Narrow Spectral Intervals Research Articles

Fast and simultaneous determination of zinc and iron using HR-CS GF-AAS in vegetables and plant material

Due to the high nutritional importance of zinc and iron, accurate methods for their determination in foodstuffs are required. Here, we present a routine-ready method for their co-determination by means of high-resolution continuum source graphite-furnace atomic absorption spectrometry. As the technique allows for monitoring a narrow spectral interval, adjacent secondary lines of Zn (307.588 nm) and Fe (307.572 nm) can be measured simultaneously in a single firing. The analysis is therefore fast (under 2 min) and its sensitivity corresponds to common concentrations of the elements in foodstuffs. The accuracy of the method was verified by means of reference materials and also by comparison with an independent technique. Analysis of a large set of vegetable samples proved the applicability of the method in a routine laboratory setting. The vegetables for analysis were obtained from both markets and local gardeners. The results indicate higher contents of zinc and iron in homegrown vegetables compared to store-bought ones.

Characterization of the H2O+CO2 continuum within the infrared transparency windows

Absorption spectra of humidified CO2 have been recorded at room temperature by cavity enhanced absorption techniques (CRDS and OFCEAS): (i) in three spectral ranges of the 1.6 µm window (5720–6045 cm−1; 6390–6460 cm−1 and 6570–6665 cm−1), (ii) in four narrow spectral intervals of the 2.3 µm window (4243–4255 cm−1; 4301.3–4302 cm−1; 4421.5–4440 cm−1 and 4518–4535 cm−1), and (iii) around 2853 cm−1. All these spectral ranges are situated in transparency windows of both H2O and CO2. The binary absorption coefficients (BCO2−H2O+BH2O−CO2)are retrieved from low pressure spectra (<1 atm) recorded with different molar fractions of water vapor in CO2 after subtracting the H2O and CO2 local monomer contributions and the self-continuum contribution of each species (i.e. H2OH2O and CO2CO2). Experimental room temperature binary coefficients are then compared to the only available empirical model based on line shape profiles with χ-factors. This model well reproduces our experimental values on the low- and high-frequency edges of the 1.6 µm window and gives a relatively good agreement for the 2853 cm−1 data point. Larger differences are observed in the 2.3 µm window where the calculated values are underestimated by a factor of 3. Around 6000 cm−1, an additional absorption peak is observed which is tentatively interpreted as a collision induced absorption band due to the simultaneous excitation of the H2O and CO2 molecules.

Eta Carinae: A Tale of Two Periastron Passages

Abstract Since 2002, the far-ultraviolet (FUV) flux (1150–1680 Å) of Eta Carinae, monitored by the Hubble Space Telescope/Space Telescope Imaging Spectrograph, has increased by an order of magnitude. This increase is attributed to partial dissipation of a line-of-sight (LOS) occulter that blocks the central core of the system. Across the 2020 February periastron passage, changes in the FUV emission show a stronger wavelength dependence than occurred across the 2003 July periastron passage. Across both periastron passages, most of the FUV spectrum dropped in flux then recovered a few months later. The 2020 periastron passage included enhancements of FUV flux in narrow spectral intervals near periastron followed by a transient absorption and recovery to pre-periastron flux levels. The drop in flux is due to increased absorption by singly ionized species as the secondary star plunges deep into the wind of the primary star, which blocks the companion’s ionizing radiation. The enhanced FUV emission is caused by the companion’s wind-blown cavity briefly opening a window to deeper layers of the primary star. This is the first time transient brightening has been seen in the FUV comparable to transients previously seen at longer wavelengths. Changes in resonance line-velocity profiles hint that the dissipating occulter is associated with material in LOS moving at −100 to −300 km s−1, similar in velocity of structures previously associated with the 1890s lesser eruption.

Iterative normalized cross-correlation method for absolute optical path difference demodulation of dual interferometers.

It is still a challenge to realize the absolute optical path difference (OPD) demodulation of multi-interference systems with a narrow spectral interval and small OPD interval. In this paper, an iterative normalized cross-correlation algorithm is firstly proposed for demodulating the multiple absolute OPDs of a dual-interference system and applied to optical fiber sensing system. By constructing a template function in combined form, the optimal solutions of its components and OPDs are solved iteratively based on the reconstruction matrix method and cross-correlation algorithm, respectively. The simulation and experiment show that the demodulation accuracies near the OPDs of 560 µm and 660 µm are both up to 5 nm in different spectral intervals from 45 to 80 nm. The simulation results show that all demodulation precisions at the spectral interval of 55 nm do not exceed 4 nm when the OPD changes in the range of 650-670 µm. Besides, the experimental verification shows the temperature accuracy (0.125 °C) with 95% confidence of T-distribution is very close to the control accuracy (0.1 °C). The proposed algorithm can improve the multiplexing capability of optical fiber sensor system and reduce its cost.

Design of an optical illumination system for a tunable source with acousto-optical filtering

Sources with the possibility to select a narrow spectral band can be applied in many fields of science and technology. One method for selecting single or multiple spectral bands with the possibility to tune their width is acousto-optical filtering in conjunction with a source of broadband unpolarized radiation. In this case a narrow spectral interval is selected owing to the diffraction in an acousto-optical tunable filter. The main problem in the development of such a source is the obtainment of a sufficient luminous flux. Possible configurations of an optical illumination system allowing an increase in the efficiency and luminous flux in a system with acousto-optical filtering are presented in this study.

Selective Elimination of Homogeneous Broadening by Multidimensional Spectroscopy in the Electromagnetically Induced Transparency Regime.

The key feature of photon echo-based spectroscopies is a selective elimination of the inhomogeneous broadening. Fast homogeneous dephasing typical for large macromolecules makes it difficult to resolve the congested spectra even if the inhomogeneous component is eliminated. We propose a novel two-dimensional spectroscopy in which a series of temporally separated probe pulses are combined with the strong narrowband control pulse. Using electromagnetically induced transparency originating from the interference between the control and probe pulses, we achieve an observation window for molecular response in narrow spectral intervals significantly smaller than the homogeneous dephasing limit.

Parameters of plasma of high-current nanosecond discharge in air with small admixtures of copper vapor

Introduction . The use of a discharge in air of atmospheric pressure opens the possibility of developing windowless UV lamps that are promising for applications with significant mechanical overloads under various extreme conditions, since they do not contain any quartz elements (windows or bulbs). The output characteristics of such point lamps strongly depend on the plasma parameters of the nanosecond discharge with small admixtures of the vapor metal from which the electrodes are made. At present, there are no data on the parameters of air plasma with small admixtures of copper vapor under the operating conditions of a point lamp, which inhibits the establishment of mechanisms for populating the excited levels of atoms and copper ions in such a plasma. This in turn inhibits the long-range optimization of the output characteristics of such point sources of UV radiation. Purpose . The purpose of the study was to determine the emission characteristics in the spectral range of 200-250 nm of the plasma of high-current nanosecond discharge, between copper electrodes in air at atmospheric pressure under conditions of strong overstress of the discharge gap and to carry out a numerical simulation of the parameters of the plasma of air with small copper impurities under conditions close to those realized in the experiment. Methods . The nanosecond discharge in air of atmospheric pressure between copper electrodes was ignited. The distance between the copper electrodes was 1 mm, which led to the ignition at atmospheric pressure air of a point diffuse discharge of a spherical shape with a diameter of 1 mm. The standard program was used to solve the Boltzmann kinetic equation for the electron energy distribution function (EEDF) for modeling air plasma atmospheric pressure parameters with small copper vapor admixtures, since under the experimental conditions the maximum value of the E/P parameter reached about 530 V cm -1 torr -1 , which is less than the critical value of E/P for nitrogen according to the local electron runaway criterion – 590 V cm -1 torr -1 . Results . Numerical simulation of the plasma parameters established that for the range of reduced electric field strength 610 Td-810 Td, at which experimental studies of the characteristics of a point nanosecond discharge were carried out, mean electron energies varied within the range of 12.2 -15.6 eV; the value of the electron concentration was 10.7·10 21 m -3 – 8.7 ·10 21 m -3 at a current density of 765 · 10 6 A / m 2 on the surface of the copper electrode. The excitation constants of the 2 P 3/2, 1/2 , 2 D 3/2 , 2 D 5/2 , 4 F 7/2 , - states of the copper atom and its ionization were in the range (7.3 – 8.5) 10 -13 m 3 /c, ( 0.29 - 0.28) 10 -13 m 3 /c, ( 0.22 - 0.22) 10 -13 m 3 /c, (0.19 – 0.22) 10 -15 m 3 /c and (0.33 -0.45) 10 -13 m 3 /c,, respectively. The maximum excitation rate constant for nitrogen molecules was for the sum of the singlet states: they were (1.4 10 -14 ) m 3 / s for an electric field strength of 810 Td. The specific losses of the discharge power in the plasma on inelastic collisions of electrons with the components of the gas mixture were maximal for nitrogen molecules and reached 32% for excitation of the sum of singlet nitrogen states. Conclusion . Radiation of the plasma is concentrated in a narrow spectral interval of 200-250 nm, is promising for use as a point UV lamp; the spectral lines 248.9 and 254.4 nm Cu II are promising for estimating the electron temperature in a plasma. Mean electron energy, electron mobility, specific electric power losses and the rate constants of elastic and inelastic scattering of electrons by copper, argon and carbon dioxide, oxygen and nitrogen molecules, the ratio of which was 3: 7.098: 0.266: 159.157: 593.438, depending on the magnitude of the reduced electric field (ratio of the electric field strength (E) to the total concentration of atoms and gas molecules (N)). The range of changes in the parameter E / N = 1-1300 Td (1 ∙10 -17 - 1 ∙ 10 -15 V ∙ cm 2 ) and included the values of the parameter E / N, which were realized in the experiment (610-810) Td.

A New Probe of Line-of-sight Magnetic Field Tangling

Abstract The Galactic neutral hydrogen (H i ) sky at high Galactic latitudes is suffused with linear structure. Particularly prominent in narrow spectral intervals, these linear H i features are well aligned with the plane-of-sky magnetic field orientation as measured with optical starlight polarization and polarized thermal dust emission. We analyze the coherence of the orientation of these features with respect to line-of-sight velocity, and propose a new metric to quantify this H i coherence. We show that H i coherence is linearly correlated with the polarization fraction of 353 GHz dust emission. H i coherence constitutes a novel method for measuring the degree of magnetic field tangling along the line of sight in the diffuse interstellar medium. We propose applications of this property for H i -based models of the polarized dust emission in diffuse regions, and for studies of frequency decorrelation in the polarized dust foreground to the cosmic microwave background (CMB).

Transparency of [formula omitted]m window of Titan's atmosphere

Titan's atmosphere is optically thick and hides the surface and the lower layers from the view at almost all wavelengths. However, because gaseous absorptions are spectrally selective, some narrow spectral intervals are relatively transparent and allow to probe the surface. To use these intervals (called windows) a good knowledge of atmospheric absorption is necessary. Once gas spectroscopic linelists are well established, the absorption inside windows depends on the way the far wings of the methane absorption lines are cut-off. We know that the intensity in all the windows can be explained with the same cut-off parameters, except for the window at 2 μm. This discrepancy is generally treated with a workaround which consists in using a different cut-off description for this specific window. This window is relatively transparent and surface may have specific spectral signatures that could be detected. Thus, a good knowledge of atmosphere opacities is essential and our scope is to better understand what causes the difference between the 2 μm window and the other windows. In this work, we used scattered light at the limb and transmissions in occultation observed with VIMS (Visual Infrared Mapping Spectrometer) onboard Cassini, around the 2 μm window. Data shows an absorption feature that participates to the shape of this window. Our atmospheric model fits well the VIMS data at 2 μm with the same cut-off than for the other windows, provided an additional absorption is introduced in the middle of the window around ≃ 2.065 μm. It explains well the discrepency between the cut-off used at 2 μm, and we show that a gas with a fairly constant mixing ratio, possibly ethane, may be the cause of this absorption. Finally, we studied the impact of this absorption on the retrieval of the surface reflectivity and found that it is significant.

Selection of the optimal combination of water vapor absorption lines for detection of temperature in combustion zones of mixing supersonic gas flows by diode laser absorption spectrometry

Determination of a gas medium temperature by diode laser absorption spectrometry (DLAS) is based on the measurement of integral intensities of the absorption lines of a test molecule (generally water vapor molecule). In case of local thermodynamic equilibrium temperature is inferred from the ratio of the integral intensities of two lines with different low energy levels. For the total gas pressure above 1 atm the absorption lines are broadened and one cannot find isolated well resolved water vapor absorption lines within relatively narrow spectral interval of fast diode laser (DL) tuning range (about 3 cm −1 ). For diagnostics of a gas object in the case of high temperature and pressure DLAS technique can be realized with two diode lasers working in different spectral regions with strong absorption lines. In such situation the criteria of the optimal line selection differs significantly from the case of narrow lines. These criteria are discussed in our work. The software for selection the optimal spectral regions using the HITRAN-2012 and HITEMP data bases is developed. The program selects spectral regions of DL tuning, minimizing the error of temperature determination δТ/T , basing on the attainable experimental error of line intensity measurement δS . Two combinations of optimal spectral regions were selected – (1.392 & 1.343 μm) and (1.392 & 1.339 μm). Different algorithms of experimental data processing are discussed.

Intracavity photoacoustic sensing of water vapor with a continuously tunable external-cavity quantum-cascade laser operating near 5.5 μm.

A gas sensor integrating a pulsed external-cavity quantum-cascade laser and a photoacoustic (PA) cell placed inside the laser cavity is described. The laser design allows a continuously tunable and mode-hop-free operation with an estimated instrument-limited linewidth of 0.063 cm-1 in Littrow or Littman-Metcalf cavity configurations. Due to higher intracavity power levels, the PA signal for the cell located within the laser cavity is 80-320 times stronger than the signal for the same cell placed outside the cavity. The sensor shows a good capability to identify individual absorption lines of isotopic H2O16, H2O8, and H2O17 species within a narrow spectral interval from 1814 to 1821 cm-1 at natural isotope abundances.

Two-photon resonance on the D2 line of Cs enhanced by optical pumping

Sub-natural-width N-type resonance, observed in the D2 line of Cs vapor has been reported recently. Three-photon, bi-chromatic excitation of Cs atomic vapor with 20 Torr of Neon was performed and a sub-natural-width resonance was observed, superimposed on the absorption profile of a set of hyperfine transitions starting from single hyperfine ground level. Here we report the study of a different three-photon, couple-probe- excitation scheme that allows the observation of a sub-natural-width resonance situated on a flat background, well outside the D2 line absorption spectrum of Cs. The lack of a noisy background could be advantageous for many applications related to the narrow N resonance. In addition, a new broader feature of enhanced transparency is demonstrated that is centered at the ground state level that does not interacting with the coupling beam. This enhanced transparency resonance can be attributed to the atomic population loss in a narrow spectral interval occurring due to excitation of energy levels situated above the 6 2P levels.

Raman hyperspectral imaging spectrometer utilizing crystalline colloidal array photonic crystal diffraction.

We fabricated a novel hyperspectral Raman imaging spectrometer that, for the first time, uses a photonic-crystal wavelength-selecting device to select a narrow-wavelength spectral interval. The photonic crystal consists of an array of highly charged, monodisperse polystyrene particles that self-assemble into a face-centered cubic crystal. The photonic crystal Bragg-diffracts a narrow spectral interval that can be tuned by altering the incident angle of collimated Raman scattered light. Our prototype spectrometer diffracts a ~200 cm(-1) interval of the 488 nm excited visible Raman spectrum of Teflon. This enabled us to select a close-lying triplet of Teflon Raman bands. We imaged the Teflon surface by focusing this narrow region onto a charge-coupled device to create a Raman image of the sample surface that spectrally details the chemical composition.

Obtaining spectral stereo images with electronic spectral tuning and polarization separation

The problem of obtaining stereo images of objects in arbitrary narrow spectral intervals is considered. A method of accomplishing this task is proposed, according to which the two light beams forming a stereopair are simultaneously filtered in one acousto-optic cell and the output beams are separated with respect to polarization. An optimum geometry of diffraction is found and requirements to the acousto-optic filter are formulated. The main characteristics of the spectral stereo system are determined.

Compact echelle spectrometer for occultation sounding of the Martian atmosphere: design and performance

The echelle spectrometer TIMM-2 is the instrument developed for the unsuccessful Russian mission Phobos-Grunt. The instrument was dedicated to solar occultation studies of the Martian atmosphere by measuring the amount of methane, by sensitive measuring of other minor constituents, and by profiling the D/H ratio and the aerosol structure. The spectral range of the instrument is 2300-4100 nm, the spectral resolving power λ/Δλ exceeds 25,000, and the field of view is 1.5×21 arc min. The spectra are measured in narrow spectral intervals, corresponding to discreet diffraction orders. One measurement cycle includes several spectral intervals. To study the vertical profiles of aerosol, the instrument incorporates four photometers in the UV to near-IR spectral range. The mass of the instrument is 2800 g, and its power consumption is 12 W. One complete flight model remains available after the Phobos-Grunt launch. We discuss the science objectives of the occultation experiment for the case of Mars, the implementation of the instrument, and the results of ground calibrations.

Cavity ring down spectroscopy with 5 × 10−13 cm−1 sensitivity

The ultimate sensitivity performances obtained with a continuous wave-cavity ring down spectroscopy setup in the near infrared are investigated. At fixed frequency, the noise of the photodetector is found to be the main limitation and the best limit of detection (about 10(-11) cm(-1)) is reached after a 10 s averaging. We show that long term baseline fluctuations can be efficiently averaged over several days allowing us to reach a detection limit as low as 5 × 10(-13) cm(-1). The achieved sensitivity is illustrated on narrow spectral intervals where the weakest lines detected so far by absorption spectroscopy are observed: (i) ultra-weak transitions of the a(1)Δ(g)(0)-X (3)Σ(g) (-)(1) hot band of (16)O(2) near 1.58 μm and (ii) first detection of an electric quadrupole transition in the second overtone band of nitrogen ((14)N(2)) near 1.44 μm.

Revisiting satellite radiative flux computations at the top of the atmosphere

Most satellite observations of radiative fluxes at the top of the atmosphere (TOA) are at narrow spectral intervals and at particular viewing angles. Critical elements in the formulation of TOA shortwave (SW) radiative fluxes are (1) the transformation from narrowband to broadband values (n/b) and (2) the application of angular distribution models (ADMs) to correct for anisotropy. In this article, the n/b transformations are based on theoretical simulations with a radiative transfer model Moderate Resolution Atmospheric Transmission (MODTRAN) 3.7 using land classification types based on the International Geosphere-Biosphere Programme (IGBP) scheme and a range of realistic atmospheric conditions. The newly developed ADMs are a combination of MODTRAN-3.7 simulations and the Clouds and the Earth's Radiant Energy System (CERES)-observed ADMs. To evaluate the impact of the proposed corrections, they are implemented with observations from the Spinning Enhanced Visible Infrared Imager (SEVIRI) on the Meteorological Satellite (METEOSAT) 8 to derive TOA fluxes and compared to similar quantities from CERES. It is shown that the estimated TOA radiative fluxes have –3% bias and 7% root mean square error (RMSE) when compared with CERES observations at a monthly timescale.

Spectral pyrometry of the objects with hot spots

Optical emission spectra of microwave discharges at the powder mixture surface are experimentally recorded, indicating spatial temperature nonuniformity the within the field of vision. Within the field of vision, the maximal and minimal temperature values calculated from the continuous spectrum (wavelength range 380–620 nm) differ by 20–25%. Modeling of the integral emission spectra of a heated surface with hot spots is performed. Spectral pyrometry of the objects with nonuniform temperature, when used within a narrow spectral interval, gives no possbility of determining the exact nature of the measured temperature because the temperature calculated from this spectrum may differ substantially from both averaged and maximal values. To determine the averaged and maximal temperature, it is necessary to significantly widen the recorded spectral interval.

Sub-Doppler spectroscopy of cesium vapor layers with nanometric and micrometric thickness

The saturation behavior of absorption and fluorescence spectra on the D2 line of Cs is presented, demonstrating a significant difference between open and closed transitions. Cs vapor is confined in an extremely thin cell (ETC) with widely tunable thickness L=(0.5-3)λ, where λ is the light wavelength. In the saturation regime, the closed transition demonstrates enhanced absorption in a narrow spectral interval due to the Dicke effect, while the open one demonstrates only a velocity-selective dip in the absorption. The fluorescence of open transitions shows reduced fluorescence dips, enhancing their contrast with ETC thickness. The closed transition exhibits only a small plateau around the optical transition center. Applying two-level theoretical modeling based on optical Bloch equations, a qualitative agreement with experimental observations is achieved. The rate of contrast enhancement with cell thickness is larger for the theoretical than for the experimental dips. In addition, for the closed transition a tiny peak in the fluorescence is theoretically predicted, with the first experimental confirmation presented. The sub-Doppler spectra of vapor layers with a thickness of several light wavelengths show potential for realization of precise frequency references and photonics sensors.

Agile acousto-optic tracking system for free-space optical communications

Laser communication is viewed as an enabling technology with great potential in both long-range and “last-mile” applications, serving the needs of individual subscribers, corporate offices, military, and possibly deep-space missions. Indeed, lasers allow for concentration of energy within tightly focused beams and narrow spectral intervals, thus offering high throughput, information security, weight and size of components, and power requirements that cannot be matched by rf systems. A key challenge is that high data rates should be complemented by high-precision wide-bandwidth position control of a laser beam. In many applications, the ability to maintain a link is affected by the complex maneuvers performed by mobile communication platforms, resident vibrations, and atmospheric effects. This work is focused on the application of acousto-optic technology as an alternative to electromechanical devices, which addresses the tracking problems encountered under such conditions. With the realization that an acousto-optic Bragg cell is only a component of the entire system, which should perform complex tasks of pointing and tracking of the remote terminal, we present an attempt to consider this problem from the “systems” point of view. We present the developed electro-optical laboratory prototype, show synthesis of the control system, and feature experimental results that demonstrate excellent tracking performance.