Diode-laser-based ultraviolet absorption sensor for nitric oxide

An all-solid-state continuous-wave laser system for ultraviolet absorption measurements of the nitric oxide (NO) molecule has been developed and demonstrated. The single-mode, tunable output of a 10-mW, 395-nm external-cavity diode laser (ECDL) is sum-frequency-mixed with the output of a 115-mW, frequency-doubled, diode-pumped Nd:YAG laser in a beta-barium-borate crystal to produce 40 nW of tunable radiation at 226.8 nm. The wavelength of the 395-nm ECDL is then scanned over NO absorption lines to produce fully resolved absorption spectra. Initial results from mixtures of NO in nitrogen in a room-temperature gas cell are discussed. The estimated NO detection limit of the system for a demonstrated absorption sensitivity of 2×10^-3 is 0.2 ppm per meter of path length for 300 K gas. The estimated accuracy of the measurements is ±10%. Received: 25 February 2002 / Revised version: 31 May 2002 / Published online: 8 August 2002     

Diode-laser-based sensor for ultraviolet absorption measurements of atomic mercury

A new sensor has been developed for measuring atomic mercury using absorption spectroscopy with 254-nm radiation generated from two sum-frequency-mixed diode lasers. Beams from a 375-nm external-cavity diode laser and a 784-nm distributed feedback diode laser are mixed in a beta-barium-borate crystal to generate approximately 4 nW of ultraviolet radiation. The development of the sensor is described along with extensive characterization experiments in a mercury vapor cell in the laboratory. An accuracy of ±6% in the absolute concentration of atomic mercury has been demonstrated by comparison with equilibrium vapor pressure calculations. The detection limit is approximately 0.1 parts per billion of atomic mercury in a meter path length for 300-K gas and a 10-s integration time. The insensitivity of the sensor to broadband attenuation is demonstrated. Measurements of collision-broadening coefficients for air, N₂, Ar, and CO₂ are reported, and implementation of wavelength-modulation spectroscopy with the sensor is demonstrated. Finally, results are presented from measurements with the sensor in situ in the exhaust stream of an actual coal-fired combustor. PACS 07.07.Df; 42.62.Fi; 42.65.Ky; 42.72.Bj     

Laser-induced fluorescence detection of hydroxyl (OH) radical by femtosecond excitation

The development of a laser-induced fluorescence detection scheme for probing combustion-relevant species using a high-repetition-rate ultrafast laser is described. A femtosecond laser system with a 1 kHz repetition rate is used to induce fluorescence, following two-photon excitation (TPE), from hydroxyl (OH) radicals that are present in premixed laminar flames. The experimental TPE and one-photon fluorescence spectra resulting from broadband excitation into the (0,0) band of the OH A(2)∑(+)-X(2)Π system are compared to simulated spectra. Additionally, the effects of non-transform-limited femtosecond pulses on TPE efficiency is investigated.     

UV cavity enhanced absorption spectroscopy of the hydroxyl radical

We present the application of a continuous-wave ultra-violet tuneable light source for detection of the hydroxyl radical (OH) using cavity-enhanced absorption spectroscopy of the Q₁₁(2) and Q₂₁(2) absorption lines in the A²⁺(v=0)²_3/2(v=0) band at ca. 308 nm. A tuneable infra-red diode laser operating at 835 nm and either an Ar⁺ laser or a single frequency continuous-wave intracavity frequency-doubled diode laser, both operating at ca. 488 nm, were used to produce 0.1–0.5 W of tuneable radiation at ca. 308 nm by sum frequency generation in a BaB₂O₄ crystal. Cavity enhanced absorption spectroscopy was used to detect OH generated by UV photolysis of water vapour in argon, nitrogen, neon and helium at atmospheric pressure. A noise-equivalent (1) absorption sensitivity of 2.1×10^-7 cm^-1Hz^-1/2 measured over 128 scans in a time of 1.16 s was demonstrated with mirrors of reflectivity 0.9963 in a cavity of length 58.5 cm for a 2 cm^-1 scanning range at a UV power of 0.5 W. An OH detection limit (1) of 3.84×10⁹ moleculecm^-3 was estimated in argon at atmospheric pressure. OH collisional broadening in humidified N₂, Ar, Ne and He was determined at atmospheric pressure . PACS 39.30.+w; 42.55.Px; 42.62.Fi; 42.68.Ca     

Exploring the reaction channels between arsine and the hydroxyl radical

The aim of this study was to present the reaction mechanism channels between arsine (AsH₃) and hydroxyl (OH) which was evaluated at CCSD(T)/CBS//CCSD/cc-pVTZ level. One potential channel is the hydrogen abstraction pathway (R1), leading to AsH₂ and H₂O products, which occurs due to the formation of an entrance complex (AsH₃OH) followed by a 1,2-hydrogen shift pathway (involving the proton transfer from the arsine group to hydroxyls, with one leading to the products). Additional channels are accessed via H-elimination pathways of the entrance complexes, forming arsinous acid (AsH₂OH; R2) and arsine oxide (AsH₃O; R3). In this respect, R2 is the only exoergic route of the three exit channels, representing the major branching ratio at 200–1000 K and, after 2000 K, R1 increases gradually becoming the major route of this reaction. In contrast, even at 4000 K, R3 is a highly unfeasible pathway. Therefore, the information predicted here provides new insights into the neutral–neutral chemical reaction dynamics regarding the Group V hydrides. On the other side, the R2 pathway may have some potential to solve the arsine oxidation puzzle as a possible primary pathway to the arsenic-oxygen species formation.     

Intensification of hydroxyl radical production in sonochemical reactors

The efficacy of sonochemical reactors in chemical processing applications has been well established in the laboratory scale of operation though at a given set of operating parameters and no efforts have been directed in terms of maximizing the free radical production. In the present work, the effect of different operating parameters viz. pH, power dissipation into the system, effect of additives such as air, haloalkanes, titanium dioxide, iron and oxygen on the extent of hydroxyl radical formation in a sonochemical reactor have been investigated using salicylic acid dosimetry. Possible mechanisms for oxidation of salicylic acid in the presence of different additives have also been established. It has been observed that acidic conditions under optimized power dissipation in the presence of iron powder and oxygen result in maximum liberation of hydroxyl radicals as quantified by the kinetic rate constant for production of 2,5- and 2,3-dihydroxybenzoic acid. The study has enabled the optimization of the conditions for maximum efficacy of sonochemical reactors where free radical attack is the controlling mechanism for the chemical processing applications.     

Sonocatalytic facilitation of hydroxyl radical generation in the presence of TiO2

The generation of hydroxyl (OH) radicals was investigated during ultrasonic irradiation and in the presence of TiO(2). The effect of TiO(2) on an ultrasonic system's oxidation power was evaluated by examining the oxidation of salicylic acid. The generation of the salicylic acid derivatives, 2,3-dihydroxybenzoic acid (DHBA) and 2,5-DHBA, was measured by high-performance liquid chromatography coupled with electrochemical detection under different experimental conditions. The presence of TiO(2) enhanced the generation of DHBA during ultrasonic irradiation, thus indicating a higher oxidation power in the ultrasonic system. Al(2)O(3) also increased the generation of DHBA during irradiation; however, the effect of TiO(2) was found to be higher than that of Al(2)O(3). The addition of OH radical scavengers such as dimethylsulfoxide (DMSO), methanol and mannitol significantly suppressed the production of DHBA, and DMSO was found to have the highest suppressive effect among all scavengers. The effects of dissolved gases on the generation of OH radicals were further studied, and their power was found to be in the order Xe > Ar > O(2) > N(2). The degassing of the irradiation solution completely suppressed the generation of OH radicals. These results indicate that the presence of TiO(2) accelerates the generation of OH radicals during ultrasonic irradiation, and that the process may be mediated through the induction of cavitation bubbles in irradiating solutions.     

Nonimaging speckle interferometry for high-speed nanometer-scale position detection

We experimentally demonstrate a nonimaging approach to displacement measurement for complex scattering materials. By spatially controlling the wavefront of the light that incidents on the material, we concentrate the scattered light in a focus on a designated position. This wavefront acts as a unique optical fingerprint that enables precise position detection of the illuminated material by simply measuring the intensity in the focus. By combining two fingerprints we demonstrate position detection along one in-plane dimension with a displacement resolution of 2.1 nm. As our approach does not require an image of the scattered field, it is possible to employ fast nonimaging detectors to enable high-speed position detection of scattering materials.     

Nasicon sensor for NO detection in the automotive exhaust

Development of a sensor where both electrodes are exposed to the same gas offers simple design with no need for a separate reference gas with a complicated sealing system. Such sensors are still in research stage. The NASICON electrolyte, which has high ionic conductivity at the temperature of the exhaust gas mixture, allows such sensors to work without a heater, making a simple and miniaturised construction technique. Using two asymmetric electrodes, where each electrode has a different catalytic activity at a given temperature, can result in anemf, which is a function of partial pressure of NO. A NASICON sensor with SmFeO₃ as a sensing electrode and Au as an oxygen electrode was calibrated as a NO sensor and tested in oxidising atmosphere in simulated exhaust gases. The sensor responded quickly with reproducibleemf values. Paper presented at the 9th EuroConference on Ionics, Ixia, Rhodes, Greece, Sept. 15 – 21, 2002.     

Nanoscale microcavity sensor for single particle detection

Recently we demonstrated a biosensor based on a two-dimensional photonic crystal microcavity for detection of proteins. We present a theoretical and experimental study of a modified structure for single particle detection. With an active sensing volume of approximately 0.15 microm(3), the device is capable of detecting approximately 1 fg of matter. Its performance is tested with latex spheres with sizes that fall in the size range of a variety of viruses.     

Coherent probe-pump-based Brillouin sensor for centimeter-crack detection

We provide a theoretical explanation for a coherent probe-pump-based Brillouin sensor system that achieves centimeter spatial resolution with high-frequency resolution. It was recently discovered that, when a combination of cw and pulsed light (the probe beam) interacts with a cw laser (the pump beam), centimeter spatial resolution with high-frequency resolution can be achieved even though the probe-pulse duration is 1.5 ns [Opt. Lett. 29, 1485 (2004)]. Our study reveals that the coherent portion inside the pulse length of these two interactions caused by the same phase is responsible for this behavior. It allows us to detect 1.5-cm outer-layer cracks on an optical ground-wire cable.     

低强度超声场中羟自由基分光光度法检测研究

研究了不同pH(7～11)、不同乙醇体积分数(45%～85%)的孔雀石绿乙醇溶液的紫外-可见光吸收特性,通过最大吸收波长为620nm处的吸光度与浓度关系工作标准曲线,在低强度超声场中,分别检测超声条件和非超声条件下孔雀石绿溶液吸光度变化,两者的差值就是超声场产生的羟自由基导致的孔雀石绿吸光度变化,从而间接测定了超声场产生的羟自由基浓度。确定了4个超声条件产生的羟自由基浓度,最小检测限达到8.4×10-6 mmol.L-1,相对标准偏差为9.4×10-5～3.7×10-4 mmol.L-1,显示出较高精确度和较好重复性,特别适于中性、碱性乙醇溶液体系中羟自由基极低浓度情况下的快速检测。由于孔雀石绿对热敏感,因此该法可以比测量温度,更好地反映超声场热效应的作用。     

Theoretical mechanistic studies on the degradation of alizarin yellow R initiated by hydroxyl radical

The degradation of azo dyes has attracted many research efforts not only due to the resulting environmental problems but also because the azo compounds with various substituents may show different degradation mechanism. It has been computationally found here, for the first time, that the HO? initiated cleavages of C–N and N–N bonds of alizarin yellow R with carboxyl group are kinetically competitive. In view of the formation of HO? adducts, the C–N and N–N bond cleavages of the hydrazone tautomer of alizarin yellow R are also kinetically competitive, but the former is more thermodynamically favorable. This result is different from that previously reported for the hydrozone tautomers of Acid Orange 7 and Acid Orange 8 containing hydroxyl and azo groups in neighboring positions, which are favorable to follow C–N bond cleavage mechanism both kinetically and thermodynamically. The decarboxylation occurs via an attack of HO? to the benzene ring carbon connecting to the carboxyl group rather than a direct attack of HO? to the carboxyl carbon atom. The anion form has higher reactivity than the neutral form in all of the reactions investigated. In addition, a water molecule as a proton relay reagent could significantly reduce the energy barrier for the N–N bond cleavage of alizarin yellow R. Copyright © 2014 John Wiley & Sons, Ltd.     

荧光光谱法研究羟基自由基诱导的酪氨酸氧化

二酪氨酸是酪氨酸氧化的标志性产物.为研究影响羟基自由基诱导酪氨酸氧化的因素,采用同步荧光光谱结合二维相关技术研究羟基自由基诱导酪氨酸氧化的反应过程.结果表明:pH变化时,二酪氨酸的荧光峰位、峰强发生变化.在羟基自由基诱导酪氨酸氧化体系中,随着酪氨酸浓度的增加,二酪氨酸产量升高;随着过氧化氧浓度的增加,二酪氨酸产量降低;...     

Cascaded frequency upconversion for high-speed single-photon detection at 1550 nm

We present a device for two-stage frequency upconversion of single-photon-level signals in the 1.55 μm telecom band to the green spectral region with low excess noise, suitable for detection by low-timing-jitter silicon single-photon avalanche photodiodes (APDs). We achieve a net conversion efficiency of 87% and a system timing jitter below 70 ps FWHM, dominated by the jitter of the APD. Modifications of our device are suitable for downconversion of single photons from visible-wavelength quantum emitters into the telecom band.     

A nano-metallic-particles-based CMOS image sensor for DNA detection

In this paper we report on a study of the CMOS image sensor detection of DNA based on self-assembled nano-metallic particles, which are selectively deposited on the surface of the passive image sensor. The nano-metallic particles effectively block the optical radiation in the visible spectrum of ordinary light source. When such a technical method is applied to DNA detection, the requirement for a special UV light source in the most popular fluorescence is eliminated. The DNA detection methodology is tested on a CMOS sensor chip fabricated using a standard 0.5 μm CMOS process. It is demonstrated that the approach is highly selective to detecting even a signal-base mismatched DNA target with an extremely-low-concentration DNA sample down to 10 pM under an ordinary light source.     

SQUID sensor application for small metallic particle detection

High-Tc superconducting quantum interference device (SQUID) is an ultra-sensitive magnetic sensor. Since the performance of the SQUID is improved and stabilized, now it is ready for application. One strong candidate for application is a detection system of magnetic foreign matters in industrial products or beverages. There is a possibility that ultra-small metallic foreign matter has been accidentally mixed with industrial products such as lithium ion batteries. If this happens, the manufacturer of the product suffers a great loss recalling products. The outer dimension of metallic particles less than 100 μm cannot be detected by an X-ray imaging, which is commonly used for the inspection. Ionization of the material is also a big issue for beverages in the case of the X-ray imaging. Therefore a highly sensitive and safety detection system for small foreign matters is required. We developed detection systems based on high-Tc SQUID with a high-performance magnetic shield. We could successfully measure small iron particles of 100 μm on a belt conveyer and stainless steel balls of 300 μm in water. These detection levels were hard to be achieved by a conventional X-ray detection or other methods.     

Multi-wavelength laser sensor for intruder detection and discrimination

An intruder detection and discrimination sensor with improved optical design is developed using lasers of different wavelengths to demonstrate the concept of discrimination over a distance of 6 m. A distinctive feature of optics is used to provide additional transverse laser beam scanning. The sample objects used to demonstrate the concept of discrimination over a distance of 6 m are leaf, bark, black fabric, PVC, wood and camouflage material. A camouflage material is chosen to illustrate the discrimination capability of the sensor. The sensor utilizes a five-wavelength laser combination module, which sequentially emits identically-polarized laser light beams along one optical path. A cylindrical quasi-optical cavity with improved optical design generates multiple laser light beams for each laser. The intensities of the reflected light beams from the different spots are detected using a high speed area scan image sensor. Object discrimination and detection is based on analyzing the Gaussian profile of reflected light at the different wavelengths. The discrimination between selected objects is accomplished by calculating four different slopes from the objects' reflectance spectra at the wavelengths 473 nm, 532 nm, 635 nm, 670 nm and 785 nm. Furthermore, the camouflage material, which has complex patterns within a single sample, is also detected and discriminated over a 6 m range by scanning the laser beam spots along the transverse direction.     

LIF diagnostics of hydroxyl radical in a methanol containing atmospheric-pressure plasma jet

In this paper, a pulsed-dc CH_3OH/Ar plasma jet generated at atmospheric pressure is studied by laser-induced fluorescence(LIF) and optical emission spectroscopy(OES). A gas–liquid bubbler system is proposed to introduce the methanol vapor into the argon gas, and the CH3OH/Ar volume ratio is kept constant at about 0.1%. Discharge occurs in a 6-mm needle-to-ring gap in an atmospheric-pressure CH_3OH/Ar mixture. The space-resolved distributions of OH LIF inside and outside the nozzle exhibit distinctly different behaviors. And, different production mechanisms of OH radicals in the needle-to-ring discharge gap and afterglow of plasma jet are discussed. Besides, the optical emission lines of carbonaceous species, such as CH, CN, and C_2 radicals, are identified in the CH_3OH/Ar plasma jet. Finally, the influences of operating parameters(applied voltage magnitude, pulse frequency, pulsewidth) on the OH radical density are also presented and analyzed.     

Study of silicon pixel sensor for synchrotron radiation detection

The silicon pixel sensor(SPS) is one of the key components of hybrid pixel single-photon-counting detectors for synchrotron radiation X-ray detection(SRD). In this paper, the design, fabrication, and characterization of SPSs for single beam X-ray photon detection is reported. The designed pixel sensor is a p+-in-n structure with guard-ring structures operated in full-depletion mode and is fabricated on 4-inch, N type, 320 μm thick, high-resistivity silicon wafers by a general Si planar process. To achieve high energy resolution of X-rays and obtain low dark current and high breakdown voltage as well as appropriate depletion voltage of the SPS, a series of technical optimizations of device structure and fabrication process are explored. With optimized device structure and fabrication process,excellent SPS characteristics with dark current of 2 n A/cm~2, full depletion voltage 50 V and breakdown voltage 150 V are achieved. The fabricated SPSs are wire bonded to ASIC circuits and tested for the performance of X-ray response to the 1W2 B synchrotron beam line of the Beijing Synchrotron Radiation Facility. The measured S-curves for SRD demonstrate a high discrimination for different energy X-rays. The extracted energy resolution is high( 20% for X-ray photon energy 10 keV) and the linear properties between input photo energy and the equivalent generator amplitude are well established. It confirmed that the fabricated SPSs have a good energy linearity and high count rate with the optimized technologies. The technology is expected to have a promising application in the development of a large scale SRD system for the Beijing Advanced Photon Source.