Wavelength modulated off-axis integrated cavity output spectroscopy in the near infrared

A novel instrument based on an improved off-axis alignment of integrated cavity output spectroscopy (OA-ICOS) in conjunction with a wavelength modulation (WM) technique, was developed using a DFB diode laser operating in the near infrared at 1.573 μm (6357.3 cm^-1). The laser-based sensor employed a 44 cm optical cavity that provided an effective absorption path length of ∼68 m. A minimum detectable absorption of approximately 3.6 ppmv Hz^-1/2 or 2.3×10^-7 Hz^-1/2 per optical pass was obtained using second harmonic detection. We demonstrated that by implementation of the WM technique to OA-ICOS in the near infrared, the detection sensitivity was improved by a factor of 14 compared to that obtained with OA-ICOS. Measurements of CO₂ mixing ratios in ambient air have been performed by using both OA-ICOS and WM-OA-ICOS techniques for performance comparison. PACS 42.62.Fi; 07.07.Df; 33.20.Ea     

A novel multiple species ringdown spectrometer for in situ measurements of methane,carbon dioxide,and carbon isotope

We have developed a standalone, user-friendly, multi-species ringdown spectrometer for in situ measurements of methane (CH₄), carbon dioxide (CO₂), and a carbon dioxide isotope (¹³CO₂). The instrument is based on near-infrared continuous-wave cavity ringdown spectroscopy (NIR cw-CRDS) and engineered to be of approximately 16 kg with dimensions of 50 cm × 40 cm × 15 cm. The instrument design, optical configuration, electronic control, and performance are described. CH₄, CO₂, and ¹³CO₂ are measured at different wavelengths that are obtained through multiplexing two distributed feedback laser diodes with central wavelengths at 1597 and 1650 nm. The spectrometer has low power consumption and runs for 4–6 h when powered by a standard car battery. The instrument is operated either locally by interacting with a 7-inch touch screen or remotely via an Internet connection. The 1-σ detection limits for CH₄ and CO₂ are 0.2 and 120 ppmv, respectively. The measurement uncertainty is better than ±4% of full-scale reading for CH₄ and CO₂ and ±1.5‰ for δ¹³C (part per thousand relative to the Pee Dee Belemnite scale). Measurement of each species is near real-time; switching from measuring one species to another takes less than one minute. This work demonstrates a novel multiple-species CRDS-instrumentation platform, which can be adopted for development of an array of ringdown spectrometers for portable, user-friendly, field analysis of a variety of gases in environmental and industrial applications. Discussion of a future version of the spectrometer with better detection sensitivity, higher accuracy, and a smaller geometry is also presented. PACS  42.62.Fi; 42.55.Px; 33.20.Ea; 07.88.+y; 07.57.Ty     

Detection of trace gases by rapidly-swept continuous-wave cavity ringdown spectroscopy: pushing the limits of sensitivity

Cavity ringdown (CRD) absorption spectroscopy enables spectroscopic sensing of gases with a high sensitivity and accuracy. Instrumental improvements result in a new high-performance continuous-wave (cw) CRD spectrometer using a rapidly-swept cavity of simple design. It employs efficient data-acquisition procedures, high-reflectivity mirrors, a low-adsorption flow cell, and various compact fibre-optical components in a single-ended transmitter-receiver configuration suitable for remote sensing. Baseline noise levels in our latest cw-CRD experiments yield a competitive noise-equivalent absorption limit of ∼5×10^-10 cm^-1Hz^-1/2, independent of whatever molecules are to be detected. Measurements in the near-infrared wavelength range of 1.51–1.56 μm yield sub-ppmv (i.e., ppbv or better) sensitivity in the gas phase for several representative molecules (notably CO₂, CO, H₂O, NH₃, C₂H₂, and other hydrocarbons). By measuring spectroscopic features in the 1.525 μm band of C₂H₂ gas, we realise detection limits of 19 nTorr (2.5×10^-11 atm) of neat C₂H₂ (Doppler-limited at low pressure) and 0.37 ppbv of C₂H₂ in air (pressure-broadened at 1 atm). Our cw-CRD spectrometer is a high-performance sensor in a relatively simple, low-cost, compact instrument that is amenable to chemical analysis of trace gases in medicine, agriculture, industry, and the environment. PACS 07.07.Df; 07.57.Ty; 42.62.Fi     

Current status of astronomical observations on possible cosmological variations of the proton-to-electron mass ratio μ=m<Subscript>p</Subscript>/m<Subscript>e</Subscript>

Astronomical constraints on a possible cosmologicalvariation of the proton-to-electron mass ratio μ=m_p/m_e are discussed. The analysis of H₂ lines observed in thespectra of distant quasars Q 0405-443 (z_em=3.02) andQ 0347-383 (z_em=3.22) is performed [1] using, partly, very precise values of H₂ frequencies from new laboratorymeasurements [2] and sensitivity coefficients from newaccurate calculations [2,3]. A possibleμ-variation of Δμ/ μ= (2.0±0.6)×10^-5over 12 Gyr is not excluded. However, the discussion of systematicerrors show that some may well be underestimated. Thus, the abovevalue should be treated as the most stringent limit the cosmologicalvariation of μ at z≈2.6 - 3.0 (12 Gyr ago).     

Fiber laser intracavity absorption spectroscopy for in situ multicomponent gas analysis in the atmosphere and combustion environments

Intracavity absorption spectroscopy with a broadband Er³⁺-doped fiber laser is applied for the measurements of several molecular species revealing quantitative information about the gas concentration, temperature and chemical reactions in flames. The spectral range of measurements extends from 6200 cm⁻¹ to 6550 cm⁻¹ with the proper choice of the fiber length and by moving an intracavity lens. With a pulsed laser applied in this experiment, the sensitivity to absorption corresponds to an effective absorption path length of 3 km assuming the cavity is completely filled with the sample. For a cw laser, the effective absorption path length is estimated to be 50 km. Absorption spectra of various molecules such as CO₂, CO, H₂O, H₂S, C₂H₂ and OH were recorded separately in the cell and/or in low-pressure methane and propane flames. The presented measurements demonstrate simultaneous in situ detection of three molecular products of chemical reactions at different flame locations. Variation of the relative strengths of OH absorption lines with the temperature enables the estimation of the local flame temperature. The sensitivity of this laser does not depend on the broadband cavity losses and it can be used for in situ measurements of absorption spectra in hostile environments such as contaminated samples, flames or combustion engines. The presented technique can be applied for various diagnostic purposes, such as in environmental, combustion and plasma research, in medicine and in the determination of stable isotope ratios.     

Fluorophotometric Determination of Hydrogen Peroxide with Fluorescin in the Presence of Cobalt (II) and Reaction Against Other Reactive Oxygen Species

A fluorophotometric method for the determination of hydrogen peroxide (H₂O₂) using fluorescin was developed. This method was based on the oxidative reaction of fluorescin, a colorless, non-fluorescent lactoid fluorescein, by H₂O₂ to give highly fluorescein fluorescence emission. In the determination of H₂O₂, the calibration curve exhibited linearity over the H₂O₂ concentration range of 1.5–310 ng mL⁻¹ at an emission wavelength of 525 nm with an excitation of 500 nm and with relative standard deviations (n = 6) of 2.51%, 2.48%, and 1.31% for 3.1 ng mL⁻¹, 30.8 ng mL⁻¹, and for 308 ng mL⁻¹ of H₂O₂, respectively. The detection limit for H₂O₂ was 1.9 ng mL⁻¹ six blank determinations was performed (ρ = 6). This proposed method was applied to detection of other reactive oxygen species and nitrogen species (ROS/RNS) such as singlet oxygen (¹O₂), hydroxyl radical (^•OH), peroxynitrite (ONOO⁻) etc., and it was possible to detect them with a high sensitivity. In addition, this proposed method was applied to the recovery tests of H₂O₂ in calf serum, human saliva, rain water, and wheat noodles; the results were satisfactory.     

High precision and continuous field measurements of δ 13C and δ 18O in carbon dioxide with a cryogen-free QCLAS

The present paper describes a compact and cryogen-free, quantum cascade laser based absorption spectrometer (QCLAS) designed for in situ, continuous and high precision isotope ratio measurements of atmospheric CO₂. The mobile instrument incorporates several new features including a novel astigmatic multi-pass cell assembly, a quasi-room temperature quantum cascade laser, thermoelectrically cooled detectors as well as a new retrieval approach. The combination of these features now makes it possible to measure isotope ratios of ambient CO₂ with a precision of 0.03 and 0.05‰ for δ¹³C and δ¹⁸O, respectively, using a 100 s integration time. A robust and optimized calibration procedure was developed to bring the retrieved isotope ratios on an absolute scale. This assures an accuracy better than 0.1‰ under laboratory conditions. The instrument performance was also assessed in a field campaign in which the spectrometer operated autonomously and provided mixing ratio values for the main three CO₂ isotopologues at one second time resolution. An accuracy of 0.2‰ was routinely obtained for both isotope ratios during the entire period. The results were in excellent agreement with the standard laboratory-based isotope ratio mass spectrometer measurements made on field-collected flask samples. A few illustrative examples are used to depict the potential of this optical method in atmosphere–biosphere research. PACS  07.57.Ty; 42.62.Fi; 92.60.Kc     

QEPAS spectrophones: design, optimization, and performance

The impact of design parameters of a spectrophone for quartz-enhanced photoacoustic spectroscopy on its performance was investigated. The microresonator of spectrophone is optimized based on an experimental study. The results show that a 4.4 mm-long tube with 0.6 mm inner diameter yields the highest signal-to-noise ratio, which is ∼30 times higher than that of a bare QTF at gas pressures between 400 and 800 Torr. The optimized configuration demonstrates a normalized noise-equivalent absorption coefficient (1σ) of 3.3×10⁻⁹ cm⁻¹W/Hz^1/2 for C₂H₂ detection at atmospheric pressure. The effect of the changing carrier gas composition is studied. A side-by-side sensitivity comparison between QEPAS and conventional photoacoustic spectroscopy technique is reported.     

Line strength and collisional broadening studies of hydrogen sulphide in the 1.58 μm region using diode laser spectroscopy

High resolution diode laser spectroscopy has been applied to the detection of hydrogen sulphide at ppm levels utilizing different transitions within the region of the ν ₁+ν ₂+ν ₃ and 2ν ₁+ν ₂ combination bands around 1.58 μm. Suitable lines in this spectral region have been identified, and absolute absorption cross sections have been determined through single-pass absorption spectroscopy and confirmed in the Doppler linewidth regime using cavity enhanced absorption spectroscopy (CEAS). The desire for a sensitive system potentially applicable to H₂S sensing at atmospheric pressure has led to an investigation on suitable transitions using wavelength modulation spectroscopy (WMS). The set-up sensitivity has been calculated as 1.73×10⁻⁸ cm⁻¹ s^1/2, and probing the strongest line at 1576.29 nm a minimum detectable concentration of 700 ppb under atmospheric conditions has been achieved. Furthermore, pressure broadening coefficients for a variety of buffer gasses have been measured and correlated to the intermolecular potentials governing the collision process; the H₂S–H₂S dimer well depth is estimated to be 7.06±0.09 kJ mol⁻¹.     

Initiation of nuclear reactions under laser irradiation of Au nanoparticles in the aqueous solution of Uranium salt

Laser exposure of a suspension of either gold or palladium nanoparticles in aqueous solutions of UO₂Cl₂ of natural isotope abundance was experimentally studied. Picosecond Nd:YAG lasers at peak power of 10¹¹–10¹³ W/cm² at the wavelength of 1.06–0.355 μm were used as well as a visible-range Cu vapor laser at a peak power of 10¹⁰ W/cm². The composition of colloidal solutions before and after laser exposure was analyzed using atomic absorption and gamma spectroscopy in the 0.06–1 MeV range of photon energy. Real-time gamma spectroscopy was used to characterize the kinetics of nuclear reactions during laser exposure. It was found that laser exposure initiated nuclear reactions involving both ²³⁸U and ²³⁵U nuclei via different channels in H₂O and D₂O. The influence of saturation of both the liquid and nanoparticles by gaseous H₂ and D₂ on the kinetics of nuclear transformations was found. Possible mechanisms of observed processes are discussed.     

Electrical conductivity response and sensitivity of ZSM-5, Y,and mordenite zeolites towards ethanol vapor

This work is an attempt to search for highly selective sensing materials for ethanol vapor. The electrical conductivity response of ZSM-5, Y, and mordenite zeolites towards ethanol vapor have been investigated for the effects of the framework, the charge balancing cation type, and the Si/Al ratio. All zeolites were characterized using XRD, FT-IR, SEM, TGA, BET, and NH₃-TPD techniques. For the effect of the zeolite framework type, H⁺Y has a higher electrical conductivity sensitivity value than that of H⁺MOR because of a greater pore volume and available surface area. For the effect of the charge balancing cation, all NH₄ ⁺ZSM-5 zeolites (Si/Al = 23, 50, 80, 280) show negative responses, whereas the H⁺Y zeolites (Si/Al = 30, 60, 80) and the H⁺MOR zeolites (Si/Al = 30, 200) show positive responses. These differing behaviors can be traced to the electrostatic field at the cation sites in zeolite micropores, and their hydrophilic–hydrophobic character, which affect the adsorption properties of the zeolites. For the effect of Si/Al ratio, the electrical conductivity sensitivity towards the ethanol decreases with increasing Si/Al ratio or decreasing Al content, and there is a lesser degree of interaction between ethanol molecules and the active sites of the zeolites due to its higher hydrophobicity and the lower amount of cations. However, the H⁺Y (Si/Al = 5.1) and the H⁺MOR (Si/Al = 19) zeolites have lower conductivity sensitivity than those of H⁺Y (Si/Al = 30) and H⁺MOR (Si/Al = 30), respectively. The interactions between the C₂H₅OH molecules and the zeolites with respect to the electrical conductivity sensitivity were investigated and verified through infrared spectroscopy.     

Sensitive absorption measurements in the near-infrared region using off-axis integrated-cavity-output spectroscopy

A novel instrument that employs a high-finesse optical cavity as an absorption cell has been developed for sensitive measurements of gas mixing ratios using near-infrared diode lasers and absorption-spectroscopy techniques. The instrument employs an off-axis trajectory of the laser beam through the cell to yield an effective optical path length of several kilometers without significant unwanted effects due to cavity resonances. As a result, a minimum detectable absorption of approximately 1.4×10^-5 over an effective optical path of 4.2 km was obtained in a 1.1-Hz detection bandwidth to yield a detection sensitivity of approximately 3.1×10^-11 cm^-1 Hz^-1/2. The instrument has been used for sensitive measurements of CO, CH₄, C₂H₂ and NH₃. Received: 6 May 2002 / Revised version: 31 May 2002 / Published online: 2 September 2002 RID="*" ID="*"Corresponding author. Fax: +1-650/965-7074, E-mail: d.baer@lgrinc.com     

Optical feedback cavity-enhanced absorption spectroscopy for in situ measurements of the ratio 13C:12C in CO2

We report on the design and laboratory performance of a portable infrared absorption spectrometer for the measurement of the isotopic ratio ¹³C:¹²C in CO₂. The design relies on optical feedback cavity-enhanced absorption spectroscopy in the 2 μm spectral region to achieve optimal performance at ambient CO₂ concentrations. The prototype instrument measures δ¹³C, relative to a standard calibration bottle, with a precision of ±0.7‰ for a 20-s integration time and with an automatic recalibration every 6 min. The absolute accuracy obtained is 0.9‰. The principal performance limitations are discussed along with improvements currently being implemented for the second generation instrument. PACS  42.62.Fi; 07.57.Ty; 33.20.Ea     

Synthesis of Novel Eu(III) Luminescent Probe Based on 9- Acridinecarboxylic Acid Skelton for Sensing of ds-DNA

Eu(III)-9-acridinecarboxylate (9-ACA) complex was synthesized and characterized by elemental analysis, conductivity measurement, IR spectroscopy, thermal analysis, mass spectroscopy, ¹H-NMR, fluorescence and ultraviolet spectra. The results indicated that the composition of this complex is [Eu(III)-(9-ACA)₂(NCS)(C₂H₅OH)₂] 2.5 H₂O and the oxygen of the carbonyl group coordinated to Eu(III). The interaction between the complex with nucleotides guanosine 5′- monophosphate (5′-GMP), adenosine 5′-diphosphates (5′-ADP), inosine (5′-IMP) and CT-DNA was studied by fluorescence spectroscopy. The fluorescence intensity of Eu(III)-9-acridinecarboxylate complex was enhanced with the addition of CT-DNA. The effect of pH values on the fluorescence intensity of Eu(III) complex was investigated. Under experimental conditions, the linear range was 9–50 ng mL⁻¹ for calf thymus DNA (CT- DNA) and the corresponding detection limit was 5 ng mL⁻¹. The results showed that Eu(III)-(9-ACA)₂ complex binds to CT-DNA with stability constant of 2.41 × 10⁴ M .     

Inner-shell excited quartet states in Li-like O<Superscript>5+</Superscript> and Ne<Superscript>7+</Superscript> ions

First laser spectroscopic measurements of the 6s5d³D₁-6s6p¹P₁ and 6s5d³D₂-6s6p¹P₁ transitions in several isotopes of atomic barium have been performed. The hyperfine structure of these transitions was optically resolved and isotope shifts for even and odd isotopes were determined. The isotope shifts show a deviation from their expected behavior for odd isotopes in an analysis based on King-plots. This observation puts atomic structure calculations at test because available theories do not predict this. A profound understanding of the wavefunctions for heavy alkaline earth systems like barium (Ba) and radium (Ra) is essential for a theoretical evaluation of their sensitivity to fundamental symmetry breaking effects such as they could be observed, e.g., through permanent electric dipole moments. Further the absolute frequency of the 6s^{2 1}S₀-6s6p³P₁ intercombination line in ¹³⁸Ba was determined to be 12 636.6232(1) cm^-1.     

Performance evaluation of a near infrared QEPAS based ethylene sensor

A sensor based on quartz-enhanced photoacoustic spectroscopy (QEPAS) was evaluated for the detection of trace levels of ethylene at atmospheric pressure using a fiber coupled DFB diode laser emitting in the 1.62 μm spectral range. A noise-equivalent QEPAS signal of ∼4 ppm C₂H₄ was achieved for a 0.7 s data acquisition time using wavelength-modulation with a second-harmonic detection scheme on the strongest C₂H₄ absorption peak at 6177.14 cm⁻¹ with an average optical power of ∼15 mW. Improved detection sensitivity of 0.5 and 0.3 ppm C₂H₄ (1σ) was demonstrated using longer averaging time of 70 and 700 s, respectively. Important characteristics for the QEPAS based sensor operation in real-world conditions are presented, particularly the influence of external temperature variations. Furthermore, the response time of the ethylene sensor was measured in different configurations and it is shown that the QEPAS technique can provide a response time in a few seconds range even without active gas flow.     

Near-infrared diode laser absorption sensor for rapid measurements of temperature and water vapor in a shock tube

A fast-response (100 kHz) tunable diode laser absorption sensor is developed for measurements of temperature and H₂O concentration in shock tubes, e.g. for studies of combustion chemistry. Gas temperature is determined from the ratio of fixed-wavelength laser absorption of two H₂O transitions near 7185.60 cm^-1 and 7154.35 cm^-1, which are selected using design rules for the target temperature range of 1000–2000 K and pressure range of 1–2 atm. Wavelength modulation spectroscopy is employed with second-harmonic detection (WMS-2f) to improve the sensor sensitivity and accuracy. Normalization of the second-harmonic signal by the first-harmonic signal is used to remove the need for calibration and minimize interference from emission, scattering, beam steering, and window fouling. The laser modulation depth for each H₂O transition is optimized to maximize the WMS-2f signal for the target test conditions. The WMS-2f sensor is first validated in mixtures of H₂O and Ar in a heated cell for the temperature range of 500–1200 K (P=1 atm), yielding an accuracy of 1.9% for temperature and 1.4% for H₂O concentration measurements. Shock wave tests with non-reactive H₂O–Ar mixtures are then conducted to demonstrate the sensor accuracy (1.5% for temperature and 1.4% for H₂O concentration) and response time at higher temperatures (1200–1700 K, P=1.3–1.6 atm). PACS 42.62.Fi; 42.55.Px; 42.60.Fc; 07.35.+k     

The hyperfine and isotope structure of the Cd intercombination line – revisited

Careful analysis of the intercombination 5¹S₀–5³P₁ line of the ¹¹³Cd isotope with two hfs components and was carried out. The hyperfine splitting of this line was determined to uncertainty less than 10^-3 cm^-1 using neon-perturbed Doppler limited spectra.     

Charged Higgs bosons in the next-to MSSM (NMSSM)

The charged Higgs boson decays H^±→W^±A₁ and H^±→W^±H_i are studied in the framework of the next-to-minimal supersymmetric standard model (NMSSM). It is found that the decay rate for H^±→W^±A₁ can exceed the rates for the τ^±ν and tb channels both below and above the top–bottom threshold. The dominance of H^±→W^±A₁ is most readily achieved when A₁ has a large doublet component and small mass. We also study the production process pp→H^±A₁ at the LHC followed by the decay H^±→W^±A₁, which leads to the signature W^±A₁A₁. We suggest that pp→H^±A₁ is a promising discovery channel for a light charged Higgs boson in the NMSSM with small or moderate tanβ and dominant decay mode H^±→W^±A₁. This W^±A₁A₁ signature can also arise from the Higgsstrahlung process pp→W^±H₁ followed by the decay H₁→A₁A₁. It is shown that there exist regions of parameter space where these processes can have comparable cross sections and we suggest that their respective signals can be distinguished at the LHC by using appropriate reconstruction methods. PACS  12.60.Fr; 14.80.Cp