Simulation of operation of multiwave remote gas-analyzer based on NH3-laser

Numerical analysis of a multiwave path gas-analyzer, based on a NH₃-laser pumped by CO₂-laser radiation, is performed for model detection of concentrations of a series of molecular species such as NH₃, HCN, phosgene, NHO₃, CO₂, and H₂O. The potentialities of the gas analyzer and uncertainty of the gas concentration detection were estimated for a 4 km horizontal atmospheric path. The estimation took into account the absorption of laser radiation by the atmospheric aerosol and molecular gases under study and distortion of the laser beam due to atmospheric turbulence.     

Multiple-photon ir excitation of electronic states of OsO4 molecule

Basic experimental characteristics of collisionless luminescence of OsO₄ molecule excited by pulsed ir laser radiation are presented. The theoretical model of the process is developed and a comparison with experiment is carried out both for one-frequency and two-frequency ir excitation. From such comparison information is obtained on the fraction of molecules excited by laser radiation as well as on the spectroscopic characteristics of highly excited states.     

Electron-driven processes in high-pressure plasmas

This review article summarizes results from selected recent studies of collisional and radiative processes initiated and driven by low-energy electron interactions with atoms and molecules in high-pressure plasmas. A special emphasis of the article is on spectroscopic studies of plasmas used as sources for non-coherent vacuum ultraviolet radiation such as rare excimer emissions and atomic and molecular emissions from plasmas in admixtures of rare gases and the molecular gases H₂ and N₂. An attempt is made to correlate the various observed emission features and their dependence on the plasma operating parameters (pressure, power, gas mixture, mode of excitation, etc.) to the underlying microscopic atomic and molecular processes.     

The Influence of Admixtures of Molecular Gases on the Efficiency of Radiation Production by Ar?Hg Mixture Plasmas used in Fluorescent Lamps

Starting from former investigations of pure Ar? Hg mixture plasmas in parameter ranges typical of fluorescent lamps we studied the influence of additional admixtures of molecular gases (N₂, H₂) on the energy transfer from the electrons heated by an electric field to the lowest excited states of Hg atoms which are the energy source for the resonance radiation production. By calculation of the different power loss rates via solving the appropriate Boltzmann equation for three component mixture plasmas it was found that already a threshold level of molecular impurities of about 10^?4 Torr leads to a marked energy dissipation by the impurities and thus to a pronounced reduction of the efficiency of the resonance radiation production. This is caused by the great effectivity of vibrational excitation of molecules in electron collisions due to the great cross sections for such collisions and their low thresholds.     

Methane activation on Ni(111) at high pressures

High pressure CH₄ decomposition experiments on Ni(111) have been conducted to discriminate between a direct reaction mechanism involving hot CH₄ molecules and a precursor-mediated mechanism. It has been found that the direct process for CH₄ decomposition is kinetically significant in depositing adsorbed carbon. Using absolute carbon coverage measurements the absolute efficiency of carbon deposition on Ni(111) at 600 K was found to be 4 × 10⁻⁸ C atom per CH₄ collision at 1.0 Torr CH₄ pressure. By the use of He, Ne, and Ar buffer gases, it has been shown that the major process leading to CH₄ dissociation involves heating CH₄ followed by return of excited CH₄ back to the surface where the CH₄ dissociates. These experiments demonstrate that CH₄ translational excitation and vibrational excitation, as studied by molecular beam methods, are the kinetically important CH₄ activation processes leading to dissociation in the high pressure regime.     

Nuclear dealignment in multiply ionized fast fluorine atoms recoiling in gases

The dealignment of the 5/2 ₁ ⁺ excited nuclear state of¹⁹F (E _x=197keV) has been determined for foil-excited fluorine ions of energies 3 and 6 MeV recoiling in He, N₂ and Ar gases using backscatter-coincidence excitation with He⁺⁺ and the recoil-distance technique. The observed decay of nuclear alignment with increasing gas pressure was interpreted in a microscopic collision model using theories of statistically perturbed angular correlations. The strong velocity dependence of the dealignment cross section indicates the dominance of charge exchange, e.g. capture, processes in producing the ionic states of strong hyperfine interaction.Work supported in part by Bundesministerium für Forschung und Technologie.     

Multiphoton excitation of molecules by single mode CW lasers

An intracavity laser interaction zone with molecules expanded in a molecular beam set-up permits multiphoton excitation under collision-free conditions by narrow band radiation. Two- and three-laser experiments are performed to study rapid adiabatic passage processes, stimulated de-excitation and hole burning effects. The detection of excitation occurs by means of a sensitive bolometric molecular beam detector. The essential information obtained for SF₆ concerns the vibrational bottleneck, the conservation of excited eigenstate characteristics for at least 1 ms, the ease with which about 15 photons can be deposited in the molecules with the help of two cw CO₂ lasers and the influence of simultaneous two-frequency radiation, yielding a significant extra-excitation. Other molecules where multiphoton excitation is observed utilizing the same set-up are CF₃I and CF₃Br.     

Review of Laser-Induced Plasma,Its Mechanism,and Application to Quantitative Analysis of Hydrogen and Deuterium

Abstract

A comprehensive review of important progress achieved over the last 30 years regarding knowledge of laser-induced plasmas generated by CO₂ and Nd:YAG lasers in a variety of ambient gases is presented in this article, as well as research results on the extension of laser-induced breakdown spectroscopy (LIBS) for quantitative analysis of light elements, especially hydrogen and deuterium. First, the formation of shock wave–induced expanding secondary plasma in low-pressure ambient gases is discussed along with the dynamic characteristics of the secondary plasma expansion process. The unique advantages of low-pressure gas plasma are explained in relation to the successful detection of the sharp H and D emission lines. The experimental results using helium ambient gas are presented with emphasis on the role of He gas plasma in introducing an additional delayed excitation mechanism involving the helium metastable excited state, which resulted in the complete resolution of H and D emission lines, separated by only 0.18 nm. The development of a laser precleaning treatment and special double-pulse techniques further produced a linear calibration line with zero intercept applicable to quantitative H and D analyses of zircaloy sample, with either low- or high-pressure ambient He gas. More recent use of a transversely excited atmospheric (TEA) CO₂ laser in place of an Nd:YAG laser has demonstrated the much desired larger excited helium plasma and thereby resulted in significant emission enhancement and improved detection sensitivity.     

氮分子离子N+2和氮原子离子N+与Ne原子碰撞激发的比较

用发射光谱法研究了同种元素氮的分子离子N⁺₂和原子离子N⁺与Ｎｅ原子碰撞产生的激发态，获得了靶激发、入射分子离子的分解激发、入射原子离子的激发以及两种离子与靶之间的电荷转移激发等信息．计算了各发射谱线的发射截面．对两种离子引起的谱线发射截面的差异进行了分析，得出一些初步结论，并对此作了些定性解释     

Towards an explanation of collisionless multiple-photon laser dissociation of SF6

The rapidity and high degree of molecular vibrational excitation by the absorption of ir laser light in SF₆ and other molecules may be due in large part to the anharmonic splitting of excited vibrational states. Anharmonic splitting of an overtone or combination vibrational level (i) is possible only in molecules with degenerate vibrational states, (ii) can be comparable in magnitude to the net anharmonic shift of the level, (iii) is generally much larger than the rotational shifts which have previously been proposed as an explanation for the dissociation of SF₆. We find that consecutive nearly resonant transitions are possible in SF₆ up to υ₃ = 5 to 10.     

Submillimeter-wave properties of thermospheric rocket plumes

The problem of detecting rocket plumes at thermospheric altitudes with satellite-borne submillimeter-wave radiometers is examined theoretically. To estimate the sizes of plume signatures contrasted against a 250-K earth background or in self-emission against the cold sky, a computer program has been developed to predict plume brightness temperatures and optical depths of rotational lines of plume molecular constituents (e.g., H₂O) as a function of distance from the nozzle. The methods employed in the computations are described in general terms, and examples are presented to indicate that detectable H₂O signatures extending to several thousand nozzle diameters should exist at plume altitudes above 250 km.     

A K-harmonics description of 16O breathing mode

We calculate the properties of the ¹⁶O breathing mode in a K_minK-harmonics calculation. The breathing mode has single particle quantum numbers that are identical to the nucleon quantum numbers in the ground state. We calculate an excitation energy of 27 MeV, and a monopole transition matrix element of 7.21 fm². Six excited monopole states are calculated to be bound in ¹⁶O, with k equal to K_min. These states exhaust 90 % of the isoscalar monopole sum rule. The first excited K_min state, the breathing mode, exhausts 68 % of the isoscalar sum rule.     

Spectroscopic Detection of Sulfur Oxides in the Aircraft Wake

The absorption/emission spectral regions of SO, SO₂, SO₃, S₂O and HSO are analyzed for the range from UV (λ ≥ 0.2 μm) to IR (λ < 30 μm) and are compared with the atmospheric transmission spectrum. It is shown that many vibrational bands of the compounds considered fall into atmospheric transmission windows. For the vibrational bands of SO, SO₂, SO₃, S₂O, and HSO molecules there are some gases which hinder the absorption diagnostics of the indicated compounds. These interfering gases are natural components of atmospheric air as well as specific gases of aircraft engine exhaust. It is found that the least influence of the interference takes place in the 2400–2700 cm⁻¹ IR region. The spectroscopic techniques used for the detection of aircraft engine exhaust compounds are briefly reviewed, with much consideration given to SO₂. The IR absorption spectra of SO₂ and other gases are calculated for the conditions of the aircraft engine nozzle exit. Narrow spectral intervals suitable for SO₂ detection in a hot flow are determined. An analysis is made for the detection capabilities of CO₂ lasers (including isotope CO₂ lasers) and CO lasers (both fundamental band and first-overtone ones) as applied to SO₂ detection in aircraft engine exhaust. Published in English as Preprint No. 5 of the P. N. Lebedev Physical Institute, Moscow (2004).     

THz radiation generation via the interaction of two‐colour ultra‐short laser pulses with SO2 and NH3 gases

In this work, using a two‐dimensional particle‐in‐cell Monte Carlo collision computation method, terahertz (THz) radiation generation via the interaction of two‐colour, ultra‐short, high‐power laser pulses with the polyatomic molecular gases sulphur dioxide (SO₂) and ammonia (NH₃) is examined. The influence of SO₂ and NH₃ pressures and two‐colour laser pulse parameters, i.e., pulse shape, pulse duration, and beam waist, on the THz radiation generation is studied. It is shown that the THz signal generation from SO₂ and NH₃ increases with the background gas pressure. It is seen that the THz emission intensity for both gases at higher laser pulse durations is higher. Moreover, for these polyatomic gases, the plasma current density increases with increase in the laser pulse beam waist. A more powerful THz radiation intensity with a larger time to peak of the plasma current density is observed for SO₂ compared to NH₃. In addition, many THz signals with small intensities are observed for both polyatomic gases. It is seen that for both SO₂ and NH₃ the generated THz spectral intensity is higher at higher gas pressures.     

Laser-induced fluorescence of the CH molecule in a low-pressure flame

The laser-induced fluorescence from [A²Δ(υ′ = 0)→X²Π(υ″ = 0)] band of the CH radical was studied in a low-pressure (20 torr) methane-oxygen flame (φ = 1.06). A time-resolved fluorescence technique was used to measure the relative CH concentration profile and the quenching of the A²Δ excited state through the flame. The pressure dependence of the quenching was also measured and used to determine an effective quenching cross section of 6 Ų in the CH₄-O₂ flame. Analysis of the fluorescence spectra scanned at different delays after the laser excitation, according to a pseudo-three-level model, yields a rotational energy transfer (RET) rate in the A²Δ(υ′ = 0) electronic state which is a factor of four faster than the electronic quenching rate of 1.57 × 10⁷ sec^-1 in the flame at 2000 K.     

Two-photon excitation of Na atoms in a flame by broad-band laser irradiation

With a flashlamp-pumped tunable dye laser the 3S→3D, 3S→4D and 3S→5S two-photon transitions of sodium are excited and the resulting fluorescence radiation at various atomic transitions is detected. The sodium is nebulized into a stoichiometric H₂-O₂-Ar flame at atmospheric pressure and at a temperature of 1800 K. Collisional population exchange between several of the higher Na-levels is found to occur. Saturation and saturation broadening of two-photon transitions are observed. Formulas for two- photon excitation rates in the case of broad-band excitation are derived and show substantial agreement with experiment.     

Utilization of rotational and vibrational temperatures for plasma diagnostics of high-pressure discharges

In the spectra of high-pressure discharges excited in molecular gases, very intensive molecular spectral bands may usually be observed. We may determine the rotational and vibrational temperatures without difficulty, however, the rotational and vibrational temperatures (T _r, T_v) do not offen equal to the temperature of neutral gas (T ₀) or to that of electrons (T _e). If the collision cross sections of electronic, atomic, and molecular excitation (deexcitation) are known, we may then calculate the dependence of the rotational and vibrational temperatures onT _e,T ₀,N _e and the pressure of the gas. The calculations have been performed for pure N₂ and for an Ar-N₂ mixture at atmospheric pressure. The computed graphs make it possible to determine some of the values 4T _e,T ₀,N _e if the temperaturesT _r andT _v are known.The author wishes to extend his thanks to Prof. V. Truneek for valuable comments and to Mr. A. Struka for the preparation of the diagrams.     

Candoluminescence and radical-excited luminescence

The literature on the emission of solids heated in flames or excited by gases containing free radicals or excited molecules is reviewed. Many different emission processes can occur, including selective thermal radiation, candoluminescence, surface chemiluminescence, adsorboluminescence, and chemisorptive luminescence. These effects occur in a wide variety of materials, including BN, various oxides (MgO, ZnO, Y₂O₃, etc.), many impurity-activated phosphors (CaO:Bi, ZnS: Cu, Zn₂SiO₄:Mn, etc.), and organic compounds. Emission may be excited by a number of radicals, including H, O, OH, N, CO and CH. In a flame, the catalytic activity of the solid surface for radical recombination or de-excitation influences both the excitation of luminescence and the temperature of the solid (and hence the incandescence). An outstanding example of high-temperature candoluminescence is the Welsbach mantle (ThO₂ : Ce).     

Vibrationally Excited Ozone Relaxation by CO

Time profiles of ozone concentration after pulsed UV laser photolysis in the O₂- O₃-Ar-CO mixture, measured using time-resolved absorption spectroscopy, are presented. The experimental results show the dominance of the stabilization channel over the reactive one for the interaction between the vibrationally excited ozone molecule O₃(υ) and carbon monoxide CO. The rate constant of the process O₃(υ)+CO→O₃ + CO, obtained by processing experimental data by the kinetic modeling method is (1.5 ± 0.2) · 10^?13 cm³/s.     

复燃对液体火箭尾焰红外辐射特性的影响

为了定量研究复燃对液体火箭尾焰红外辐射特性的影响,建立了一个可以计算液体火箭尾焰复燃流场和红外辐射特性的模型.首先,使用FLUENT软件计算液体火箭尾焰复燃流场,其中尾焰中的复燃反应使用有限速率化学反应模型计算;然后,使用基于HITEMP数据库的窄带模型计算尾焰内气体的辐射参量;最后,使用有限体积法求解尾焰中的辐射传输方程.通过比较该模型计算的Titan IIIB尾焰光谱辐射强度与(美国)国家航空航天局公布结果的一致性,证明了该模型的正确性.最后,利用该模型计算了复燃对某液体火箭尾焰光谱和波段红外辐射强度的影响,结果表明,复燃反应可以显著增加尾焰红外光谱辐射强度,在2.5~3.0 μm和4.2~4.7 μm两个主要辐射波段平均辐射强度的增加比例分别达到了30.8%和28.3%,所以,在计算液体火箭尾焰准确的红外辐射特性时,需要考虑复燃的影响.