Diode-laser-based ultraviolet-absorption sensor for high-speed detection of the hydroxyl radical

A new diode-laser-based UV-absorption sensor for high-speed detection of the hydroxyl radical (OH) is described. The sensor is based on sum-frequency generation of UV radiation at 313.5 nm by mixing the output of a 763-nm distributed-feedback diode laser with that of a 532-nm high-power, diode-pumped, frequency-doubled Nd:YVO4 laser in a beta-barium borate crystal. Approximately 25 microW of UV radiation is generated and used to probe rotational transitions in the A2 Sigma+ -X2II (v' = 0, v" = 0) electronic transition of OH. Single-sweep, single-pass measurements of temperature and OH concentration in a stoichiometric C2H4-air flame are demonstrated at rates up to 20 kHz.     

Quasi-monochromatic measurements of homogeneous arc plasmas

The refined diagnostic information obtainable by high-order spectrometry is illustrated by the results of quantitative measurements of a few rotational lines of OH in the ultraviolet spectrum of water-vapor plasmas generated in a wall-stabilized arc. Because of the high spectral and spatial resolution achieved in end-on measurements, the emission and also the absorption coefficients pertaining to homogeneous arc regions were obtained directly from measured line spectra—although the absorption was not measured explicitly—leading to the occupation of the upper and the lower state for the transition. The gas temperature was determined from the halfwidth of the Doppler-broadened rotational lines. The measured resolving power of the spectrometer was of the order of 400,000 in these measurements.     

Energy transfer in the A2Σ+ state of OH following v’ = 1 excitation in a low pressure CH4/O2-flame

2 Σ⁺(v’=1) level of OH. Measurements were performed in a laminar premixed flame at 10 Torr total pressure. The low pressure allowed the spatial variation of the effective quenching rate to be determined through the flame front. In addition, the dependence of the quenching rate on rotational quantum number was measured by exciting a series of rotational lines in the range N’=0–16. The results show that the total quenching rate decreases only 17% through the flame front, in the region where OH can be detected. Nevertheless, the absolute value of the quenching rate Q is required if absolute concentrations are to be determined from LIF-signals. The variation both of Q and of the rotational relaxation rate with excited rotational quantum state must be known for quantification of LIF-temperature measurements via the Boltzmann relation. Finally, the rotational and vibrational energy transfer (RET, VET), was investigated by recording the spectrally and temporally resolved fluorescence. For all excited rotational lines, efficient RET to neighbouring rotational states was observed, but only very little VET. Total RET rates were determined from the difference between the time-resolved broadband (total fluorescence) and narrowband (fluorescence from the laser excited level) curves. The experimental results were compared with simulations using a dynamic model, which describes the energy transfer for flame conditions. With the available input data (temperature, major species concentrations and collision-partner specific RET cross sections), good agreement was obtained. Received: 3 February 1997/Revised version: 3 September 1997     

OH rotational temperature and number density measurements in high-pressure laminar flames using double phase-conjugate four-wave mixing

OH number densities and rotational temperatures up to 9 bar have been measured using double-phase-conjugate four-wave mixing (DPCFWM) in flat laminar premixed methane/air flames. By phase conjugating the backward pump to the forward pump with a phase-conjugate mirror conventional degenerate four-wave mixing (DFWM) becomes DPCFWM and quantitative measurements of OH radicals and OH rotational temperatures in flames at high pressure are possible. Our results show that conventional DFWM with a standard mirror is a low-biased measurement at high pressure primarly due to a fluctuating interaction volume which results from large fluctuating density gradients at the edge of the flame and from flame movement. A comparison is made between a laser-saturated fluorescence technique, conventional DFWM, and DPCFWM at 1, 5 and 9 bar.     

Collisional depolarization of OH(A) studied by Zeeman quantum beat spectroscopy

The depolarization of the rotational angular momentum of electronically excited OH(2Σ) radicals through collisions with water molecules has been measured using Zeeman quantum beat spectroscopy. The new data have permitted the evaluation of OH(A) state-specific quenching and angular momentum depolarization cross-sections for superthermal OH(A) radicals with mean relative velocities centred around 3500 m s-1. The quenching cross-sections are compared both with values available in the literature, and with predictions based on a simple harpoon model, and are found to be in good qualitative accord with previous findings. For the lowest rotational levels studied, the depolarization cross-sections (which include contributions from both elastic and inelastic processes in OH(A)) are found to approach ～100 Å2, only slightly below the high-temperature cross-sections for rotational energy transfer determined elsewhere. The data suggest that under the present conditions rotational energy transfer is accompanied by significant depolarization. The cross-section for the translational moderation of superthermal OH(X) by water is also determined in the present study.     

Ar/H2O大气压等离子体射流放电特性

为了研究水蒸气体积分数对大气压等离子体射流放电机理及放电效率的影响,进而产生高活性低温等离子体并优化其效率。通过对大气压氩水等离子体射流的电压电流波形和Lissajous图形等电气特性的测量及发射光谱和发光图像等光学特性诊断,研究了不同水蒸气体积分数时,等离子体射流的放电特性。通过计算放电功率、传输电荷量、电子激发温度、分子振动温度和分子转动温度等主要放电参量,研究了它们随水蒸气体积分数的变化趋势,并结合放电机理对所得实验结果进行分析。结果表明,Ar/H2O等离子体射流除了产生N2和Ar,还有OH和O,气体温度在525~720 K之间变化,为典型的低温等离子体;随着水蒸气体积分数的增加,等离子体羽喷出管口的长度减小,放电功率减小,发光强度减弱,转动温度和振动温度增加;相同功率下,水蒸气体积分数为0.5%时,产生的OH达到最大。     

Photochemical effects of KrF excimer excitation in laser-induced fluorescence measurements of OH in combustion environments

This paper presents experimental evidence that using the KrF excimer laser for quantitative laser-induced fluorescence (LIF) studies of the OH A-X (3,0) system is highly problematic if the effects of both photobleaching and photochemistry are not included for laser spectral irradiances greater than 20 MW/cm² cm^-1. Pump-probe and time-resolved measurements of the OH LIF signal in an atmospheric pressure, premixed CH₄-air flame at low- and high-laser-spectral-irradiance conditions show that a significant amount of OH is produced from photofragments resulting from the simultaneous 2-photon predissociation of H₂O molecules in the C-X system. A 5+2-level rate-equation model that includes the effects of both photobleaching and photochemical OH production is shown to satisfactorily predict the data using a single adjustable parameter given by the effective, spectrally integrated 2-photon cross-section of H₂O near 248 nm. The time-integrated OH LIF signal was found to depend on both the laser spectral irradiance and the local concentration of H₂O. Additionally, use of the KrF excimer laser for 2-line rotational thermometry can produce temperature errors as great as +550 K at high laser-pulse energies. Received: 21 August 2000 / Revised version: 30 October 2000 / Published online: 21 February 2001     

On the absorption saturation in the vibrational-rotational bands of molecules

Analytical expressions describing saturation of the stationary absorption coefficient in a vibrational-rotational band in the spectrum of a molecular gas are obtained taking into account that many other bands are involved in the absorption process as well. The formulas are derived for an arbitrary spectral composition of the pumping radiation with an allowance for a difference between the rotational and vibrational relaxation rates and between the relaxation rates of the lower and upper vibrational levels. The influence of saturation on the appearance of the partial inversion and the negative absorption effects is considered. The general formulas are simplified and the vibrational and rotational contributions to the saturation are separated within the frame-work of the local Elsasser model for the rotational band structure. Explicit relationships between the dimensionless parameters describing the vibrational and rotational saturation mechanisms are obtained and analyzed, and conditions under which one of these mechanisms is dominating are determined. Particular calculations were performed for the (000)–(010) and (000)–(001) absorption bands of the O₃ molecule and features of the saturation effect in these bands were established.     

大气压Ar射频容性放电模式转变的温度表征

在大气压条件下Ar气流中实现了容性射频放电α和γ两种模式及其转变与共存.由于放电处于开放大气环境中,放电发射光谱中清晰地存在 N₂C³Π_u→Β³Π_g跃迁产生的第二正带和OH自由基 Α²Σ→Χ²Π跃迁的(0,0)带光谱.为了获得放电区域的宏观温度,针对氮的第二正带(0,1),(1,2)两个谱带,自编了拟合程序,用温度拟合方法获得了氮分子的转动温度和振动温度,研究了转动温度随放电功率的变化趋势,得到了温度突变与放电模式转变的相关性.利用Lifbase的发射光谱模拟功能,进行了OH自由基Α²Σ→Χ²Π(0,0)带光谱的模拟,通过与实验光谱对比,得到了与N₂光谱拟合结果相符的OH转动温度,以及相似的随放电功率的变化趋势,这说明放电空间内的中性物种达到了热平衡状态.根据放电伏安特性变化,放电模式转变对应的转动温度变化趋势得到确认,并且与放电形态的照片符合. 关键词： 大气压等离子体 放电模式 转动温度     

A far-infrared laser magnetic resonance spectrometer for the investigation of radicals of atmospheric relevance

A far-infrared laser magnetic resonance spectrometer is described. A superconducting magnet is used for the tuning of the energy levels. Pressure broadening measurements of OH and NO have been performed. The measurements demonstrate that the laser magnetic resonance technique is well suited for this kind of measurements. Based on this results the design and expected performance of a compact laser magnetic resonance spectrometer is presented. The sensitivity as well as the mechanical ruggedness of this spectrometer is sufficient in order to be used aboard an aircraft for the in situ detection of OH and HO₂ in the lower stratosphere and upper troposphere.     

Measurements of temperatures and OH-concentrations in a lean methane-air flame using high-resolution laser-absorption spectroscopy

Line profile measurements of selected spectral lines in the 1-0 band of the A²Σ⁺-X²π system of OH near 2830 Å, using a tunable, frequency-doubled dye laser, have been made to determine simultaneously the translational temperature and hydroxyl concentration in a laminar, flat methane-air flame. The rotational temperature was determined from the ratio of two line-absorption coefficients. The rotational degree of freedom was found to be in equilibrium with the translational degree of freedom. Values deduced for the collision broadening parameter in the 1-0 band depend on OH-concentration, ground state rotational quantum number and, possibly, on the vibrational quantum number.     

Effect of laser intensity and of lower-state rotational energy transfer upon temperature measurements made with laser-induced fluorescence

4 –air flame, with OH at 2000 K. We calculate the ratio of LIF intensities that would be induced by doubled dye-laser light near 283 nm, by means of the A←X, 1←0, P₁(7), and Q₂(11) transitions in OH. Here we show that the ratio of LIF signals from those two transitions, and thus the deduced temperature, is sensitive to laser intensity. That is caused mainly by the competition between laser-pumping of molecules out of the lower rotational state and of rotational energy transfer (RET) collisions into that state. A-state collisional effects are normally important, but are minimized here by assuming that they are the same for both transitions. The laser spectral intensity dependence of the fluorescence ratio depends heavily upon the value of the RET coefficients within the X-state. While RET reduces the sensitivity of the observed signal to the laser spectral intensity, the conversion of a measured fluorescence ratio to temperature is particularly difficult. That is because RET rates, and quenching rates, can be a function of local conditions and of the rotational state being populated. Two different models are used to demonstrate these effects, and both predict large effects upon temperature. Received: 19 February 1998/Revised version: 16 June 1998     

硝基甲烷与硝基乙烷266 nm光解动力学

利用单光子激光诱导荧光技术，在硝基甲烷与硝基乙烷气相条件下266 nm光解过程中，对初生态的解离产物OH进行了测量．结果表明，两个解离过程中产生的初生态OH都没有振动激发．硝基甲烷光解过程中产生的OH的转动分布可以用玻尔兹曼温度简单的表示，相对于自旋轨道分裂的2|3=2与2|1=2两个态，其转动温度分别为2045§150与1923§150 K．硝基乙烷光解过程中产生的OH不是简单的玻尔兹曼分布，因此不能用玻尔兹曼温度来描述，这一结果 意味着硝基甲烷与硝基乙烷在生成OH的过程中经历了不同的解离过程．硝基甲烷     

State to state rotational transfer in collisions between IF molecules and helium atoms

We report measurements of the rate for transfer of population between rotational levels of the B state of iodine monofluoride in collisions with helium atoms. The measured rates are much larger than those observed for transfer of population between vibrational levels, indicating that the rotational population distribution in the collisionally pumped IF laser should be rapidly thermalized. The rates show a dependence on the quantum number of the initial level, in contrast to the results of theoretical calculations.     

Relaxation-based distance measurements between a nitroxide and a lanthanide spin label

Distance measurements by electron paramagnetic resonance techniques between labels attached to biomacromolecules provide structural information on systems that cannot be crystallized or are too large to be characterized by NMR methods. However, existing techniques are limited in their distance range and sensitivity. It is anticipated by theoretical considerations that these limits could be extended by measuring the enhancement of longitudinal relaxation of a nitroxide label due to a lanthanide complex label at cryogenic temperatures. The relaxivity of the dysprosium complex with the macrocyclic ligand DOTA can be determined without direct measurements of longitudinal relaxation rates of the lanthanide and without recourse to model compounds with well defined distance by analyzing the dependence of relaxation enhancement on either temperature or concentration in homogeneous glassy frozen solutions. Relaxivities determined by the two calibration techniques are in satisfying agreement with each other. Error sources for both techniques are examined. A distance of about 2.7 nm is measured in a model compound of the type nitroxide–spacer–lanthanide complex and is found in good agreement with the distance in a modeled structure. Theoretical considerations suggest that an increase of the upper distance limit requires measurements at lower fields and temperatures.     

Simultaneous one-dimensional fluorescence lifetime measurements of OH and CO in premixed flames

A method for simultaneous measurements of fluorescence lifetimes of two species along a line is described. The experimental setup is based on picosecond laser pulses from two tunable optical parametric generator/optical parametric amplifier systems together with a streak camera. With an appropriate optical time delay between the two laser pulses, whose wavelengths are tuned to excite two different species, laser-induced fluorescence can be both detected temporally and spatially resolved by the streak camera. Hence, our method enables one-dimensional imaging of fluorescence lifetimes of two species in the same streak camera recording. The concept is demonstrated for fluorescence lifetime measurements of CO and OH in a laminar methane/air flame on a Bunsen-type burner. Measurements were taken in flames with four different equivalence ratios, namely ? = 0.9, 1.0, 1.15, and 1.25. The measured one-dimensional lifetime profiles generally agree well with lifetimes calculated from quenching cross sections found in the literature and quencher concentrations predicted by the GRI 3.0 mechanism. For OH, there is a systematic deviation of approximately 30 % between calculated and measured lifetimes. It is found that this is mainly due to the adiabatic assumption regarding the flame and uncertainty in H₂O quenching cross section. This emphasizes the strength of measuring the quenching rates rather than relying on models. The measurement concept might be useful for single-shot measurements of fluorescence lifetimes of several species pairs of vital importance in combustion processes, hence allowing fluorescence signals to be corrected for quenching and ultimately yield quantitative concentration profiles.     

On the quantification of OH*, CH*, and C2* chemiluminescence in flames

Absolute concentrations of all important chemiluminescent species, OH–A, CH–A, CH–B, and C₂-d have been measured for the first time in methane-oxygen flames at low pressure. The optical detection system for chemiluminescence measurements has been calibrated with Rayleigh and Raman scattering of a cw laser, with the latter approach yielding superior results. The measured ratio between the concentration of CH–B and CH–A suggests that the electronically excited CH* is formed close to thermal equilibrium. Introduction of different rate constants for reactions leading to CH–A and CH–B were not necessary to explain the experimental results. Results are compared with a recent numerical model. Deviations in profile shape and peak positions are relatively small for stoichiometric flames, but become more pronounced in richer mixtures. Larger discrepancies are observed for the absolute concentrations, depending on the chemiluminescent species and the stoichiometry. In an attempt to find an alternative method for the quantification of chemiluminescent species, MIR-CRDS has been performed around 3.9 μm. While H₂O and OH–X could be measured, the sensitivity was not high enough to detect the low sub-ppb concentration of OH–A—in part due to the limited reflectivity of mirrors in the MIR, in part due to a significant background of hot H₂O lines.     

On the rotational diffusion of rod-like macromolecules in lyotropic-mesomorphic phases

The rotational diffusion of rod-like macromolecules, idealized as monodisperse rods, in concentrated solution is considered. The Doi-Edwards model is modified to explain the experimentally observed increase of the rotational relaxation time with increasing concentration in the lyotropic-nematic phase of rigid rod-like molecules in solution. Our approach consists in (i) reconsidering the original Doi-Edwards model to eliminate unphysical behaviour of the rotational relaxation time at the limit of perfect order, and (ii) including concentration effects on the translational diffusivity of rods in the Doi-Edwards model. Predictions of the corrected model are compared with steady flow viscosity data for poly(n-alkylisocyanates). In the lyotropic-nematic phase a very good qualitative agreement between the theory and experiment is observed. Additionally, the model applied to a highly concentrated isotropic phase explains in a natural way the viscosity behaviour as the concentration is increased towards the critical value for formation of the lyotropic-nematic phase.