Trace detection of atmospheric NO2 by laser-induced fluorescence using a GaN diode laser and a diode-pumped YAG laser

We report on the development of a highly sensitive detection system for measuring atmospheric NO(2) by means of a laser-induced fluorescence (LIF) technique at 473 nm using a diode-pumped Nd:YAG laser. A GaN-based laser diode emitting at 410 nm is also used as an alternative fluorescence-excitation source. For laboratory calibrations, standard NO(2) gas is diluted with synthetic air and is introduced into a fluorescence-detection cell. The NO(2) LIF signal is detected by a photomultiplier tube and processed by a photon-counting method. The minimum detectable limits of the NO(2) instrument developed have been estimated to be 0.14 ppbv and 0.39 ppbv (parts per billion, 10(-9), by volume) in 60 s integration time (signal-to-noise ratio of 2) for 473 and 410 nm excitation systems, respectively. Practical performance of the instrument has been demonstrated by the 24 hour continuous measurements of ambient NO(2) in a suburban area.     

A compact diode laser cavity ring-down spectrometer for atmospheric measurements of NO3 and N2O5 with automated zeroing and calibration

A compact rack-mounted cavity ring-down spectrometer (CRDS) for simultaneous measurements of the nocturnal nitrogen oxides NO(3) and N(2)O(5) in ambient air is described. The instrument uses a red diode laser to quantify mixing ratios of NO(3) (at its absorption maximum at 662 nm) and of N(2)O(5) following its thermal dissociation to NO(3) in a second detection channel. The spectrometer is equipped with an automated zeroing and calibration setup to determine effective NO(3) absorption cross-sections and NO(3) and N(2)O(5) inlet transmission efficiencies. The instrument response was calibrated using simultaneous measurements of NO(2), generated by thermal dissociation of N(2)O(5) and/or by titration of NO(3) with excess NO, using blue diode laser CRDS at 405 nm. When measuring ambient air, the (2σ, 10 s) precision of the red diode CRDS varied between 5 and 8 parts-per-trillion by volume (pptv), which sufficed to quantify N(2)O(5) concentrations under moderately polluted conditions. Sample N(2)O(5) measurements made on a rooftop on the University of Calgary campus in August 2010 are presented. A maximum N(2)O(5) mixing ratio of 130 pptv was observed, corresponding to a steady-state lifetime of less than 50 min. The NO(3) mixing ratios were below the detection limit, consistent with their predicted values based on equilibrium calculations. During the measurement period, the instrument response for N(2)O(5) was 70% of the theoretical maximum, rationalized by a slight mismatch of the laser diode output with the NO(3) absorption line and a N(2)O(5) inlet transmission efficiency less than unity. Advantages and limitations of the instrument's compact design are discussed.     

Investigation of optical fibers for gas-phase, ultraviolet laser-induced-fluorescence (UV-LIF) spectroscopy

We investigate the feasibility of transmitting high-power, ultraviolet (UV) laser pulses through long optical fibers for laser-induced-fluorescence (LIF) spectroscopy of the hydroxyl radical (OH) and nitric oxide (NO) in reacting and non-reacting flows. The fundamental transmission characteristics of nanosecond (ns)-duration laser pulses are studied at wavelengths of 283 nm (OH excitation) and 226 nm (NO excitation) for state-of-the-art, commercial UV-grade fibers. It is verified experimentally that selected fibers are capable of transmitting sufficient UV pulse energy for single-laser-shot LIF measurements. The homogeneous output-beam profile resulting from propagation through a long multimode fiber is ideal for two-dimensional planar-LIF (PLIF) imaging. A fiber-coupled UV-LIF system employing a 6 m long launch fiber is developed for probing OH and NO. Single-laser-shot OH- and NO-PLIF images are obtained in a premixed flame and in a room-temperature NO-seeded N(2) jet, respectively. Effects on LIF excitation lineshapes resulting from delivering intense UV laser pulses through long fibers are also investigated. Proof-of-concept measurements demonstrated in the current work show significant promise for fiber-coupled UV-LIF spectroscopy in harsh diagnostic environments such as gas-turbine test beds.     

Laser-induced fluorescence of green plants. 3: LIF spectral signatures of five major plant types

A technique amenable to remote sensing use which utilizes laser-induced fluorescence (LIF) properties of plants has been successfully used in the laboratory to identify five major plant types. These included herbaceous dicots, herbaceous monocots, conifers, hardwoods, and algae. Each of these plant types exhibited a characteristic LIF spectra when excited by a pulsed N2 laser emitting at 337 nm. Although monocots and dicots possess common fluorescence maxima at 440, 685, and 740 nm, they could be differentiated from one another by using the ratio of the square of the fluorescence intensity at 440 nm to the nonsquared intensity at 685 nm, i.e., (440)2/685. In all cases, monocots yielded a significantly higher ratio. Conifers have fluorescence maxima at 440, 525, and 740 nm but none at 685 nm. Hardwoods exhibited fluorescence at 440, 525, 685, and 740 nm. Algae had very low fluorescence at 440 nm, no fluorescence at 525 nm, and fluorescence maxima at 685 and 740 nm. For algae, the ratio of the fluorescence intensity at 685 nm to that at 740 nm was much greater than that for monocots, dicots, and hardwoods. The potential use of the LIF technique for individual species identification is suggested.     

Strategies for laser-induced fluorescence detection of nitric oxide in high-pressure flames. III. Comparison of A-X excitation schemes

Laser-induced fluorescence (LIF) has proven a reliable technique for nitric oxide (NO) diagnostics in practical combustion systems. However, a wide variety of different excitation and detection strategies are proposed in the literature without giving clear guidelines of which strategies to use for a particular diagnostic situation. We give a brief review of the high-pressure NO LIF diagnostics literature and compare strategies for exciting selected transitions in the A-X(0, 0), (0, 1), and (0, 2) bands using a different detection bandpass. The strategies are compared in terms of NO LIF signal strength, attenuation of laser and signal light in the hot combustion gases, signal selectivity against LIF interference from O2 and CO2, and temperature and pressure sensitivity of the LIF signal. The discussion is based on spectroscopic measurements in laminar premixed methane-air flames at pressures between 1 and 60 bars and on NO and O2 LIF spectral simulations.     

Two-photon nitric oxide laser-induced fluorescence measurements in a diesel engine

A two-photon nitric oxide (NO) laser-induced fluorescence (LIF) technique was developed and applied to study in-cylinder diesel combustion. The technique prevents many problems associated with in-cylinder, single-photon NO planar-laser-induced fluorescence measurements, including fluorescence interference from the Schumann-Runge bands of hot O2, absorption of a UV excitation beam by in-cylinder gases, and difficulty in rejecting scattered laser light while simultaneously attempting to maximize fluorescence signal collection. Verification that the signal resulted from NO was provided by tuning of the laser to a vibrational off-resonance wavelength that showed near-zero signal levels, which resulted from either fluorescence or interference at in-cylinder pressures of as much as 20 bar. The two-photon NO LIF signal showed good qualitative agreement with NO exhaust-gas measurements obtained over a wide range of engine loads.     

Strategies for laser-induced fluorescence detection of nitric oxide in high-pressure flames. I. A-X(0,0) excitation

Three different high-pressure flame measurement strategies for NO laser-induced fluorescence (LIF) with A-X(0,0) excitation have been studied previously with computational simulations and experiments in flames up to 15 bars. Interference from O2 LIF is a significant problem in lean flames for NO LIF measurements, and pressure broadening and quenching lead to increased interference with increased pressure. We investigate the NO LIF signal strength, interference by hot molecular oxygen, and temperature dependence of the three previous schemes and for two newly chosen excitation schemes with wavelength-resolved LIF measurements in premixed methane and air flames at pressures between 1 and 60 bars and a range of fuel/air ratios. In slightly lean flames with an equivalence ratio of 0.83 at 60 bars, the contribution of O2 LIF to the NO LIF signal varies between 8% and 29% for the previous schemes. The O2 interference is best suppressed with excitation at 226.03 nm.     

Comparisons of laser-saturated, laser-induced, and planar laser-induced fluorescence measurements of nitric oxide in a lean direct-injection spray flame

We report quantitative, spatially resolved laser-saturated fluorescence (LSF), linear laser-induced fluorescence (LIF), and planar laser-induced fluorescence (PLIF) measurements of nitric oxide (NO) concentration in a preheated, lean direct-injection spray flame at atmospheric pressure. The spray is produced by a hollow-cone, pressure-atomized nozzle supplied with liquid heptane, and the overall equivalence ratio is unity. NO is excited by means of the Q(2)(26.5) transition of the gamma(0, 0) band. LSF and LIF detection are performed in a 2-nm region centered on the gamma(0, 1) band. PLIF detection is performed in a broad ~70-nm region with a peak transmission at 270 nm. Quantitative radial NO profiles obtained by LSF are presented and analyzed so as to correct similar LIF and PLIF profiles. Excellent agreement is achieved among the three fluorescence methodologies.     

A REMPI method for the ultrasensitive detection of NO and NO2 using atmospheric pressure laser ionization mass spectrometry

We report on the development of a quasi-simultaneous highly selective method for NO and NO2 detection at the ultratrace level. Atmospheric pressure laser ionization (APLI), recently introduced by our group, is used to detect both compounds at low parts per trillion by volume (pptv) mixing ratios. APLI is based on resonance-enhanced multiphoton ionization mass spectrometry. Two-color pump-probe experiments employing a single excimer pumped dye laser combination allow for the ultrasensitive measurement of NO and NO2 within a narrow range of maximum pumping efficiency of the laser dye Coumarin 120. NO is detected via excitation of the long-lived A 2sigma+ (nu' = 1) level at 215.36 nm and subsequently ionized with 308-nm radiation provided by the excimer pump laser. NO2 is ionized after double resonant excitation of the A2B1 and 3psigma manifolds in a (1 + 1' + 1(')) process using 431.65 + 308 nm. The selectivity of the NO measurement exceeds 2,000 with respect to NO2 and N2O5. For NO2, a selectivity of >3,000 with respect to N2O5 and organic nitrates is observed. The current APLI detection limit of NO and NO2 is 0.5 and 5 pptv, respectively, with a 20-s integration time.     

Laser photofragmentation-fragment detection and pyrolysis-laser-induced fluorescence studies on energetic materials

Trace concentrations of energetic materials such as 2, 4, 6-trinitrotoluene (TNT), pentaerythritol tetranitrate (PETN), and hexahydro-1, 3, 5-trinitro-s-triazine (RDX) are detected by laser photofragmentation-fragment detection (PF-FD) spectrometry. In this technique, a single laser operating near 227 nm photofragments the parent molecule and facilitates the detection of the characteristic NO fragment by means of its A (2)Sigma(+)-X (2)Sigma (0, 0) transitions near 227 nm. Fragment detection is accomplished by resonance-enhanced multiphoton ionization with miniature electrodes and by laser-induced fluorescence (LIF) with a photodetector. Experiments are also conducted in the visible region by use of 453.85-nm radiation for photofragmentation and fragment detection. Sand samples contaminated with PETN and RDX are analyzed by a pyrolysis-LIF technique, which involves pyrolysis of the energetic material with subsequent detection of the pyrolysis products NO and NO(2) by LIF and PF-LIF, respectively, near 227 nm. The application of these techniques to the trace analysis of TNT, PETN, and RDX at ambient pressure in room air is demonstrated with limits of detection (signal-to-noise ratio, 3) in the low parts-in-10(9) to parts-in-10(6) range for a 20-s integration time and 10-120 microJ of laser energy at 226.8 nm and approximately 5 mJ at 453.85 nm. An increase in detection sensitivity is projected with an increase in laser energy and an improved system design. The analytical merits of these techniques are discussed and compared with those of other laser-based techniques.     

Laser-induced fluorescence of se, as, and sb in an electrothermal atomizer

Trace detection of Se, As, and Sb atoms has been performed by electrothermal atomization laser-induced fluorescence (ETA-LIF) approaches. Production of far-UV radiation necessary for excitation of As atoms at 193.696 nm and Se atoms at 196.026 nm was accomplished by stimulated Raman shifting (SRS) of the output of a frequency-doubled dye laser operating near 230 nm. Both wavelengths were obtained as second-order anti-Stokes shifts of the dye laser radiation and provided up to 10 μJ/pulse, which was shown through power dependence studies to be sufficient for saturation in the ETA. An excited-state direct line fluorescence approach using excitation at 206.279 nm was also investigated for the LIF detection of Se. High-sensitivity LIF of Sb atoms was accomplished using 206.833-nm excitation and detection at 259.805 nm. The accuracy of the ETA-LIF approaches was demonstrated by determining the As and Se content of aqueous reference samples. The limits of detection (absolute mass) were 200 fg by ground-state LIF and 150 fg by excited-state direct line fluorescence for Se, 200 fg for As, and 10 fg for Sb; these LODs compare favorably with results reported previously in the literature for ETA-LIF, GFAAS, and ICP-MS methods.     

Effect of pulsed dye-laser wavelength stabilization on spectral overlap in atmospheric NO fluorescence studies

We introduce an inexpensive application of a Fabry-Perot etalon to control long-term UV-laser line drift in atmospheric NO laser-induced fluorescence (LIF) measurements by monitoring the visible fundamental of a pulsed dye laser. A linear image sensor captures the interference pattern, and the dye grating can be adjusted to maintain a fixed wavelength through an interface with labview software. Results indicate that the laser wavelength can be fixed to an accuracy of +/-0.0001 nm in the dye fundamental and +/-0.00003 nm in the UV beam. Hence the average error in the LIF signal owing to fluctuations in spectral overlap between the laser and the NO absorption transition decreases from ~5 to ~0.05%, which results in improved measurement accuracy.     

Frequency-domain multiplexing system for in vivo diffuse light measurements of rapid cerebral hemodynamics

A novel frequency-domain multiplexing system has been developed for in vivo measurements of rapid cerebral hemodynamics. The instrument operates in the frequency-domain with three optical wavelengths, six source positions, and two detectors. Frequency-division multiplexing was used to modulate three wavelengths (690, 786, and 830 nm) at slightly different frequencies around 70 MHz. The three laser output beams were combined and switched into different source positions by use of fast optical switches (switch time <10 ms). Three narrowband, in-phase and in-quadrature demodulators decode the modulated signals. Our full-frame-acquisition rate is 2.5 Hz, with flexibility for acquisition rates greater than 50 Hz with smaller detection areas. We evaluate the performance of the instrument with tissue phantoms, and then employ the system to measure in vivo cerebral blood oxygenation during forepaw stimulation of a rat's brain.     

Two-Line Laser-Induced Fluorescence Imaging of Vibrational Temperatures in a NO-Seeded Flame

Two-dimensional temperature fields are measured in lean and sooting flames by means of two-color laser-induced fluorescence (LIF) imaging that uses seeded NO. Vibrational thermometry is performed by the probing of different vibrational ground-state levels. Spectral properties of the excited transitions within the A (2)?(+)-X (2)? system are well known from previous studies. The energy difference of 1974 cm(-1) between the (0, 0)Q(1) + P(21)(33.5) and the (0, 2)O(12)(5.5) lines offers great sensitivity in the temperature range that is relevant for combustion processes. Excitation is possible by use of a tunable KrF excimer laser on its fundamental (248-nm) and Raman shifted (in H(2), 225-nm) wavelengths. An excitation scheme for instantaneous two-line measurements by use of a single laser is developed. The possibility of single-shot measurements is discussed.     

Development of a selective light-emitting diode photolytic NO(2) converter for continuously measuring NO(2) in the atmosphere

A photolytic converter of nitrogen dioxide (NO(2)) to nitric oxide (NO) using light-emitting diodes (LEDs) has been designed to measure NO(2) in the troposphere. The typical electrical power consumption of the photolytic converter (PLC) is only 44 W. The maximum conversion efficiency of NO(2) to NO of the photolytic converter is around 90%, which is higher than that of metal halides or high-pressure Xe arc lamps (up to ～70%). The conversion efficiency of the PLC was almost constant for at least 2.5 months. The conversion efficiency of peroxyacetyl nitrate by the LED-PLC was measured to be 2.6 ± 0.1% (1σ). The interference of HONO using the PLC was experimentally estimated to be less than 3%, which is within the uncertainty of the instrument. An intercomparison of NO(2) measurements between the PLC-CLD and the laser-induced fluorescence (LIF) technique was conducted, and the NO(2) concentrations measured by the PLC-CLD method were in agreement with those obtained by the LIF technique, within the uncertainties of the instruments. Continuous observations were made on Fukue Island, a remote area. These results demonstrate the performance of the PLC for continuous ambient measurements.     

Crank-angle-resolved laser-induced fluorescence imaging of NO in a spark-ignition engine at 248 nm and correlations to flame front propagation and pressure release

Inside the combustion chamber of a spark-ignition engine, NO fluorescence is excited with a narrow-band tunable KrF excimer laser. The fluorescence light is detected by an intensified CCD camera that yields images of the NO distributions. Rotational-vibrational transitions of NO are excited by the A(2)Σ+ ? X(2)Π (0, 2) band system around 248 nm. Single laser shot planar NO distributions are obtained with good signal-to-noise ratio at all crank angles and allow us to locate areas of NO formation during combustion. The pressure within the combustion chamber is measured simultaneously with the NO distributions, which allows the evaluation of correlations between indicated work and NO formation. The crank-angle-resolved sequences of two-dimensional NO distributions and averaged pressure traces are presented for different engine-operating conditions. In addition, laser-induced predissociation fluorescence of OH excited by the same laser source is measured in order to visualize the corresponding flame front propagation and to compare the time of formation of NO relative to that of OH.     

1064 nm和532 nm激光对HfO2/SiO2非规整高反膜的激光诱导损伤研究

规整膜系是常见的薄膜结构,但往往无法满足理想激光传输效果和仪器使用效果,由此在满足激光系统及传输的窗口要求下,以规整膜系为设计基础优化得到非规整膜系,以获得良好的激光传输效果。采用电子束热蒸发法制备了HfO₂/SiO₂非规整高反射率薄膜,该薄膜的激光诱导损伤分别通过1064 nm、10 ns脉冲和532 nm、10 ns脉冲进行测试。损伤形貌分别使用尼康L150光学显微镜和ZYGO白光干涉仪进行表征。对两种波长下的激光诱导损伤阈值和损伤形貌进行比较。得到以下结果:两种波长下激光诱导损伤的典型损伤形貌在低能量脉冲辐照下表现出凹坑,周围有一定面积的烧蚀区域,在高能量脉冲辐照下表现出材料分层现象;两个波长对薄膜损伤测试过程中,1064 nm激光损伤阈值为5.64 J/cm²,532 nm激光损伤阈值为1.54 J/cm²,损伤通常起始于电场的峰值附近;不同波长下损伤形貌可能与缺陷吸收引起的热应力有关,1064 nm损伤测试中损伤形貌近似圆形,532 nm损伤形貌不规则。分析可得以下结论:该薄膜于1064 nm激光下的薄膜抗激光损伤能力优于532nm激光;1064 nm和532 nm激光下损伤形貌均为凹陷状损伤;1064 nm激光下薄膜界面场强值和峰值场强均高于532 nm激光。     

Photochemical effect in two-photon laser-induced fluorescence detection of carbon monoxide in hydrocarbon flames

The CO formation as a result of the CO(2) photodissociation at 230.08 nm was observed by using the two-photon laser-induced fluorescence (LIF) method. The measurements were performed in a propane-air combustion product flow and in mixtures of CO(2) and O(2). The temperature dependence of the fluorescence signal caused by CO molecules, produced in the photodissociation of CO(2) molecules under the action of laser radiation at a wavelength of 230.08 nm, was measured at temperatures ranging from 1300 to 2000 K. It is shown that consideration of CO(2) photodissociation under the action of the probing radiation is necessary when one applies the two-photon LIF method for the measurement of small CO concentrations in high-temperature gas mixtures containing CO(2). As an example, a correction is given of the CO concentration profiles measured by the LIF method in the combustion product flow around a cooled metallic plate.     

Confocal, two-photon laser-induced fluorescence technique for the detection of nitric oxide

We describe a confocal two-photon laser-induced fluorescence scheme for the detection of gaseous NO. Excitation from a simple YAG-pumped Coumarin 450 dye system near 452.6 nm was used to promote the two-photon NO(A (2)?(+), nu? = 0 ? X (2)?, nu? = 0) transition in the gamma(0, 0) band. Subsequent fluorescence detection in the range 200-300 nm permitted almost total rejection of elastic and geometric scatter of laser radiation for excellent signal/noise ratio characteristics. The goal of the research was to apply NO fluorescence to a relatively realistic limited optical access combustion environment. A confocal optical arrangement was demonstrated for single-point measurements of NO concentration in gas samples and in atmospheric-pressure flames. The technique is suitable for applications that offer only a single direction for optical access and when significant elastic scatter is present.     

Quantitative temperature measurements in high-pressure flames with multiline NO-LIF thermometry

An accurate temperature measurement technique for steady, high-pressure flames is investigated using excitation wavelength-scanned laser-induced fluorescence (LIF) within the nitric oxide (NO) A-X(0, 0) band, and demonstration experiments are performed in premixed methane/air flames at pressures between 1 and 60 bars with a fuel/air ratio of 0.9. Excitation spectra are simulated with a computational spectral simulation program (LIFSim) and fit to the experimental data to extract gas temperature. The LIF scan range was chosen to provide sensitivity over a wide temperature range and to minimize LIF interference from oxygen. The fitting method is robust against elastic scattering and broadband LIF interference from other species, and yields absolute, calibration-free temperature measurements. Because of loss of structure in the excitation spectra at high pressures, background signal intensities were determined using a NO addition method that simultaneously yields nascent NO concentrations in the postflame gases. In addition, fluorescence emission spectra were also analyzed to quantify the contribution of background signal and to investigate interference in the detection band-width. The NO-LIF temperatures are in good agreement with intrusive single-color pyrometry. The proposed thermometry method could provide a useful tool for studing high-pressure flame chemistry as well as provide a standard to evaluate and validate fast-imaging thermometry techniques for practical diagnostics of high-pressure combustion systems.