. The resulting rate data are discussed in terms of a curve-crossing mechanism based on existing spectroscopic data for the noble gas oxides. The absolute rate constants are compared with previous relative rate data for the deactivation of O(2¹D₂) by the noble gases.     

Time-of-flight mass spectroscopy of femtosecond and nanosecond laser ablated TeO2 crystals

Single-pulse femtosecond (fs) (pulse duration ～200 fs, wavelength 398 nm) and nanosecond (ns) (pulse duration 4 ns, wavelength 355 nm) laser ablation have been applied in combination with time-of-flight mass spectrometer (TOFMS) to analyze the elemental composition of the plasma plume of single-crystalline telluria (c-TeO₂, grown by the balance controlled Czochralski growth method). Due to the three-order difference of the peak intensities of the ns and fs-laser pulses, significant differences were observed regarding the laser-induced species in the plasma plume. Positive singly, doubly and triply charged Te ions (Te⁺, Te₂⁺, Te₃⁺) in the form of their isotopes were observed in case of both irradiations. In case of the ns-laser ablation the TeO⁺ formation was negligible compared to the fs case and there was no Te trimer (Te₃⁺) formation observed. It was found that the amplitude of Te ion signals strongly depended on the applied laser pulse energy. Singly charged oxygen ions (O⁺) are always present as a byproduct in both kinds of laser ablation.     

Sampling modulation technique in radio-frequency helium glow discharge emission source by use of pulsed laser ablation

An emission excitation source comprising a high-frequency diode-pumped Q-switched Nd:YAG laser and a radio-frequency powered glow discharge lamp is proposed. In this system sample atoms ablated by the laser irradiation are introduced into the lamp chamber and subsequently excited by the helium glow discharge plasma. The pulsed operation of the laser can produce a cyclic variation in the emission intensities of the sample atoms whereas the plasma gas species emit the radiation continuously. The salient feature of the proposed technique is the selective detection of the laser modulation signal from the rest of the continuous background emissions, which can be achieved with the phase sensitive detection of the lock-in amplifier. The arrangement may be used to estimate the emission intensity of the laser ablated atom, free from the interference of other species present in the plasma. The experiments were conducted with a 13.56 MHz radio-frequency (rf) generator operated at 80 W power to produce plasma and the laser at a wavelength of 1064 nm (pulse duration:34 ns, repetition rate:7 kHz and average pulse energy of about 0.36 mJ) was employed for sample ablation. The measurements resulted in almost complete removal of nitrogen molecular bands (N₂⁺ 391.44 nm). Considerable reduction (about 75%) in the emission intensity of a carbon atomic line (C I 193.03 nm) was also observed.     

Crossed-beám chemiluminescence. II. Kinetics and mechanisms for reactions of group IIA Metals in ground and metastable states with fluorine

Visible chemiluminescence technique under crossed-beam conditions has been applied to the study of the reactions of the group IIA metal atoms Mg, Ca, Sr and Ba in their ground state (¹S₀) or in an excited metastable state (³P_i, ³D_i or ¹D₂) with F₂. The monofluoride emission bands are the most prominent features in the chemiluminescence spectrum. Using higher fluorine densities radiation is observed from excited alkaline earth difluorides, which are shown to principally originate from secondary reactions. Formations of MF* are determined to be first order with respect to both the metal atom and the fluorine molecule. Total cross sections for removal of ground state metal atoms from the beam by F₂ are 115 ± 15Ų for Ca, 125 ± 15Ų for Sr, and 160 ± 15Ų for Ba, which is consistent with an electron jump model. Chemiluminescence cross sections are reported for the reactions involving electronically excited reactants M*. Photon yields of 12 ± 3% for Mg*, 18 ± 5% for Ca*, 20 ± 5% for Sr*, and 15 ± 8% for Ba* reacting with F₂ are measured. These high photon yields are remarkable when compared with absolute photon yields for the ground state reactions which indicate that less than 2% of the products are MF* molecules. It was possible to obtain vibrational state distributions for some of the excited monofluorides which are found to be populated in a non-thermal manner. This strongly suggests that the dynamics of the reactions are governed by a direct mechanism. From the crossed-beam chemiluminescence spectra the dissociation energies of the ground state monofluorides are estimated. In addition, improved spectroscopic constants, dissociation energies and dissociation products of some of the excited electronic states of MF are given.     

Dynamics of excited and ionic states of N,N,N′,N′-tetramethyl-p-phenylenediamine in poly(methyl methacrylate) under ablation condition

The nanosecond transient dynamics of N,N,N′,N′-tetramethyl-p-phenylenediamine in poly(methyl methacrylate) has been investigated under ablation condition. Time-resolved spectroscopic data can be interpreted in terms of the dynamics of the lowest excited singlet and triplet states and the cation of TMPD. The decay processes of all the detected transients became rapid with an increase in the laser fluence. It is shown directly that TMPD absorbs about eight 248 nm photons during a nanosecond laser pulse at the ablation threshold of 220 mJ/cm². These results are well explained by cyclic multiphotonic absorption mechanism where dopant transients work as repetitive absorbers of laser photons.     

Cyanides,Isocyanides, and Hydrides of Zn,Cd and Hg from Metal Atom and HCN Reactions: Matrix Infrared Spectra and Electronic Structure Calculations

Zinc and cadmium atoms from laser ablation of the metals and mercury atoms ablated from a dental amalgam target react with HCN in excess argon during deposition at 5 K to form the MCN and MNC molecules and CN radicals. UV irradiation decreases the higher energy ZnNC isomer in favor of the lower energy ZnCN product. Cadmium and mercury atoms produce analogous MCN primary molecules. Laser ablation of metals also produces plume radiation which initiates H-atom detachment from HCN. The freed H atom can add to CN radical to produce the HNC isomer. The argon matrix also traps the higher energy but more intensely absorbing isocyanide molecules. Further reactions with H atoms generate HMCN and HMNC hydrides, which can be observed by virtue of their C−N stretches and intense M−H stretches. Computational modeling of IR spectra and relative energies guides the identification of reaction products by providing generally reliable frequency differences within the Zn, Cd and Hg family of products, and estimating isotopic shifts using to ¹³C and ¹⁵N isotopic substitution for comparison with experimental data.     

Rate coefficient measurements for reactions between uracil and N(2D0) atoms

Rate coefficients of the reactions between uracil molecules and excited nitrogen atoms (N(²D₀)) have been measured at room temperature using the dynamic flowing afterglow method. The measured rate coefficients are 3.3 and 2.7 × 10⁻¹⁰ cm³ s⁻¹.     

Two-photon excitation of mercury atoms by photodissociation of mercury halides

Photodissociation of HgCl₂, HgBr₂, and Hgl₂ with an ArF laser at 193 nm produces strong fluorescence from highly excited Hg atoms. The experiments indicate that single photon dissociation of the mercuric halide, HgX₂, is followed by the two-photon dissociation of HgX (X² Σ) molecules to produce electronically excited Hg atoms.     

Thermoemission of singlet oxygen and chemical structure of copper oxides

Thermal treatment of copper oxides (CuO, Cu₂O) is accompanied by large-scale emission of singlet oxygen molecules (¹Σ⁺ _g ). Electron spectroscopy for chemical analysis (ESCA) and electronic and IR spectroscopy were used to show that the thermoemission of electronically excited molecules results from dark generation of electronically excited states which contain in their structure isolated metal-metal bonds and oxygen associates. The anomalous diamagnetic response of the samples and reduced thermoemission activity (Cu₂O) are associated with cooperative interaction of electronically excited states.     

四叔丁基萘酞菁铅化合物的非线性和光限幅特性

合成了四叔丁基萘酞菁铅[(t-Bu)₄NcPb]化合物. 利用元素分析、红外光谱、核磁共振氢谱等方法, 验证了化合物的分子结构. 应用调Q倍频ns/ps Nd:YAG脉冲激光系统, 在波长为532 nm下, 研究了化合物的非线性和光限幅特性. 测得化合物的非线性折射率n₂和三阶非线性极化率χ⁽³⁾分别为2.42×10^－11和7.91×10^－12 esu, 通过计算得到分子极化率γ'为3.4×10^－29 esu. 在透过率69%时限幅阈值为1522 mJ/cm², 箝位值为553 mJ/cm², 有效激发态与基态吸收截面比为3.16.     

Investigation of the dynamic of an expanding laser plume by a shadowgraphic technique

The new generation of EUV sources for lithography, based on a high current z-pinch, exploits the laser ablation of a Sn target for triggering of a discharge and delivery of working material. The dynamic of the Sn plume expansion during the first 120 ns, which strongly affects the later behavior of z-pinch was investigated by a shadowgraphic technique. Radiation in a spectral range from 18 to 20 nm was used for detection of shadow images of the Sn plume because 20 nm radiation is absorbed by the inner shells of neutrals and first ions. Thus, the probing beam is efficiently absorbed by the species most important in the formation and evolution of z-pinch. Images of the Sn plume were detected at 22 ns, 55 ns, 88 ns and 120 ns delays after the plume ignition. The technique enabled the observation of the dynamic of Sn species expansion within a range of 2 mm from the target surface. A software for the processing the detected images was developed. The estimated total number of ablated Sn neutrals and first ions varied from ∼ 2–4 × 10¹⁴ for intensities of the focused ablating pulse in the range 8 × 10⁸–10¹⁰ W/cm². The experimentally detected dynamic of Sn plume expansion and total number of ablated Sn species coincide reasonably with simulated data.     

Vibrational and rotational relaxation of NH3 studied in a molecular jet by infrared-infrared double resonance

Infrared-infrared double resonance is applied to an expanding jet of NH₃. Molecules in the a(3,3) rotational level of the vibrational ground state are excited with a CO₂-laser into the s(4,3) level of the υ₂ vibrational state, 2s(4,3). Rotational distributions in the ground and υ₂ state are probed by the infrared absorption of color-center-laser radiation producing transitions to the υ₁ and υ₁ + υ₂ states. The time delay between pumping and probing is determined by the distance between the CO₂ laser focus and the color-center-laser focus, along the jet axis. The results indicate an average relaxation time of vibration to rotation/translation of 280 (50) ns Torr* over the temperature range 50–110 K. In that temperature range the population difference between ortho and para species comes into equilibrium within 425(190) ns Torr*, and the relaxation between (3,3) and other ortho ground state levels occurs within 56(20) ns Torr* and between the 2s(4,3) and 2s(3,3) levels within 20(8) ns Torr*. The inversion relaxation time between a(3,3) and s(3,3) is determined to 7.6 (20) ns Torr*, at 50 K. (Here 1 Torr* indicates a density, 1 Torr* = 3.27 × 10²² molecules/m³).     

Emission spectra of the radical cations of 1,3-dichlorobenzene, 1,4-dichlorobenzene and 1,3,5-trichlorobenzene in the gas phase

Emission spectra of the radical cations of 1,3-dichlorobenzene, 1,4-dichlorobenzene-h₄ (and -d₄), and 1,3,5-trichlorobenzene, excited in the gas phase by controlled electron impact, are presented. The band systems, which lie in the 500–750 nm wavelength region, are attributed to the B?(π^?1) → X?(π^?) electronic transition of the cations on the basis of photoelectron spectroscopic data. The NeI excited photoelectron spectra and the ionisation energies of chloro-,o-,m-,p-dichloro- and 1,3,5-trichlorobenzene have been obtained. The information acquired from the emission and photoelectron spectra is discussed and compiled to deduce the symmetry of the B? states. Emission, with quantum yield > 10^?5, could not be detected with electronically excited radical cations of chloro-,o-dichloro-, 1,2,4- and 1,2,3-trichloro- and tetrachloro-benzenes. This is attributed to the nature of the B? states, which arise by σ^?1 ionisation processes. The lifetimes of the zeroth and some vibrationally excited levels of the B?(π^?1) states were also measured and found to be 22 ± 2 ns for 1,3,5-trichlorobenzene cation and < 6 ns for 1,3- and 1,4-dichlorobenzene cations. The lifetimes of the latter two electronically excited cations are estimated to be two orders of magnitude shorter than 6 ns from the measurement of the relative emission intensities of the B? → B? band systems of the three title cations.     

Temporal evolution study of the plasma induced by CO2 pulsed laser on targets of titanium oxides

This paper reports studies on time-resolved laser induced breakdown spectroscopy (LIBS) of plasmas induced by IR nanosecond laser pulses on the titanium oxides TiO and TiO₂ (anatase). LIBS excitation was performed using a CO₂ laser. The laser-induced plasma was found strongly ionized yielding Ti⁺, O⁺, Ti^2 +, O^2 +, Ti^3 +, and Ti^4 + species and rich in neutral titanium and oxygen atoms. The temporal behavior of specific emission lines of Ti, Ti⁺, Ti^2 + and Ti^3 + was characterized. The results show a faster decay of Ti^3 + and Ti^2 + ionic species than that of Ti⁺ and neutral Ti atoms. Spectroscopic diagnostics were used to determine the time-resolved electron density and excitation temperatures. Laser irradiation of TiO₂-anatase induces on the surface sample the polymorphic transformation to TiO₂-rutile. The dependence on fluence and number of irradiation pulses of this transformation was studied by micro-Raman spectroscopy.     

Potentiometric Study of Reactions of Titanium Complexes in Phosphate–Perchlorate Acid Solutions

The potentiometric method is used to measure the equilibrium potential in the Ti(IV)/Ti(III) system and determine that monophosphate Ti(IV) complexes and Ti³⁺hydrated complexes dominate in phosphate–perchlorate acid solutions, 4M(H, Na)ClO₄, at of 5 × 10^–2to 4 × 10^–1M. Equations that describe the total electrode reaction are proposed. Decreasing the concentration of free hydrogen ions from 3 to 0.12 M results in the deprotonation of TiO(H₂PO₄)⁺complexes and the formation of TiO(HPO₄) complexes. Equilibrium constants for reactions of the formation of Ti(IV) monophosphate complexes and the protonation of TiO(HPO₄) complex are calculated.