High-resolution infrared and laser photoacoustic spectroscopy of monochloroacetylene

High-resolution vibration—rotation spectra of monochloroacetylene (HCCCl) have been recorded in two different wavenumber regions. Fourier transform infrared interferometer measurements in the 4000–10000cm^?1 range have been performed with a resolution of 0.005–0.016cm^?1 and titanium: sapphire ring laser intracavity photoacoustic measurements in the 12500–12600cm^?1 and 12800–12860cm^?1 regions have been carried out with a Doppler-limited resolution of about 0.02 cm^?1. Altogether, 40 vibrational bands belonging to the HCC³⁵ Cl and 15 bands belonging to the HCC³⁷ Cl isotopic species have been rotationally assigned. A vibrational model, based on the conventional rectilinear normal coordinate theory including anharmonic Fermi, Darling—Dennison and vibrational l-type doubling resonances, has been used to attach vibrational labels to the bands. This approach is found to describe well the rovibrational energy level structure at least in the energy range covered in this work. The model shows that the CH oscillator becomes decoupled from the rest of the molecule as vibrational energy increases.     

Analysis of liquid surface films by pulsed laser photoacoustic spectroscopy

The photoacoustic generation of plane acoustic waves in strongly absorbing or opaque liquids by pulsed laser radiation is discussed both experimentally and theoretically. The regimes of a confined and a free surface of the liquid are considered. The model which takes the temporal shape of the laser pulses applied in the experiments into account, implies that spectroscopic studies are feasible with direct photoacoustic generation and detection also for opaque liquids. The experiments are performed with a tunable hybrid CO₂ laser and piezoelectric detection. For the first time liquid/liquid interfaces are studied by this technique. We demonstrate that the presence of an absorbing liquid film with a thickness of >1 m on the surface of another liquid amplifies the acoustic signal which is detected in the bottom liquid. The enhancement depends on the thickness and the optical and thermal properties of the film medium. The surface layer can be analyzed on the basis of the photoacoustic spectrum. It is also shown that this non-contact method is surface-film selective and should thus prove useful for pollution analysis of liquid surfaces.     

菊酯农药的太赫兹时域光谱定性和定量检测

拟除虫菊酯类农药是一类能防治多种害虫的广谱杀虫剂,主要用于卫生杀虫和农田害虫防治。利用太赫兹时域光谱技术对室温状态下的三种常用于农田害虫防治的菊酯农药高效氯氰菊酯、高效氯氟氰菊酯及溴氰菊酯进行了研究,三种菊酯农药根据其在0.2～2.2THz的吸收谱可清晰地分辨;采用线性回归和偏最小二乘回归对溴氰菊酯、聚乙烯混合物进行定量分析。偏最小二乘回归取得更好结果,其中溴氰菊酯含量的检出限达2.0%,获得的均方差为0.55%,绝对偏差均小于0.8%。结果表明THz-TDS技术在农药分子识别及含量检测方面有很大潜力。     

灵敏的基于分布反馈式半导体激光波长调制光声光谱

利用室温下单模运行的近红外半导体二极管激光,报导了波长调制共振光声光谱结合二次谐波探测技术.实验系统应用到乙炔探测,在1个标准大气压和3毫瓦平均光功率以及3毫秒锁相积分时间条件下其探测灵敏度可达10ppm(体积比),归一化到激光功率和系统带宽最小可探测吸收为4.0×10-8Wcm-1/Hz,并且实验中发现系统最佳压力响应值在2.66×104Pa附近.本实验装置可有效的消除光声光谱系统中常见的窗片和光声腔壁吸收入射光而引起的背景噪声.此外,相对于其他方法我们描述的基于半导体激光共振光声光谱具有很大的优点,为进一步发展便利、实用、便携式环境监测仪器奠定了坚实的基础.     

Optical absorption coefficient of laser irradiated wheat seeds determined by photoacoustic spectroscopy

Present study is concerned with the photoacoustic (PA) assessment of optical absorption coefficient (β) for treated (soaking in the methylene blue dye) and untreated seeds of wheat. The determination of β, based on the Rosencwaig and Gersho model enables the optical absorption coefficient to be assessed directly from the measured amplitude of PA signal. The obtained β values were statistically analyzed. The outcome of the analysis shows significant differences in β values: in the wavelength range from 600 to 700 nm treated (with dye) seeds have higher β than untreated seeds. These seeds were also irradiated (varying time and intensity) with a laser diode emitting at 650 nm. Germination tests were carried out on both, irradiated and control (no irradiated) seeds. The percentage of normal seedlings from treated seeds was found higher (p ≤ 0.05) than from control seeds.     

Mid-infrared photoacoustic spectroscopy of solids using an external-cavity quantum-cascade laser

We describe the use of a pulsed external-cavity quantum-cascade laser (EC-QCL) for the acquisition of mid-IR photoacoustic (PA) spectra of solids. The EC-QCL employed in this work operates from 990 to 1075 cm(-1) (9.30-10.10 microm). A gas-microphone PA cell was used as the detector, and the signal was demodulated using a lock-in amplifier. PA EC-QCL spectra of solids display bands significantly narrower than those in corresponding PA Fourier transform infrared spectra.     

应用近红外漫反射光谱对猪肉肉糜进行定性定量检测研究

利用傅里叶变换近红外漫反射光谱结合不同数学建模算法对不同部位取样的猪肉肉糜进行定性判别建模,并建立猪肉肉糜品质指标脂肪、蛋白质和水分含量的定量检测模型。结果表明:不同部位取样猪肉肉糜判别分析PLSDA模型性能良好,最优模型校正集判别正确率为100%,预测集判别正确率为96%;比较两种方法结合,不同光谱预处理建立各品质指标的定量模型,LS-SVM模型性能优于PLSR模型,脂肪和水分含量最佳预测模型校正及预测相关系数r均高于0.9,蛋白质含量最优模型校正及预测相关系数r,RMSEC,RMSEP和RMSECV分别为0.722,0.593,1.595,1.550和1.888,模型精度需进一步提高。研究表明利用傅里叶变换近红外漫反射光谱快速判别不同部位猪肉肉糜的方法是可行的,脂肪和水分含量定量分析模型从预测精度、稳定性及适应性考虑均具一定的通用性,具有良好的市场应用前景。     

Formaldehyde sensor using interband cascade laser based quartz-enhanced photoacoustic spectroscopy

A novel continuous-wave mid-infrared distributed feedback interband cascade laser was utilized to detect and quantify formaldehyde (H₂CO) using quartz-enhanced photoacoustic spectroscopy. The laser was operated at liquid-nitrogen temperatures and provided single-mode output powers of up to 12 mW at 3.53 m (2832.5 cm^-1). The noise equivalent (1) detection sensitivity of the sensor was measured to be 2.2×10^-8 cm^-1W(Hz)^-1/2 for H₂CO in ambient air, which corresponds to a detection limit of 0.6 parts in 10⁶ by volume (ppmv) for a 10 s sensor time constant and 3.4 mW laser power delivered to the sensor module. PACS 42.62.Fi; 72.50.+b     

Room-temperature diode laser photoacoustic spectroscopy near 2.3 μm

We describe results obtained with a new type of GaSb-based semiconductor distributed feedback multiple quantum well lasers operating in continuous-wave mode at room temperature and above. The lasers show good tunability both by the excitation current and the temperature from -1 °C to +60 °C (4170 to 4360 cm^-1). They operate in a high-power (more than 1 mW) single-mode regime and can be tuned without hopping in the methane and ammonia absorption spectral range of up to 15 cm^-1. Application to photoacoustic detection is also described. Potential use lies in the areas of analytical chemistry of gases and in atmospheric pollution research. PACS 82.80.Kq; 42.62.Fi; 42.55.Px     

Breath ammonia detection based on tunable fiber laser photoacoustic spectroscopy

A new detection method for ammonia in high concentration of CO₂ and H₂O is reported, which uses a wavelength modulated photoacoustic spectrometer based on a near-infrared tunable erbium-doped fiber laser in combination with an optical fiber amplifier. The multi-wavelength (1522.44 nm, 1522.94 nm and 1545.05 nm) photoacoustic signal measurement is established to detect multi-spectrum signal in samples. The problem of ammonia detection in high concentration of CO₂ and H₂O is resolved at atmospheric pressure. The minimum detection limit of 16 ppb (signal-to-noise ratio = 1) in simulated breath samples (5.3% CO₂ and 6.2% H₂O (100% relative humidity at 37°C)) is achieved.     

Fast simultaneous measurement of multi-gases using quantum cascade laser photoacoustic spectroscopy

The authors developed a fast simultaneous method in detecting multi-gases using quantum cascade laser (QCL) based photoacoustic (PA) spectroscopy. We demonstrated the simultaneous measurement of CO and SO₂ concentrations using two QCLs working at 4.56 and 7.38 μm, corresponding to the absorption bands of CO and SO₂, respectively. The modulation frequencies of the two QCLs were 234 and 244 Hz. The response time was 0.6 seconds. A computer sound card was used to process the PA signals. Fast Fourier transform was an essential step to get the amplitudes of the PA signals at different frequencies. The concentration of each gas can be obtained from the PA signal amplitude at the corresponding modulation frequency.     

基于7.6μm量子级联激光的光声光谱探测N_2O气体

近年来,气候变化对地球的生态环境产生严重影响,而大气温室气体在气候变化中具有重要的作用.一氧化二氮(N_2O)作为一种重要的温室气体,其浓度变化对大气环境产生重要影响,因此对其浓度的探测在大气环境研究中具有重要意义.本文开展了基于中国自主研发的7.6μm中红外量子级联激光的共振型光声光谱探测N_2O的研究,建立了N_2O光声光谱传感实验系统.此系统在传统的光声光谱探测的基础上优化改进,采用双光束增强的方式,增加了有效光功率,进一步提高了系统的探测灵敏度.探测系统以1307.66 cm~(-1)处的N_2O吸收谱线作为探测对象,结合波长调制技术对N_2O气体进行探测研究.通过对一定浓度的N_2O气体在不同调制频率和调制振幅的光声信号的探测,确定了系统的最佳调制频率和调制振幅分别为800 Hz和90 mV.在最优实验条件下对不同浓度的N_2O气体进行了测量,获得了系统的信号浓度定标曲线.实验表明,在锁相积分时间为30 ms时,系统的浓度探测极限为150×10~(-9).通过100次平均后,系统噪声进一步降低,实现了大气N_2O的探测,浓度探测极限达到了37×10~(-9).     

Quantitative laser photoacoustic detector spectroscopy: HD 5-0 transitions

Intracavity photoacoustic spectroscopy of HD in the 5-0 R(0), R(1), and R(2) transitions demonstrates the use of a transition of known strength to calibrate the response of a photoacoustic detector by simultaneous in situ measurements. Excellent agreement in line strengths for the HD 5-0 transitions is found with the earlier determination due to McKellar et al. Our experiments, however, were conducted with a pressure-pathlength product only 10^-6 of that used by McKellar et al.     

Broadly tunable quantum cascade laser in cantilever-enhanced photoacoustic infrared spectroscopy of solids

An external cavity quantum cascade laser (EC-QCL) is applied in the photoacoustic detection of solid samples. The EC-QCL used has a broad tuning range of 676 cm^?1 (970–1,646 cm^?1) in the mid-infrared region, which enables accurate broadband spectroscopy of large molecules. The high spectral power density of the EC-QCL is combined with an extremely sensitive optical cantilever microphone of the photoacoustic detector to achieve an ultimate sensitivity. The carbon black, polyethylene, and hair fiber samples were measured with the EC-QCL photoacoustic detection using electrical amplitude modulation to demonstrate the possibilities of the setup. The same measurements were repeated with a Fourier transform infrared (FTIR) spectrometer combined with a photoacoustic detector for a comparison. The EC-QCL photoacoustic setup yielded roughly a decade better signal-to-noise ratios than the FTIR setup with the same measurement time.     

Trace gas sensor based on quartz tuning fork enhanced laser photoacoustic spectroscopy

A compact photoacoustic gas sensor based on a quartz tuning fork and fiber-coupled distributed feedback (DFB) diode laser for detection of trace gas at atmospheric pressure has been developed. The sensor performance was evaluated by detection of water vapor in ambient air at normal atmospheric pressure. A normalized noise equivalent absorption coefficient of 1.68×10⁻⁸ cm⁻¹ W/Hz^1/2 was achieved. Influence of different acoustic microresonators and sample pressure on the sensor performance, and the characterization of the sensor response time were investigated. Approaches to improve the current sensor performance are discussed.     

Sensitive detection of methane with a 1.65 μm diode laser by photoacoustic and absorption spectroscopy

Received: 18 September 1997/Revised version: 11 November 1997     

Study of nanodispersed aluminum and iron alcosols by photoacoustic spectroscopy

Nanodispersed aluminum and iron alcosols were prepared by ultrasonic dispersion of nanodispersed aluminum and iron powders in absolute ethanol. The photoacoustic signal (PAS) produced in modulated CO₂ laser irradiation (1.026 and 1.096 kHz) of alcosols depends on the nature and method of nanoparticle fabrication and does not depend on their concentration in ethanol (within 1-5 g/l). Chemical interaction between metal nanoparticles and ethanol activated by laser irradiation or/and ultrasound is considered as the cause of the PAS.     

Theory of X-ray photoacoustic spectroscopy

PhotoAcoustic Spectroscopy (PAS) in the X-ray region is becoming a new field in PAS research and poses some new problems, such as heat production mechanisms and non-exponential heat distribution due to multi-excitation processes, additional signals caused by escaped fluorescence and electrons, special backing and fronting setup for comparison with absorption experiment, etc. In this paper we treat these problems and include them in an extended theoretical model which can be reduced to the RG model in a special case. The results indicate that the non-exponential heat distribution has no effect on the PA phase. The contribution by gas heating of the escaped electrons can be neglected. The thermally thin gas layers between the sample and the Be windows are sensitive factors to both amplitude and phase. The PA signal generated in the backing gas layer by the backing surface of the sample should be considered to understand the frequency dependence of the PA signal. The expansion of the backing gas layer is the reason for the phase change in PA-EXAFS.     

The carotenoid content in seedlings of maize seeds irradiated by a 650 nm diode laser: Qualitative photoacoustic study

The aim of the present study was to obtain qualitative content of carotenoids in the leaves of maize seedlings the seeds of which were irradiated by the 650 nm diode laser. Four different irradiation doses have been applied; and the control (non-irradiated) seeds used for comparison purposes. Optical absorption spectra of leaves from seedlings were recorded by means of photoacoustic spectroscopy in the 400 to 500 nm range. The experimental unit comprised 10 seeds, among which one seedling was selected at random for each repetition. Data was subjected to variance analysis (ANOVA) using the SAS GLM procedures (SAS, 1998 version). The ANOVA revealed significant (p < 0.05) differences among treatments in the 471 to 478 nm range.