The fate of the OH radical in molecular beam sampling experiments

The collisional history of ionized molecules in a molecular beam mass spectrometric flame experiment is target of our present investigation. Measurements in a double imaging photoelectron photoion coincidence spectroscopy (i²PEPICO) were performed at the Swiss Light Source (SLS) of the Paul Scherrer Institute to use the ion imaging device for separating the molecular beam ions from rethermalized ions. This enables the precise composition study of the individual types of ions. Results show clearly for the OH radical that the complete signal is obtained from the molecular beam, while the signal from other combustion compounds features additional rethermalized molecules. As for OH radicals, the mole fraction is reduced by sampling effects and contact with the ionization vessel walls significantly. Consequently, this leads to signal loss and lower mole fractions, when using ionization cross sections for the quantification. To improve on this, a beam fraction (BF) factor is presented. The factor describes the ratio of the separated beam signal without rethermalized ions with the total ion signal, consisting of the mass to charge ratio from the molecular beam and additional rethermalized ions. Since the detected OH radicals are solely from the molecular beam, a new method of comparing two molecular beam alignments using the OH to H₂O signal ratio is presented. This method has a decent potential for the optimization of the quality of molecular beams. Finally, the separated beam signal (without the rethermalized ions) was used to determine mole fraction profiles for the OH radical using ionization cross sections. These profiles are in good agreement with model predictions of the USC-II and the Aramco Mech 2.0 mechanisms, while the total signal leads to factor of 12 smaller OH mole fractions.     

A1等离子体与甲醇团簇的反应

利用激光烧蚀-分子束法(LA-MB)对激光烧蚀金属铝靶产生的Al等离子体与脉冲分子束超声膨胀产生的(CH3OH)n团簇在气相条件下的反应进行了研究.烧蚀激光相对于脉冲分子束之间的延时不同,观测到团簇离子序列及团簇尺寸的变化,反映出脉冲分子束状态对反应条件及激光烧蚀等离子体状态的影响.在激光烧蚀发生于脉冲分子束的前段,主要反应产物为Al (CH3OH)n,但团簇尺寸较小;在烧蚀发生于脉冲分子束的中段,主要产物为H (CH3OH)n,团簇尺寸增大,强度减弱;在烧蚀发生于脉冲分子束的后段,观测到尺寸更大的水合质子化团簇H (H2O)m(CH3OH)n.结合团簇离子速度分布的特征,对团簇的产生机理进行了讨论.     

激光烧蚀Al等离子体与甲醇团簇反应中(CH3OH)nCH3O-的形成

激光烧蚀一分子束(LA-MB)法是研究金属离子与分子团簇反应的有效方法.在气相条件下,用飞行时间质谱观测到激光烧蚀Al等离子体与脉冲分子束超声膨胀产生的甲醇团簇碰撞反应生成的(CH30H)nCH3O-(n=3～23)团簇负离子.实验发现,此序列的团簇负离子主要生成于烧蚀激光作用于脉冲分子束的后段,(CH3OH)3CH3O-离子强度始终远远大于其后的离子,并且这些离子随尺寸大小的分布变化平缓.结合量化计算,在133LYP/6-31G(d)水平上得到(CH3OH)nCH3O-(n=1～4)的可能几何构形,并推断(CH3OH)3CH3O-为-幻数结构.     

Deceleration and electrostatic trapping of OH radicals

A pulsed beam of ground state OH radicals is slowed down using a Stark decelerator and is subsequently loaded into an electrostatic trap. Characterization of the molecular beam production, deceleration, and trap loading process is performed via laser induced fluorescence detection inside the quadrupole trap. Depending on the details of the trap loading sequence, typically 10(5) OH (X2Pi(3/2),J=3/2) radicals are trapped at a density of around 10(7) cm(-3) and at temperatures in the 50-500 mK range. The 1/e trap lifetime is around 1.0 s.     

Formation of CO2 gas and OH groups in CR-39 plastics due to gamma-ray and ions irradiation

FT-IR spectral studies have been made for CR-39 detectors irradiated by gamma-rays, protons and carbon ions. An absorption peak for CO₂ appeared and grew with the fluence. The peak height was found to decrease gradually by storing in air. Absorbance of the OH group produced by ion bombardment was observed separately from that of adsorbed water using an FT-IR system on the beam line. The concentration of OH group would govern the rate of penetration of chemical agents along the track.     

Interaction of excess electrons with water molecules at the early stage of laser-induced plasma generation in water

Forward and backward stimulated Raman scattering (SRS) of OH stretching vibrations are measured when an intense 40 ps pulsed beam is focused into water. Characteristic features related to the interaction of OH groups with excess electrons generated by the strong laser irradiation are observed. The SRS spectra are indicative of the important role which excess electrons play in transient enhancement of the SRS at the initial stage of laser-induced plasma generation in water. The hydration structures around the excess electrons in water are also discussed.     

平面激光诱导荧光实验中激励激光的光束整形

分析了激励激光光强分布对平面激光诱导荧光（PLIF）实验中荧光强度的影响。基于柱面微透镜列阵设计了一套激光片状光束匀滑整形系统,并根据PLIF实验的具体要求,通过光线追迹方法优化了系统参数。建立了片状光束整形实验系统,对染料激光进行了匀滑整形,获得了不均匀性〈4%的均匀片状光束,满足了PLIF实验所需。在此基础上建立了PLIF实验系统,获得了酒精灯火焰和CH4/air预混火焰中OH的二维荧光分布。     

New insights into the oxidation chemistry of pyrrole,an N-containing biomass tar component

Pyrrole, the smallest molecule with a nitrogen atom in the heterocycle ring, is an important tar component from coal and nitrogen-rich biomass devolatilization. Understanding the combustion chemistry of pyrrole can help to elucidate the pollutant formation chemistry from fuel nitrogen, thus enabling cleaner biomass energy utilization technologies. Experimental measurements were performed in a jet stirred reactor coupled with time of flight molecular beam mass spectrometry using synchrotron vacuum ultraviolet beam as photon ionization source, and gas chromatography-mass spectrometry to provide comprehensive measurements of 31 species including nine C₄ and C₅ N-containing compounds. Based on the evidence from the experiments and aiming to improve the kinetic model performance, possible formation routes are proposed with OH addition as the entrance reaction. Reaction rate coefficients for the OH addition channel as well as those for key H-atom abstraction reactions (H, OH, CH_3, and HO₂) were calculated by quantum chemical methods and updated in the model. The updated model can qualitatively predict the identified C₄ N-containing species and perform reasonably well for a large set of experimental data considered for validation, overall improving the performance of the previous model. The influence of the investigated reactions on the predictions of fuel reactivity and pollutant formation motivates further investigations of N-containing fuel chemistry.     

Molecular photodetachment microscopy

The photodetachment microscopy technique, which was originally used with atomic negative ions, is now applied to a molecular anion. The interferograms of several rotational thresholds corresponding to transitions from OH- X (1)Sigma(+) v=0 states to OH X (2)Pi(3/2,1/2) v=0 states have been recorded. No effect due to the 1/r(2) dipolar potential of the neutral molecule appears. Using a double-pass scheme of the laser on the negative ion beam, we measure the energy of the first few detachment thresholds with improved accuracy. The new recommended value of the electron affinity of 16OH is 14,740.996(13) cm(-1), or 1.827 650 3(17) eV.     

Point and planar ultraviolet excitation/detection of hydroxyl-radical laser-induced fluorescence through long optical fibers

We demonstrate an all-fiber-coupled, UV, laser-induced-fluorescence (LIF) detection system of the hydroxyl radical (OH) in flames. The nanosecond-pulsed excitation of the (1,0) band of the OH A(2)∑(+)-X(2)Π system at ～283 nm is followed by fluorescence detection from the (0,0) and (1,1) bands around 310 nm. The excitation-laser beam is delivered through a 400 μm core UV-grade optical fiber of up to 10 m in length, and the fluorescence signal collected is transmitted through a 1.5 mm core 3 m long fiber onto the remote detector. Single-laser-shot planar LIF (PLIF) imaging of OH in flames is also demonstrated using fiber-based excitation. The effects of delivering intense UV beams through long optical fibers are investigated, and the system improvements for an all-fiber-coupled OH-PLIF imaging system are discussed. Development of such fiber-based diagnostics and imaging systems constitutes a major step in transitioning laser diagnostic tools from research laboratories to reacting flow facilities of practical interest.     

Direct measurement of the radiative lifetime of vibrationally excited OH radicals

Neutral molecules, isolated in the gas phase, can be prepared in a long-lived excited state and stored in a trap. The long observation time afforded by the trap can then be exploited to measure the radiative lifetime of this state by monitoring the temporal decay of the population in the trap. This method is demonstrated here and used to benchmark the Einstein A coefficients in the Meinel system of OH. A pulsed beam of vibrationally excited OH radicals is Stark decelerated and loaded into an electrostatic quadrupole trap. The radiative lifetime of the upper Lamda-doublet component of the Chi2Pi3/2, v=1, J=3/2 level is determined as 59.0+/-2.0 ms, in good agreement with the calculated value of 58.0+/-1.0 ms.     

激光诱导等离子体对水OH伸缩振动受激拉曼散射的影响

利用532 nm脉冲激光进行水的受激拉曼散射研究,通过改变激光焦点与水-空气界面的距离,获得截然不同的OH伸缩振动受激斯托克斯和反斯托克斯谱线.焦点距水-空气界面大于20 mm时,只存在±3400cm-1的斯托克斯和反斯托克斯谱线;焦点距离水-空气界面小于20 mm时,存在±3000和±3400cm-1的斯托克斯和反斯托克斯谱线;继续缩小焦点与水-空气界面的距离,3000 cm-1谱线被增强,而3400 cm-1谱线被削弱.研究结果表明,激光诱导水产生的等离子体增强了局部水分子的氢键,导致OH伸缩振动红移,同时过剩电子增强了水的OH伸缩振动受激拉曼散射.     

Formation of advanced tokamak plasmas without the use of an ohmic-heating solenoid

A new operational scenario of advanced tokamak formation was demonstrated in the JT-60U tokamak. This was accomplished by electron cyclotron and lower hybrid waves, neutral beam injection, and the loop voltage supplied by the vertical field and shaping coils. The Ohmic heating (OH) solenoid was not used but a small inboard coil (part of the shaping coil), providing less than 20% of total poloidal flux, was used. The plasma thus obtained had both internal and edge transport barriers, with an energy confinement time of 1.6 times H-mode scaling, a poloidal beta of 3.6, and a normalized beta of 1.6, and a large bootstrap current fraction (>90%). This result opens up a possibility to reduce, and eventually eliminate, the OH solenoid from a tokamak reactor, which will greatly improve its economic competitiveness.     

PLIF法定量测量甲烷-空气火焰二维温度场分布

利用平面激光诱导荧光(PLIF)技术,通过选择适合的OH自由基激励线,定量测量了甲烷-空气燃烧火焰的二维温度场分布。给出炉面中心上方火焰温度随离炉面高度的变化和距炉面12 mm高处沿炉面水平方向变化的实验测量结果并进行了讨论与分析。与利用相干反斯托克斯喇曼散射（CARS）技术进行测温的实验结果相比,该测量的相对不确定度优于5％。     

PLIF法定量测量甲烷-空气火焰二维温度场分布

利用平面激光诱导荧光(PLIF)技术,通过选择适合的OH自由基激励线,定量测量了甲烷-空气燃烧火焰的二维温度场分布。给出炉面中心上方火焰温度随离炉面高度的变化和距炉面12 mm高处沿炉面水平方向变化的实验测量结果并进行了讨论与分析。与利用相干反斯托克斯喇曼散射（CARS）技术进行测温的实验结果相比,该测量的相对不确定度优于5％。     

Efficient Stark deceleration of cold polar molecules

Stark deceleration has been utilized for slowing and trapping several species of neutral, ground-state polar molecules generated in a supersonic beam expansion. Due to the finite physical dimension of the electrode array and practical limitations of the applicable electric fields, only molecules within a specific range of velocities and positions can be efficiently slowed and trapped. These constraints result in a restricted phase space acceptance of the decelerator in directions both transverse and parallel to the molecular beam axis; hence, careful modeling is required for understanding and achieving efficient Stark decelerator operation. We present work on slowing of the hydroxyl radical (OH) elucidating the physics controlling the evolution of the molecular phase space packets both with experimental results and model calculations. From these results we deduce experimental conditions necessary for efficient operation of a Stark decelerator.     

Stochastic electromagnetic beams for LIDAR systems operating through turbulent atmosphere

With the help of the generalized Huygens–Fresnel integral and the ABCD matrix approach a bistatic LIDAR system involving a rough target at a distant location in a turbulent atmosphere is modeled. The system operates by means of an optical beam which has arbitrary spectral composition, and states of coherence and polarization. The rough target is modeled as a combination of a Gaussian mirror and a thin phase screen which induces phase perturbations of the components of the electric field. The analytical form of the cross-spectral density matrix of the returned beam is determined, from which the effect of the rough target on the spectral density (intensity) and polarization of the returned wave is analyzed. E. Watson’s work performed while assigned to the Ladar and Optical Communications Institute, University of Dayton, Dayton, OH 45469, USA.     

Effect of pressure on hydrogen bonding in liquid methanol

We have investigated the effect of pressure on the hydrogen bonding in liquid methanol using Raman spectroscopy. Specifically, we have measured the OH and CO stretching modes and assigned the bands, in agreement with recent IR and crossed molecular beam experiments on methanol clusters. At about 7 to 8 kbar, we note indications that the intrinsic nature of the methanol clusters in our samples has changed. Our results provide support for and extend conclusions derived from Monte Carlo simulations, explain anomalies observed by previous researchers, and provide new insights into general hydrogen-bonding phenomena.     

Infrared spectroscopic characterization of the properties of oxide films grown on Zr surfaces doped under ion-beam irradiation in a vapor-water medium

Infrared spectroscopy is shown to be applicable to the nondestructive check of the state of oxide films on metal substrates doped under irradiation by an ion beam. The reflection spectra of oxide films on zirconium surfaces doped under ion-beam irradiation are obtained. Oxide films were grown by means of oxidation in a vapor-water medium. The analysis of the fine structure of IR spectra has revealed that, at wavelengths of 1–10 μm, the spectra contain resonance lines corresponding to ZrH-and ZrOH-type molecular compounds, OH groups, and other components of oxide.     

Transmission of broadband saturable absorbing mixture for 10.6 μm nanosecond pulses

The transmission through a mixture consisting of sulphur hexafluoride (SF₆), ethyl alcohol (C₂H₅OH) and freon 502 (R502) was investigated with the 3-ns pulses of a TEA CO₂ laser operating at the 10P(20) line. The transmission characteristics of this mixture were measured. Thus, it was confirmed that the radiation beam traversing the gas-filled absorption cell exhibits self-interaction effects in a certain range of gas pressure, cell length and incident energy fluence. A more effective behavior of such an absorber was confirmed for mixtures under lower pressures and proportionally longer cells. Buffering behavior of the additives (C₂H₅OH and R502) was also assessed.