Ba原子吸收系数的光腔衰荡光谱测量

建立了一套脉冲激光腔衰荡光谱实验装置,并将光腔衰荡光谱技术用于原子束吸收光谱的测量,得到了Ba原子的6s6p^1P1←6s6s^1S0吸收线,进一步得出Ba原子在553．548nm附近的吸收系数,同时对吸收系数与原子炉温度的关系进行了分析,得到了较好的实验结果。     

CRDS-CARS-PLIF技术精确定量测量火焰OH浓度实验研究（英文）

在预混甲烷/空气燃烧的平面火焰炉上，采用脉冲式光腔衰荡光谱技术(cavity ring-down spectroscopy, CRDS)实现了对OH分子浓度的定量测量。根据光腔衰荡吸收光谱理论，选取OH的A₂Σ⁺-X₂Π(0,0)电子跃迁带中的P₁(2)吸收谱线构搭建了一套激光波长在308.6 nm的脉冲CRDS实验装置。脉冲CRDS装置中的衰荡光腔是由一对反射率为99.7%的高反射镜组成且其衰荡腔的腔长为270 cm,并测量空腔(光腔中无火焰)的衰荡时间为2.33μs。通过理论分析影响浓度精确测量的实验参数，分别采用平面激光诱导荧光(planar laser induced fluorescence, PLIF)、相干反斯托克斯拉曼散射(coherent anti-stokes Raman scattering, CARS)和脉冲CRDS三种技术精确测量OH的有效吸收长度、高温火焰的温度和有效的光腔衰荡时间。当在平面火焰炉上燃烧预混的甲烷(1.1 L·min^-1)和空气(15...     

CO2/CO干扰下基于腔衰荡吸收光谱的痕量H2S浓度测量

H₂S作为一种有毒且腐蚀性较强的气体污染物,实现其浓度的准确测量意义重大.实际工业过程中,H₂S测量常受其他排放产物的干扰,本文基于腔衰荡吸收光谱技术(CRDS)通过扫描6336—6339 cm^-1范围内的吸收光谱,实现了H₂S/CO₂/CO三组分物质浓度的同步测量,为实际工业过程中物质干扰下的H₂S浓度测量提供新思路.首先,对不同采样长度下提取衰荡时间的准确性进行分析,发现衰荡信号的采样长度约为衰荡时间的8倍时,衰荡时间提取效果最好;通过不同压力对比实验确定最佳实验压力工况为50 kPa,并将最佳采样长度与压力工况应用于H₂S浓度测量.随后,改变H₂S浓度对CO₂/CO干扰下系统对痕量H₂S浓度的测量效果进行检验,并对不同稀释比例下浓度测量结果的线性度进行分析.最后,对本文CRDS系统的检测限进行分析,通过对4组低浓度H₂S光谱的信噪比进行分析,...     

铯6S1/2 -6P3/2 -8S1/2阶梯型系统中超精细能级的多重电磁感应透明

基于铯原子6S_1/2 -6P_3/2 -8S_1/2的阶梯型能级系统,对室温下铯原子气室中的电磁感应透明(EIT)谱进行了研究.由于探测光的频率锁定于基态6S_1/2(F=3)到中间态6P_3/2的超精细跃迁线上,耦合光在中间态6P_3/2和激发态8S_1/2之间扫描,得到的EIT谱具有平坦的背景,提高了光谱的精度.理论上,采用了一个多能级的EIT模型,将其计算结果与所观察的实验现象进行了比较,二者符合得比较好. 关键词： 电磁感应透明 光抽运 超精细结构 阶梯型系统     

基于光声光谱技术的1.5 μm波段 NH3吸收谱线测量与分析

NH₃的检测具有广泛的应用,采用光声法检测NH₃是当前研究的热点,而确定NH₃的吸收谱线则是实现光声法检测NH₃的前提。采用外腔可调谐半导体激光器构造了光声气体检测系统,检测了NH₃的近红外吸收谱线,获得了常温常压下NH₃在1 515~1 532 nm范围内的吸收光谱。实验确认了NH₃在1 515.2,1 516.0,1 518.0,1 519.9,1 522.4,1 527.0,1 531.7 nm处存在强吸收。HITRAN 2004光谱数据库在近红外1.5 μm波段NH₃的吸收谱线数据未见报道,该结果为研究光声法检测NH₃提供了更多可选择的吸收谱线。     

基于高精细度光腔锁频激光的分子吸收光谱测量

利用高精细度光腔锁定激光频率,实现了对分子吸收光谱的高精度测量.光腔采用低热膨胀系数的殷钢结构设计和温度控制,实现了腔长度的稳定;通过将激光频率锁定在光腔纵模上,实现了高频率精度和高灵敏度的光腔衰荡光谱测量.利用该装置示范性地测量了二氧化碳分子在6470.42 cm~(-1)附近的光腔衰荡光谱和色散光谱,得到了高精度的谱线参数,并和数据库谱线参数进行了对比.     

铯原子7S1/2态磁偶极超精细常数的测量

在室温下的原子气室中, 基于铯原子6S_1/2-6P_3/2-7S_1/2(852.3 nm+1469.9 nm) 阶梯型能级系统, 利用电光调制器的主频和±1级边带分别产生的三套双共振吸收光谱, 当驱动电光调制器的信号源频率严格等于7S_1/2态超精细分裂的能级间隔时, 三套谱线中的一些超精细跃迁谱线重叠且线宽最窄, 利用这一现象很好地避免了激光器频率扫描时非线性效应的影响, 测量出了7S_1/2 态超精细分裂能级间隔: 2183.72 MHz±0.23 MHz, 并计算出该态的磁偶极超精细常数: A_hfs= 545.93 m MHz±0.06 MHz, 与文献中报道的测量结果一致.     

SF6分子的10.6μm高分辨射流冷却激光吸收光谱

六氟化硫(SF₆)是一种长寿命的温室气体,其红外吸收光谱对模拟大气辐射平衡非常重要. SF₆也是研究激光分离同位素原理和技术的典型体系之一.由于SF₆分子较重,其室温下的红外光谱非常密集,给利用吸收光谱技术监测不同SF₆同位素分子的相对浓度带来很大困难.本文利用超声射流冷却和像散型多程吸收池技术,测量了32SF₆和33SF₆同位素分子在10.6μm波段的高分辨红外激光吸收光谱.处于振动基态的32SF₆和33SF₆分子在狭缝型超声射流中的转动温度约为10 K,谱线线宽约为0.0008 cm^–1.在此条件下观测到了SF₆一个新的热带,其Q支的位置在941.0 cm^–1附近.将其初步归属为32SF₆的(v1+v2+v3)–(v1+v2)带,对该热带进行简化的转动分析,并讨论利用该热带和33SF₆的v3基频带进行...     

基于光腔衰荡光谱技术的压力计校准方法

压强是工业生产过程中的一个重要参数,其准确测量是过程控制的关键。气体分子光谱线型和线宽取决于分子间相互作用和温度、气压等因素,利用窄线宽气体吸收光谱的压力展宽效应,可通过高分辨地测量气体吸收谱线得到压强信息,实现压力计校准。提出了一种基于光腔衰荡光谱技术和气体吸收谱线压力展宽效应的压力计校准方法。采用5.2 μm可调谐量子级联激光器,基于连续光腔衰荡光谱技术建立了压力计校准实验装置。室温下,测量水汽在1 877 cm-1附近的一吸收谱线,线宽为0.084 21 cm-1,重复性测量误差小于1.53×10-4 cm-1,对应的压强大小为98.12 kPa,检测灵敏度优于0.18 kPa,与高精度压力计读数98.14 kPa一致。利用测试谱线线宽与压强的关系得到压力展宽系数(0.087 12±0.000 965) cm-1·atm-1,与HITARN数据库参考值0.087 1 cm-1·atm-1一致。实验校准了一小量程压力计。结果表明基于光腔衰荡光谱的高分辨吸收谱线测量在压强检测和压力计校准领域具有很好的应用前景。     

光腔衰荡技术与高灵敏吸收探测

腔衰荡光谱技术是一种新兴的主灵敏吸收光谱探测技术,已经被广泛地应用于原子、分子、团簇等吸收光谱的测量,且可实现10^-6－10^-14cm^-1量级吸收的测量,文章综述了腔衰荡光谱技术的发展及在吸收光谱探测上的应用。     

Doppler-limited CW infrared cavity ringdown spectroscopy of the ν1+ν3 OH + CH stretch combination band of jet-cooled methanol

A high resolution cavity ringdown spectrometer (CRDS) has been constructed using a 1.5 μm continuous-wave external-cavity tunable diode laser, a mode-matched near-confocal ringdown cavity, and 2 cm pulsed slit jet. Without signal averaging, the RMS noise in the absorption signal is 1.7 × 10⁻⁹ cm⁻¹. The rotationally resolved overtone spectrum of the OH(ν₁) + CH(ν₃) stretch combination band of methanol between 6510 and 6550 cm⁻¹ has been observed for J^′=0-8 and K^′=0-3 at sub-Doppler resolution. In total, 418 lines are assigned and global fits yield molecular torsion-rotation parameters for the upper state. Four K^′-localized perturbations are analyzed and the pattern of residuals is discussed.     

Continuous-wave cavity-ringdown detection of stimulated Raman gain spectra

Cavity ringdown (CRD) spectroscopy, with its high sensitivity, provides a novel way to perform continuous-wave (cw) stimulated Raman gain (SRG) spectroscopy, rather than by conventional optically detected coherent Raman techniques. Tunable cw laser light at ∼1544 nm is used to probe ringdown decay from a rapidly-swept, high-finesse optical cavity containing a gas-phase sample of interest and itself located inside the cavity of a cw single-longitudinal-mode Nd:YAG ring laser operating at ∼1064.4 nm. This approach is used to measure cw SRG spectra of the ν ₁ fundamental rovibrational Raman band of methane gas at ∼2916.5 cm⁻¹. The resulting SRG-CRD resonances have ringdown times longer than in the off-resonance case, in contrast to the usual shorter ringdown times arising from absorption and other loss processes. Previously reported noise-equivalent sensitivities have been substantially improved, by using a second ringdown cavity to facilitate subtraction of infrared-absorption background signals. Moreover, by employing a ringdown cavity in the form of a ring, the SRG-pump and CRD-detected Stokes beams can co-propagate uni-directionally, which significantly reduces Doppler broadening.     

Band model analysis of the H2O2ν6(b) band

A Mayer-Goody random band model with a Voigt line profile is used to derive interval absorption coefficients, mean line spacings, and pressure-broadening coefficients for the ν₆(b) H₂O₂ band centered at 1265 cm^?1 at 285°K.     

A new optical method of measuring electron impact excitation cross section of atoms: Cross section of the metastable 6s6p P0 level of Hg

We report a new method that is potentially applicable to the measurement of electron impact excitation cross section of any atoms. Measurement of the cross section of the metastable 6s6p ³P₀ level of mercury is conducted to demonstrate the method, which involves using cavity ringdown spectroscopy to determine the absolute number density of mercury atoms in the 6s6p ³P₀ energy level. The measured cross section is 1.7×¹⁰−17 cm² and in agreement with the literature values. Compared with the optical methods that have been used during the last three decades, this new approach not only serves as an alternative optical method, but also is applicable to the atoms under both high and low pressure conditions.     

Far-infrared absorption intensities of H2O and D2O in C6H6

Far-i.r. absorption intensities have been measured for H₂O and D₂O in C₆H₆ solutions. Beer's law plots were found to be nonlinear. From the plots, the equilibrium constants of dimer formation in benzene were estimated to be 2.4M^-1 and 3.3M^-1 for H₂O and D₂O at 20°C, respectively. Based on Onsager's reaction field and including explicitly the effect of differences in molecular size between solute and solvent molecules, the internal moment of a water molecule and the volume ratio of a molecule to a cavity were estimated from the integrated absorption intensity of a D₂O monomer in C₆H₆ as 1.98 D and 0.7, respectively     

Rapidly swept, continuous-wave cavity ringdown spectroscopy with optical heterodyne detection: single- and multi-wavelength sensing of gases

Spectroscopic sensing of gases can be performed with high sensitivity and photometric precision by cavity ringdown (CRD) absorption spectroscopy. Our cavity ringdown spectrometer incorporates continuous-wave (cw) tunable diode lasers, fibre-optic coupling and standard photonics and optical telecommunications components. It comprises a rapidly swept optical cavity in a single-ended optical heterodyne transmitter–receiver configuration, enabling optical absorption of gases to be recorded either as single-frequency scanned spectra or as simultaneous, multi-wavelength tailored spectra. By measuring weak near-infrared rovibrational spectra of carbon dioxide gas (CO₂), with high resolution in the vicinity of 1.53 μm, we have realised a noise-limited absorption sensitivity of 2.5×10^-9 cm^-1 Hz^-1/2. Analytical sensitivity limits (both actual and projected) and prospective gas-diagnostic applications are discussed. Our approach to cw-CRD spectroscopy offers high performance in a relatively simple, low-cost, compact instrument that is amenable to chemical analysis of trace gases in medical, agricultural, industrial and environmental situations. Received: 16 May 2002 / Revised version: 3 June 2002 / Published online: 21 August 2002 RID="*" ID="*"Corresponding author. Fax: +61-2/9850-8313, E-mail: brian.orr@mq.edu.au     

Fast, low-noise, mode-by-mode, cavity-enhanced absorption spectroscopy by diode-laser self-locking

A new technique of cavity enhanced absorption spectroscopy is described. Molecular absorption spectra are obtained by recording the transmission maxima of the successive TEM_oo resonances of a high-finesse optical cavity when a Distributed Feedback Diode Laser is tuned across them. A noisy cavity output is usually observed in such a measurement since the resonances are spectrally narrower than the laser. We show that a folded (V-shaped) cavity can be used to obtain selective optical feedback from the intracavity field which builds up at resonance. This induces laser linewidth reduction and frequency locking. The linewidth narrowing eliminates the noisy cavity output, and allows measuring the maximum mode transmissions accurately. The frequency locking permits the laser to scan stepwise through the successive cavity modes. Frequency tuning is thus tightly optimized for cavity mode injection. Our setup for this technique of Optical-Feedback Cavity-Enhanced Absorption Spectroscopy (OF-CEAS) includes a 50 cm folded cavity with finesse ∼20 000 (ringdown time ∼20 μs) and allows recording spectra of up to 200 cavity modes (2 cm⁻¹) using 100 ms laser scans. We obtain a noise equivalent absorption coefficient of ∼5×10⁻¹⁰ cm⁻¹ for 1 s averaging over scans, with a dynamic range of four orders of magnitude.     

Flexible laser diode trace gas sensor based on cavity ringdown spectroscopy with an optical fiber-coupled high-finesse external-cavity diode laser

A flexible and portable trace nitrogen dioxide sensor based on cavity ringdown spectroscopy using an optical fiber-coupled high-finesse cavity was successfully demonstrated. Tailoring the spatial mode matching condition of the core of an optical fiber and high-finesse external cavity allows for effective optical feedback into an antireflection-coated laser diode for stable resonant enhancement of the external cavity. The external cavity, which works as a ringdown cavity, could be remotely located from the light source and receiver section by only a single mode optical fiber. The sensitivity was found to be 1.0×10⁻⁷ cm⁻¹ in a compact 1-cm³ ringdown cavity volume.     

Three-level depletion by cavity ringdown absorption spectroscopy: proof of concept

Theoretical quantitative considerations as well as experimental data are presented based on absorption population depletion coupled with cavity ringdown spectroscopy. The absorbing number densities inside the cavity are determined by numerical integration of the coupled rate equations. The number of photons involved in absorption, cavity losses due to mirror reflectivity and stimulated emission are taken into account. The principle is to monitor a first transition by cavity ringdown spectroscopy while a second transition, with a state in common, is resonantly excited by the decaying radiation of different frequency also trapped inside the optical cavity. A numerical example is given for atomic lines of neon and the measurements carried out in a supersonic slit-jet expansion discharge demonstrate the feasibility of the technique. The technique is also proven to work with two resonant transitions of C₂. Translational velocity of the jet modifying the rate equations is included in the model.