Time evolution of the pulsed HF chemical laser system. II. Irreversible thermodynamic analysis

The time rates of change of level populations and radiation densities derived from a detailed kinetic model of the F + H₂ → HF + H laser are employed as input data for a time dependent thermodynamic analysis of this system. The laser is regarded as an irreversible heat engine generating thermodynamic work in the form of laser light. The development in time of the thermodynamic functions, efficiency and irreversible entropy production is determined by computing the contributions of pumping, radiation and relaxation to the entropy and energy of the lasing molecules. Effects of specific rate processes are evaluated by considering different kinetic schemes, i.e. different combinations of kinetic processes and initial conditions. It is shown, among others, that a laser without relaxation processes (“frictionless”) has poor efficiency despite the absence of energy losses and the low irreversible entropy production. On the other hand, the efficiency is high in lasers governed by fast rotational relaxation. This is because rotational relaxation, though leading to some energy losses and irreversible entropy production, compensates for the entropy decrease of the system (while lasing under partially inverted populations) by increasing the bath entropy. The major general conclusion of the analysis is that the thermodynamic constraints related to the kinetic scheme and not the extent of irreversibility of the lasing process is the crucial factor in determining the laser efficiency.     

The effect of two isotopic species on the performance of an HX chemical laser

The influence of the presence of two isotopic on the performance of an HX chemical laser is investigated by means of a computer simulation of the system. The isotopic effect is shown to be important at low gas pressures and high threshold values, but of less importance for lasers operating under high output power conditions.     

The significance of transfer reactions in pulsed laser polymerization experiments

In this work simulation calculations are presented, which carefully analyze pulse initiated polymerization experiments of butyl acrylate in bulk in an extended temperature range published in the recent literature. Taking into account entire data sets of experimental results, a model has been developed which describes the experiments using a single parameter set. Especially the inadequacies in experiments are also captured by the model, that occur evaluating propagation rate coefficients using the pulsed laser polymerization technique at low laser repetition rates and elevated temperatures. Enhanced chain transfer to small species is identified to be responsible for these effects and transfer rate constants are derived from the simulations. The model is then used to test experimental strategies in order to expand the k_p determination towards temperatures higher than 35°C, the maximum temperature for which k_p values of butyl acrylate are available so far. Performing pulsed laser experiments at high laser repetition rates (200 Hz) and initial radical concentrations (1 · 10⁻⁴ mol/L) should prevent the formation of the characteristic structure in the molecular weight distribution to be suppressed by this competing process.     

Surface-interface investigations of an ultrathin pulsed laser deposited NiO/ZnO bilayer structure

We hereby report detailed structural and morphological studies for an ultrathin NiO/ZnO bilayer structure grown on sapphire (001) substrate using pulsed laser deposition technique. The combined X-ray reflectivity (XRR) and grazing incidence X-ray fluorescence (GIXRF) studies revealed formation of a low-density defective ZnO interfacial layer of thickness ~32 Å at the ZnO/sapphire interface prior to growth of main ZnO layer. Our results further indicate that the variation of electron density across the NiO/ZnO bilayer structure is smooth and we do not observe presence of any interface layer between them. X-ray diffraction measurements show that deposited ZnO layer is epitaxial in nature whereas NiO is highly oriented along (100) direction. The angle dependent X-ray absorption near edge fine structure (XANES) measurements at Ni–K edge has been utilized to determine depth-resolved oxidation state of Ni and the results have been correlated with the depth-resolved electron density of NiO layer. The method described here offers nondestructive determination of the microstructural parameters as well as depth-resolved mapping of oxidation state of a thin film-based heterojunction device. It extends several advantages over destructive methods which are abundantly reported in literature.     

Ternary molybdenum cluster sulfides: electrochemical and chemical behavior of in situ pulsed laser deposited thin films

The electrochemical deinsertions and insertions in recently synthesised PLD molybdenum cluster sulfide films are presented. We prove that Cu_xMo₆S₈ can be electrochemically converted to Mo₆S₈. The copper, cadmium, zinc and lead intercalation performed by electrochemistry on deintercalated films has succeeded. During this study, we also report that the use of a chemical process allows the formation of PbMo₆S₈ and SnMo₆S₈ thin films, that means that the intercalation of large cations is effective like the one of small cations.     

Time evolution of the pulsed HF chemical laser system. I. Kinetic modeling - rotational nonequilibrium

A detailed model of a pulsed F + H₂ → HF + II laser is used to investigate the kinetic behaviour of the system and to provide input data for a thermodynamic analysis of its time evolution. The rate equations for all relevant vib-rotational level populations and photon densities are solved for various initial conditions without using the rotational equilibrium assumption. The results of the kinetic model are in good agreement with the experimental results of Berry in which high inert gas pressure was used to enhance rotational relaxation. The effects of specific kinetic processes on the time evolution of the molecular populations and the laser output are evaluated by varying the inert gas pressure and by “switching on and off” the influence of various kinetic factors. The laser efficiency ranges between ≈ 1 to 2 photons per molecule. The major rate process enhancing the efficiency is rotational relaxation. The reduction in laser efficiency due to vibrational relaxation is ≈ 20%.     

Monte Carlo simulation of pulsed laser polymerization

Pulsed laser polymerization (PLP) has been simulated using a Monte Carlo procedure. From the results of numerous simulations it has been shown that the molecular weight distribution (MWD) consists primarily of two superimposed distributions. One distribution, a relatively broad background, represents the termination reactions during the dark period; the other, a rather sharply peaked distribution, represents the termination reactions occurring as a consequence of the large number of small radicals produced during the laser pulse. The postulate that the inflection point on the sharp peak can be used to calculate that the propagation rate constant was tested and found to be accurate to within 3%. The relative position of the broad and sharp distributions on the chain length scale determines the qualitative appearance of the overall MWD and is in turn governed by the rate of photoinitiation and the relative values of termination and propagation rate constants. This explains the qualitatively different shapes of MWD which have been experimentally observed. Finally, it is shown that the occurrence of chain length dependent termination reactions precludes the use of an analytical expression to deduce quantitative or qualitative information about the termination reaction from PLP data.     

Alignment of molecules in pulsed resonant laser fields

We investigate by numerical simulations the dynamics of alignment of linear molecules in resonant pulsed laser fields and its dependence on pulse length, field strength, and molecular parameters. We propose an analytical short-time approximation for the time-dependent wave packets. We provide a theoretical basis for the occurrence of saturation in the rotational pumping. We present a formula to predict the time at which the maximum alignment occurs. We discuss the magnitude of the laser-induced alignment and we relate it to a theoretical upper limit.     

The effect of benzyl alcohol on pulsed laser polymerization of styrene and methylmethacrylate

The homo- and copolymerizations of styrene (STY) and methylmethacrylate (MMA) have been studied in the presence of several levels of benzyl alcohol (BA). From pulsed laser polymerizations it has been found that the apparent propagation rate constant increased with increasing BA for all systems. In copolymerization it has been found that the reactivity ratios for STY decrease somewhat, but those for MMA change little with increasing BA. The tacticity of poly(MMA) formed in the presence of BA is affected, the amount of mm diads increasing with increasing BA. The results are interpreted as supporting the hypothesis that BA forms a strong complex with radical chain ends terminating in MMA and a weak complex with those terminating in STY. © 1997 John Wiley & Sons, Inc.     

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.     

Water expansion dynamics after pulsed IR laser heating

A nanosecond pulsed IR (1.9 mum) laser rapidly heated water, in an open vessel, to temperatures well below the boiling point. The subsequent dynamics of volume expansion were monitored using time-resolved interferometry in order to measure the increase in the water level in the heated area. The water expanded at less than the speed of sound, taking just less than 100 ns to increase its height by approximately 500 nm at surface temperature jumps of 20 K. The initial expansion was followed by an apparent contraction and then a re-expansion. The first expansion phase occurred more slowly than the timescale for bulk H-bond re-structuring of the water, as determined from vibrational bands in the Raman spectra, and represents the limit to the rate at which the overpressure caused by sudden heating can be released. The second phase of the expansion was caused by hydrodynamic effects and is accompanied by morphological changes resulting in light scattering as well as droplet spallation.     

Vacuum ultraviolet laser pulsed field ionization-photoelectron study of cis-dichloroethene

The vacuum ultraviolet (VUV) laser pulsed field ionization photoelectron (PFI-PE) spectrum of cis-dichloroethene (cis-ClCH[Double Bond]CHCl) has been measured in the energy region of 77 600-79 500 cm(-1). On the basis of the semiempirical simulation of the origin PFI-PE band, we have obtained the IE(cis-ClCH[Double Bond]CHCl) to be 77 899.5+/-2.0 cm(-1) (9.658 39+/-0.000 25 eV). The assignment of the vibrational bands resolved in the VUV-PFI-PE spectrum are guided by high-level ab initio calculations of the vibrational frequencies for cis-ClCH[Double Bond]CHCl(+) and the Franck-Condon factors for the ionization transitions. Combining the results of the present VUV-PFI-PE measurement and the recent VUV-infrared-photoinduced Rydberg ionization study, the vibrational frequencies for eleven of the twelve vibrational modes of cis-ClCH[Double Bond]CHCl(+) have been experimentally determined: nu(1) (+)(a(1))=181 cm(-1), nu(2) (+)(a(2))=277 cm(-1), nu(3) (+)(b(2))=580 cm(-1), nu(4) (+)(b(1))=730 cm(-1), nu(5) (+)(a(1))=810 cm(-1), nu(6) (+)(a(2))=901 cm(-1), nu(8) (+)(a(1))=1196 cm(-1), nu(9) (+)(b(2))=1348 cm(-1), nu(10) (+)(a(1))=1429 cm(-1), nu(11) (+)(b(2))=3067 cm(-1), and nu(12) (+)(a(1))=3090 cm(-1)). These values are compared to theoretical anharmonic vibrational frequencies obtained at the MP2/6-311G(2df,p) and CCSD(T)/6-311G(2df,p) levels. The IE prediction for cis-ClCH[Double Bond]CHCl has also been calculated with the wave function based CCSD(T)/CBS method, which involves the approximation to the complete basis set (CBS) and the high-level correlation corrections. The theoretical IE(cis-ClCH[Double Bond]CHCl)=9.668 eV thus obtained is found to have a deviation of less than 10 meV with respect to the experimental IE value.     

Phosphorescence of 1,1,1-trifluoroacetone using pulsed nitrogen laser excitation

Phosphorescence decay times of 1,1,1-trifluoroacetone in the vapour phase have been measured as a function of pressure of ketone with low-power excitation from a pulsed N₂ laser. A strong pressure dependence is observed, yielding a rate constant for self-quenching of (4.6 ± 0.5) × 10⁶ ? mol^?1 s^?1, and an extrapolated zero-pressure lifetime of 60 μs, considerably shorter than that measured earlier in a higher-energy flash photolysis experiment. Reasons for these differences are discussed.     

Ultra-fast polymerization of epoxy-acrylate resins by pulsed laser irradiation

The 337.1-nm emission of a pulsed nitrogen laser was shown to initiate the crosslinking polymerization of epoxy-acrylate photoresists effectively. We evaluated the extent of curing from the amount of insoluble polymer formed and by the decrease in infrared (IR) absorption of the reactive double bond at 810 cm^?1. With the large power density available in the laser pulse (0.5 MW cm^?2) rates of polymerization as high as 10⁸ mol L^?1 s^?1 were observed in the presence of air. Quantum yield measurements indicated that each photon absorbed can create as many as 450 crosslinks; the kinetic chain length was calculated as ca. 4000 double bonds polymerized per initiating radical. During the induction period due to oxygen inhibition each photoinitiator radical consumed 1 O₂ molecule. The influence of the monomer and photoinitiator used on the sensitivity of the resin was examined; the best performing formulation contained the epoxy-acrylate oligomer, pentaerythritol triacrylate, as monomer and 2,2 dimethoxy-2-phenyl-acetophenone as photoiniatior. All the formulations studied can be cured by a single 500-kW laser pulse of 8 ns duration, provided that the irradiation is carried out in an inert atmosphere or with a focused laser beam.     

Infrared laser induced chemical reactions

It is shown that a high power infrared laser can enhance a chemical reaction (by lowering the activation energy) even if the reactants are infrared inactive.     

The pulsed glow discharge as an elemental ion source

A pulsed glow discharge, rather than a conventional constant dc voltage discharge, is used as an ion source for a quadrupole mass spectrometer. Both sputter yield and ion signal are enhanced by using the pulsed system because of an increase in the voltage necessary to maintain a constant average current at the cathode over the pulse period. Irregularities are seen in the pulse spectrum that appear as rapid surges in the ion signal for both sputtered and contaminant gas species. These peaks appear at the beginning of the pulse for gaseous species but are limited to the postpulse period for sputtered species. Differences in the signal forms allow for the discrimination against selected types of ion signals by using narrow data collection gates placed over different portions of the pulse period.     

The subterranean waters of tenerife-I Origin and chemical characteristics

A systematic study has been made of the chemical characteristics of the subterranean waters of Tenerife from all sources, including galleries, wells, and springs. All the constituents and characteristics of agricultural interest have been considered. A marked dependence was observed between the characteristics of the water and the situation of the sources, and a study was made of the principal types of geological formation which determine the presence of subterranean accumulations of water at different levels of the island.     

The effect of pulsed laser radiation on polyelectrolyte capsule shells modified with fluorescent dyes

Polyelectrolyte capsules containing rhodamine 6G and fluorescein isothiocyanate in their shells are obtained by successive adsorption on spherical microscopic CaCO₃ particles followed by the dissolution of the latter. Suspensions of the capsules are irradiated with a laser operating at a wavelength corresponding to the absorption bands of the dyes, and it is shown that shell modification with the selected dyes promotes photosensitized disruption of these structures. The mechanism proposed for this disruption is realized via energy transfer from photoexcited dye molecules to the polymer matrix. Therewith, the dye-modified capsules are disrupted due to their nonuniform local heating.     

Intracavity laser atomic absorption spectrometry with graphite furnace atomizer or pulsed laser sampler

An absorption intracavity laser spectrometer with two types of sample vaporization systems (graphite furnace electrothermal atomizer or laser sampler) is described that can be used for the determination of trace amounts of metals (Al, Cr, Fe and Mn) in liquid samples and at the surface of solid targets. The limits of detection for the elements tested are lower than those obtained by modern conventional spectrometers. The examined technique provides a wide dynamic range of linear standard calibration curves.