Excitation,inversion, and relaxation mechanisms of the HCN FIR discharge laser

Millimeter/submillimeter rotational absorption spectroscopy has been used as a diagnostic probe of a cw HCN discharge laser. This sensitive technique allowed in situ absolute population measurements of a number of vibrational states of HCN (including the upper lasing state) and other pertinent molecules. This unique set of data, which was obtained under a variety of discharge and laser conditions, is directly related to excitation, inversion, and relaxation processes. Along with elucidating several fundamental aspects of inversion and relaxation these results also strongly indicate that the primary laser excitation mechanism is near-resonant energy transfer from vibrationally excited nitrogen, N₂ (v=1), to the fundamental symmetric stretching mode of hydrogen cyanide, HCN(100), and subsequent thermal population exchange between HCN (100) and the upper laser state, HCN (110).Work supported by ARO Contract DAAG-29-83-K-0078     

N2O molecular tagging velocimetry

A new seeded velocity measurement technique, N₂O molecular tagging velocimetry (MTV), is developed to measure velocity in wind tunnels by photochemically creating an NO tag line. Nitrous oxide “laughing gas” is seeded into the air flow. A 193 nm ArF excimer laser dissociates the N₂O to O(¹D) that subsequently reacts with N₂O to form NO. O₂ fluorescence induced by the ArF laser “writes” the original position of the NO line. After a time delay, the shifted NO line is “read” by a 226-nm laser sheet and the velocity is determined by time-of-flight. At standard atmospheric conditions with 4% N₂O in air, ∼1000 ppm of NO is photochemically created in an air jet based on experiment and simulation. Chemical kinetic simulations predict 800–1200 ppm of NO for 190–750 K at 1 atm and 850–1000 ppm of NO for 0.25–1 atm at 190 K. Decreasing the gas pressure (or increasing the temperature) increases the NO ppm level. The presence of humid air has no significant effect on NO formation. The very short NO formation time (<10 ns) makes the N₂O MTV method amenable to low- and high-speed air flow measurements. The N₂O MTV technique is demonstrated in air jet to measure its velocity profile. The N₂O MTV method should work in other gas flows as well (e.g., helium) since the NO tag line is created by chemical reaction of N₂O with O(¹D) from N₂O photodissociation and thus does not depend on the bulk gas composition.     

Development of a cw-laser-based cavity-ringdown sensor aboard a spacecraft for trace air constituents

The progress in the development of a sensor for the detection of trace air constituents to monitor spacecraft air quality is reported. A continuous-wave (cw), external-cavity tunable diode laser centered at 1.55 μm is used to pump an optical cavity absorption cell in cw-cavity ringdown spectroscopy (cw-CRDS). Preliminary results are presented that demonstrate the sensitivity, selectivity and reproducibility of this method. Detection limits of 2.0 ppm for CO, 2.5 ppm for CO₂, 1.8 ppm for H₂O, 19.4 ppb for NH₃, 7.9 ppb for HCN and 4.0 ppb for C₂H₂ are calculated. Received: 3 April 2002 / Revised version: 3 June 2002 / Published online: 21 August 2002 RID="*" ID="*"Corresponding author. Fax: +1-202/994-5873, E-mail: Houston@gwu.edu     

激光等离子体光丝中太赫兹频谱的调控

研究了双色激光场激发空气成丝产生太赫兹辐射频谱的变化规律.实验观察到随驱动光功率和光丝长度增加,太赫兹光谱主要发生红移的现象.分析表明,由于等离子体密度的增加,太赫兹辐射的趋肤深度减小,等离子体吸收主导了红移的发生.在光丝足够短的条件下,趋肤深度远大于光丝长度,从而产生等离子体振荡主导的太赫兹辐射光谱蓝移.本研究为超快宽带太赫兹辐射的频谱调控提供了新思路.     

Near- and mid-infrared laser monitoring of industrial processes, environment and security applications

The advantages of infrared laser monitoring in terms of sensitivity, selectivity and the ability of non-intrusive detection of gases are reviewed. Emphasis is laid on direct absorption spectroscopy and evanescent-field spectroscopy. The performance of the latter for gas detection in the near-infrared is demonstrated for the analysis of volcanic gases. For industrial process control, direct mid-infrared absorption spectroscopy is used to detect CO in the high-temperature atmosphere of a glass melting furnace. For both applications portable, stable, rugged and easy-to-handle laser systems are needed. Mid-infrared absorption spectroscopy is also applied to detect different explosives. Material evaporation is achieved by plasma generation with a pulsed laser at high repetition rate. Energetic materials contain high concentrations of nitrogen; therefore NO is present in the generated plasma. However, the rate at which NO is produced varies in a highly characteristic manner for different energetic materials. This enables the distinction between different types of explosives.     

Experimental confirmation of high-stability of fluorescence in a femtosecond laser filament in air

Femtosecond laser filamentation is particularly interesting for remote sensing pollutant in the atmosphere. In this work, we investigate the local shot-to-shot stability of the filament induced fluorescence of nitrogen in air. It is found that the root-mean square fluctuation of the fluorescence signal is at least one order of magnitude lower than that of the linear propagation case. In practice, it would contribute to improve the robustness of long distance spectroscopic analysis of the fluorescence of pollutant molecules inside the filament. We further point out that this unique property of filament induced fluorescence spectroscopy is because of the intensity clamping, a profound phenomenon of filamentation.     

近红外波段CO高灵敏检测的稳定性研究

本文采用中心波长1566.64 nm的DFB激光器, 结合光程长度为56.7 m 的多次反射池, 对不同浓度的CO气体进行了长时间测量, 分析了系统的稳定性和线性度. 通过计算Allan方差, 预测了在积分时间为30 s时, 系统检测极限为0.25 ppmv, 基本上实现了在近红外波段CO的高灵敏检测. 关键词： 可调谐半导体激光吸收光谱 多次反射池 CO检测 Allan方差     

CH4/空气混合物中含N、S组分对生成污染物的影响

本文采用详细的CH4/空气反应机理,在良搅拌反应器的条件下,计算了初始含NH3、HCN和H2S对污染物NO和SOx(SO、SO2)生成规律的影响.结果表明,初始含N、S的组分可使NO和SOx的排放量明显增加.同时还利用敏感性分析对污染物形成的重要反应进行讨论.     

Backward supercontinuum emission from a filament generated by ultrashort laser pulses in air

Backward emission of the supercontinuum from a light filament induced by high-intensity femtosecond laser pulses propagating in air has been observed to be enhanced compared with linear Rayleigh-Mie scattering. This enhancement is interpreted as a nonlinear scattering process onto longitudinal refractive-index changes induced by the laser pulse itself. The spectral dependence of the supercontinuum angular distribution is also investigated.     

两种后处理方法对HfO2薄膜性能的影响

利用电子束蒸发和光电极值监控技术制备了氧化铪薄膜,并分别用两种后处理方法(空气中退火和氧等离子体轰击)对样品进行了处理.然后,对样品的透过率、吸收和抗激光损伤阈值进行了测试分析.实验结果表明,两种后处理方法都能不同程度地降低了氧化铪薄膜的吸收损耗、提高了抗激光损伤阈值.实验结果还表明,氧等离子体轰击的后处理效果明显优于热退火,样品的吸收平均值在氧等离子体后处理前后分别为34.8 ppm和9.0 ppm,而基频(1 064 nm)激光损伤阈值分别为10.0 J/cm2和21.4 J/cm2.     

CH3与NO2的反应

用时间分辨傅立叶红外光谱法和量子化学计算，研究了CH3自由基与NO2的基元反应．由248 nm激光光解CH3Br或CH3I得到CH3自由基．首次观测到了振动激发的产物OH、HNO和CO2．另一产物NO也被证实．由此确定了反应通道CH3O+NO，CH2NO+OH 和HNO+H2CO．其中CH3O+NO是主要的反应通道．还用CCSD(T)/6-311++G(df,p)//MP2/6-311G(d,p)的方法对上述通道的机理在理论上做了研究．理论计算的结果与实验观察相符．     

Time resolved simultaneous detection of 14NO and 15NO via mid-infrared cavity leak-out spectroscopy

We present a ring-down absorption spectrometer based on a continuous-wave CO laser in the mid-infrared spectral region near lambda = 5 microm. Using a linear ring-down cavity (length: 0.5 m) with high reflective mirrors (R = 99.988 %), we observed a noise-equivalent absorption coefficient of 3 x 10(-10) cm(-1)Hz(-1/2). This corresponds to a noise-equivalent concentration of 800 parts per trillion (ppt) for (14)NO and 40 ppt for (15)NO in 1 s averaging time. We achieve a time resolution of 1 s which allows time resolved simultaneous detection of the two N isotopes. The delta(15)N value was obtained with a precision of +/-1.2 per thousand in a sample with a NO fraction of 11 ppm. The simultaneous detection enables the use of (15)NO as a tracer molecule for endogenous biomedical processes.     

Application of the overtone CO laser to atmosphere gas analysis

In this paper, we consider the applicability of the overtone CO laser to atmosphere gas analysis. The atmosphere transmittance was calculated for lasing lines at the overtone frequencies of the CO laser. The results of the search for informative wavelengths for sensing the minor gas components of the atmosphere N₂O, NO₂, H₂CO, CH₄, C₂H₂, HCl, HBr, and HCN are presented.     

激光诱导荧光技术在高焓空气氮原子测量中的应用

高超声速飞行器再入大气层时,受到激波的压缩和激波层内粘性阻滞作用,周围绕流流场的空气温度在4 000～15 000 K之间,使空气中的氧、氮分子发生离解,从而出现高温气体效应,形成高焓化学非平衡流。飞行器表面防热材料基本都含有大量的碳元素,通常情况下主要是氧原子与碳发生反应,但在焓值大于18 MJ·kg-1的情况下,氮原子与飞行器表面碳反应的无量纲烧蚀因子BC_N＞0.172 5,此时碳在高焓空气介质中的无量纲质量烧蚀因子BC_air＞0.345,因此,碳的氮化烧蚀变得非常剧烈,和氧化烧蚀相当;同时离解的氮原子也会在飞行器表面发生催化复合反应放出大量的热,使飞行器表面承受更多的热载荷。因此,分析高焓化学非平衡流流场中的氮原子具有现实意义。在地面模拟设备中建立高焓化学非平衡流场,对氮原子进行测量技术,可以很好的对其进行研究。双光子吸收激光诱导荧光（TALIF）技术作为一种非接触测量,在不干扰流场环境的情况下,可以直接获得氮原子的浓度分布。对流场氮原子激发荧光,通过布置在风洞试验段外与流场和激光形成的平面呈垂直方向的ICCD获取到二维氮原子的荧光信号。为确保荧光图像的清晰及视场合适,选择了Nikon f=50 mm F/1.4镜头作为前级收光设备。实验成像采用50次曝光的累计结果,以消除气流扰动及激光能量抖动造成的不确定性。实验中,在理论激发波长附件进行测试,优化选择出206.717 nm作为正式实验中脉冲激光的最佳激发波长。在最佳激光波长条件下,从小到大调整激发能量,获得了该环境下的氮原子激发的非饱和线性区为1.8 mJ以下。正式实验过程中激光能量为1.6 mJ,处于线性区。对所获取的荧光图像进行处理,提取激光中心线上的荧光强度进行分析,发现无论是亚声速还是超声速,荧光强度沿径向都呈驼峰状分布,与之前氧原子测试结果比较,发现流场中氮分子还未完全离解,这符合实验风洞流场特性。     

Differentiation of Positional Isomers of Propyl Alcohols Using Filament-Induced Fluorescence

We experimentally demonstrate the recognition of positional isomers of propyl alcohol vapor through nonlinear fluorescence induced by high-intensity femtosecond laser filaments in air.By measuring characteristic fluorescence of n-propyl and isopropyl alcohol vapors produced by femtosecond filament excitation,it is found that they show identical spectra,that is,those from molecular bands of CH,C_2,NH,OH and CN,while the relative intensities are different.By comparing the ratios of the CH and C_2 signals,the two propyl alcohol isomers are differentiated.The different signal intensities are ascribed to different ionization potentials of the two isomer molecules,leading to different production efficiencies of fluorescing fragments.     

Infrared intensity enhancement of the CN stretch of HCN by coadsorbed CO on the Cu(100) surface

Reflection absorption infrared spectra reveal a strong enhancement in the intensity of the CN stretch in a mixed ordered overlayer of HCN and CO on the Cu(100) surface. Various combinations of HCN and CO isotopomers show that the intensity enhancement decreases with increasing frequency difference between nu(CN) and nu(CO). The intensity of the 2092 cm(-1) band of H12C14N is enhanced by a factor of 155+/-20 through coupling to the 2077 cm(-1) band of 12C16O. A simple two-state coupling model explains the isotopomer dependence of the degree of enhancement.     

Chemical pumping of the far infrared HCN laser

This paper reports the observation of 337 μm and 311 μm stimulated emission from HCN in which the (11¹0)?(04⁰0) inversion has been established by photopumping of HCN and by chemical pumping with reactions between CN and H₂ or saturated hydrogen-rich organic compounds. Similarities in output pulse behavior between the discharge and chemical versions of the HCN laser are suggestive that the pumping mechanism in the discharge is the chemical reaction, CN + H₂ → HCN² + H. The well-known inefficiency of this laser is then due to the fact that the reaction is a slow one and its exothermicity does not match the energy of the upper lasing level, but depends for inversion on (randomizing) relaxation into (11¹0) among others. Substantial improvements in the power of the HCN laser cannot be made from this route.     

Propagation offset characteristics of annular laser beams from confocal unstable resonators through the natural atmosphere

Based on the laser field from a positive confocal unstable resonator (ab initio), the propagation characteristics of the beam through atmosphere are investigated by means of fast Fourier transform (FFT), and it is assumed that the refractive index and absorption of atmosphere are only decided by the air molecules. Considering the thermal effects of the laser beam and wind velocity of air, we evaluate the beam offsets, beam spreading, and beam qualities of the laser field transmitting from the resonator through the natural atmosphere. Meanwhile, the far-field intensity distributions of the laser beam with various intensities in the atmosphere are obtained. It is shown that the far-field diffraction patterns are deformed due to absorption of air and wind velocity. β parameter and Strehl ratio, which are usually used for high-power lasers, are introduced to estimate the beam quality characteristics. With increasing the intensity of the beam, the far-field diagram patterns are outspreaded, the peak intensities reduced, the peak centers shifted, and the beam quality characteristics greatly degraded.     

原位红外光谱法研究HMX炸药的热分解过程

原位红外光谱法是一种新兴的动态研究方法.该方法具有原位实时监控和红外光谱精确分析物质化学结构的优点,能够实时跟踪材料在不同温度下的化学变化,测定材料的微观结构与温度的关系.文章采用原位红外光谱研究了炸药1,3,5,7-四硝基-1,3,5,7-四氮杂环辛烷(HMX)在5℃·min-1升温条件下的热分解过程.研究结果表明:HMX在205℃发生C-N键和N-N键的断裂,随着温度的升高,C-N键的断裂速率远高于N-N键的断裂速率,表明C-N键的断裂是HMX的主要断键方式,在C-N键的断裂中伴随着N-N键的断裂.同时环的张力增大,表明断键的HMX产生分子内重新结合.检测到HMX的分解所释放出的CO2,N2O,CO,NO,HCHO,HONO,NO2和HCN等八种气体.根据HMX分解中凝聚相结构的变化和气相产物,推出HMX的分解机理:HMX产生C-N键的断裂,会释放出HCHO和N2O以及HONO和HCN;N-N键的断裂会释放出NO2.     

利用光谱技术研究硝基甲烷快速反应波传播

本文利用研制的光谱技术，在爆炸激波管中成功地研究了硝基甲烷和氧混合物快速反应波传播的微观特性。光谱测量结果说明：ＯＨ基和Ｈ_２Ｏ、ＮＨ_３的辐射没有发生明显改变，ＣＨ_３Ｏ、ＣＨ、ＮＯ_２、ＣＯ_２、ＮＯ、ＨＣＮ的辐射随反应波传播距离的增加而增长；但是ＣＯ和ＣＨ_２Ｏ的辐射呈倒“Ｕ”型曲线。本文的研究为我国开展含能材料快速反应微观机理的研究探索到一个方法，这种方法投资不大，而且行之有效。