Moderate-resolution Spectrometer Research Articles

Real-time continuous calibration method for an ultraviolet camera.

The accuracy of SO2 cameras is significantly determined by the ability to obtain an accurate calibration. This work presents a real-time continuous calibration method for SO2 cameras with a moderate resolution spectrometer by taking realistic radiative transfer into account. The effectiveness and accuracy of the proposed method have been verified through simulations and experiments. The calibration error can be reduced by about 20-80% compared with the commonly used cell calibration, especially for situations of long distance, poor visibility, or optically thick plumes.

Quantitative measurement of hydrogen isotopes in titanium using laser-induced breakdown spectroscopy.

Laser-induced breakdown spectroscopy (LIBS) has been considered a promising technology for nuclear safeguard inspection, especially for isotope content ratio determination, since it can be easily designed for portable, fast, and in situ measurement. However, it was a challenge to determine hydrogen isotopes in metal samples due to the unfavorable spectral interference, the poor calibration of the hydrogen content, and the small difference between the atomic emission intensity of hydrogen isotopes at around 656.28 nm. This paper presents the determination of hydrogen isotope contents ratio using LIBS under partially baseline-resolved conditions. The results show that by introducing a proper buffer atmosphere for the LIBS measurement, the resolution of the hydrogen and deuterium emissions could be improved, but still not enabled, by a baseline resolution with a moderate resolution spectrometer. However, with the method of integral intensity correction, the accurate quantitative measurement of hydrogen and deuterium contents in a metal matrix could be achieved. This work provided the possibilities for the further development of LIBS in hydrogen isotopes in in situ measurement for nuclear safeguards.

Resonance broadening of argon lines in a micro-scaled atmospheric pressure plasma jet (argon μAPPJ)

Optical emission from atmospheric pressure micro-jet operating with pure argon (argon μAPPJ) flow has been detected with a moderate resolution spectrometer. Large broadening of the several argon (Ar) lines has been observed in the near infrared spectral region. This effect was attributed to resonance broadening of the s2 (Paschen notation) level in 3p54s configuration. In the present work, corresponding line profiles are suggested for plasma diagnostics. For this, a general case of resonance broadening coefficient of noble gases is discussed. As broadening reflects the Ar density, and the static gas pressure of the jet is in equilibrium with the ambient, the local gas temperature can be inferred. An estimation of gas temperature from the width of the 750 nm Ar line is in agreement with rotational temperature of OH radicals determined from the A2Σ+ → X2Π (0, 0) band. At low temperatures (300–600 K) and at partial Ar pressure near atmospheric, the resonance width of the suggested lines is very sensitive to small temperature variations. High temperature sensitivity and large width make the resonance broadened lines very attractive for diagnostics of low temperature discharges at elevated pressure, e.g., as they are used in plasma-medicine.

On the effect of tilted roof reflectors in Martin–Puplett spectrometers

In this paper we analyze theoretically and experimentally the effect of tilt of the roof mirrors in a double pendulum Martin–Puplett Polarizing Interferometer (MPI), focusing on the polarization of the interfering beams. In principle, the tilt affects the efficiency and polarimetric properties of the interferometer. The case of a moderate resolution spectrometer is analysed in detail. Using the Stokes formalism we recover the analytical expressions for the orientation angle and the ellipticity of the beam reflected from a metallic surface, and we compute these quantities for the roof-mirror of a MPI. We find that the polarization rotation and depolarization are small. Using the Jones formalism we propagate their effect on the measured interferogram and spectrum, and demonstrate that the performance degradation is small compared to other systematic effects.

Theoretical description of functionality, applications, and limitations of SO<sub>2</sub> cameras for the remote sensing of volcanic plumes

Abstract. The SO2 camera is a novel device for the remote sensing of volcanic emissions using solar radiation scattered in the atmosphere as a light source for the measurements. The method is based on measuring the ultra-violet absorption of SO2 in a narrow wavelength window around 310 nm by employing a band-pass interference filter and a 2 dimensional UV-sensitive CCD detector. The effect of aerosol scattering can in part be compensated by additionally measuring the incident radiation around 325 nm, where the absorption of SO2 is about 30 times weaker, thus rendering the method applicable to optically thin plumes. For plumes with high aerosol optical densities, collocation of an additional moderate resolution spectrometer is desirable to enable a correction of radiative transfer effects. The ability to deliver spatially resolved images of volcanic SO2 distributions at a frame rate on the order of 1 Hz makes the SO2 camera a very promising technique for volcanic monitoring and for studying the dynamics of volcanic plumes in the atmosphere. This study gives a theoretical basis for the pertinent aspects of working with SO2 camera systems, including the measurement principle, instrument design, data evaluation and technical applicability. Several issues are identified that influence camera calibration and performance. For one, changes in the solar zenith angle lead to a variable light path length in the stratospheric ozone layer and therefore change the spectral distribution of scattered solar radiation incident at the Earth's surface. The varying spectral illumination causes a shift in the calibration of the SO2 camera's results. Secondly, the lack of spectral resolution inherent in the measurement technique leads to a non-linear relationship between measured weighted average optical density and the SO2 column density. Thirdly, as is the case with all remote sensing techniques that use scattered solar radiation as a light source, the radiative transfer between the sun and the instrument is variable, with both "radiative dilution" as well as multiple scattering occurring. These effects can lead to both, over or underestimation of the SO2 column density by more than an order of magnitude. As the accurate assessment of volcanic emissions depends on our ability to correct for these issues, recommendations for correcting the individual effects during data analysis are given. Aside from the above mentioned intrinsic effects, the particular technical design of the SO2 camera can also greatly influence its performance, depending on the setup chosen. A general description of an instrument setup is given, and the advantages and disadvantages of certain specific instrument designs are discussed. Finally, several measurement examples are shown and possibilities to combine SO2 camera measurements with other remote sensing techniques are explored.

Water vapour in BALTIMOS: a comparison with MODIS measurements

In this paper, we compare integrated water vapour (IWV) retrievals from the Moderate Resolution Spectrometer (MODIS) instrument on board the polar-orbiting Terra platform with those from the coupled regional climate model system BALTIMOS for a period of 2 years (October 2001 to October 2003). The comparison was made for the whole drainage basin of the Baltic Sea as well as major parts of Central Europe. The qualitative comparison between the two data sets of the integrated water vapour shows a good agreement. The patterns in the spatial distribution of the averaged integrated water vapour in both data sets are quite similar. However, significant differences occur in the Hungarian Lowlands, along the Po River and the Wallachia (Romania) in the order of 2.5 to 7.0 kg/m2. For these areas, the BALTIMOS model is dryer than the MODIS observations. This could be an indication for the known summer drying effect of BALTIMOS but needs further investigations. The annual cycle as well as a diurnal developing of integrated water vapour from 09:00 to 12:00 UTC is well pronounced in both data sets. For both data sets, the overall annual variations are 17.5 kg/m2. The observed overall diurnal variability are 1.4 kg/m2 for MODIS and 1.04 kg/m2 for BALTIMOS, respectively.

Intensity distributions in R and P branches of (0–0) band of the A1Π→ X1Σ+ electronic transition of the BH molecule and determination of gas temperature in non-equilibrium plasmas

Relative transition probabilities of spontaneous emission in the R and P branches of the A 1 Π→ X 1 Σ + (0–0) band of BH molecule have been obtained for the first time. It was observed that measured ratios of the line strengths are in agreement with corresponding ratios of Hönl–London factors. Thus, the non-adiabatic effect of perturbations is negligibly small and Hönl–London formulas may be used for derivation of rovibronic population densities from measured line intensities of the (0–0) R branch of BH. The (0–0) P branch is less prospective because too much overlapping by other BH emissions cannot be avoided with the moderate resolution spectrometers usually used in plasma spectroscopy. General considerations are illustrated on the example of a low-pressure plasma of a planar microwave discharge in a H 2–Ar–B 2H 6 (64:33:3) gas mixture (total pressure 1– 2.5 mbar , power 1.2– 2.4 kW ). The values of the rotational temperatures obtained directly for the excited electronic-vibrational states d 3Π u −, v′=2 (T d2 rot ( H 2)) and A 1Π +, v′=0 (T A0 rot ( BH)) and those calculated in the framework of excitation-deactivation models for the ground X 1Σ u +, v=0 vibronic states ( T X0 rot(H 2) and T X0 rot(BH)) have been analysed. It has been observed that T X0 rot(H 2) and T X0 rot(BH) are in excellent agreement, while compared to these values T A0 rot(BH) is noticeably higher and T d2 rot(H 2) is about two times lower. Thus, the simultaneous measurements of intensity distributions in the rotational structure of BH and H 2 emission bands can be used as a prospective method for determination of gas temperature in non-equilibrium boron and hydrogen containing plasmas.

Complex resonance absorption structure in the X-ray spectrum of IRAS 13349+2438

The luminous infrared-loud quasar IRAS 13349+2438 was observed with the XMM - Newton Observatory as part of the Performance Verification program. The spectrum obtained by the Reflection Grating Spectrometer (RGS) exhibits broad (FWHM - 1400 km/s) absorption lines from highly ionized elements including hydrogen- and helium-like carbon, nitrogen, oxygen, and neon, and several iron L - shell ions (Fe XVII - XX). Also shown in the spectrum is the first astrophysical detection of a broad absorption feature around lambda = 16 - 17 A identified as an unresolved transition array (UTA) of 2p - 3d inner-shell absorption by iron M-shell ions in a much cooler medium; a feature that might be misidentified as an O VII edge when observed with moderate resolution spectrometers. No absorption edges are clearly detected in the spectrum. We demonstrate that the RGS spectrum of IRAS 13349+2438 exhibits absorption lines from two distinct regions, one of which is tentatively associated with the medium that produces the optical/UV reddening.

A simple soft X-ray (1 < λ < 11 nm) spectrometer for relative line intensity measurements in charge exchange studies

We describe a simple plane crystal spectrometer specially built for charge exchange study through soft X-ray deexcitation of highly charged ions. The use of low energy, highly charged ions incident on He and H 2 targets decreases the number of processes leading to photon emission. Thus, the X-ray spectra are simple and we demonstrate that a moderate resolution spectrometer is sufficient. The instrumental relative sensitivity has been measured in the 1 to 11 nm wavelength range and a good agreement has been found with calculated values. Using the “linear mix” deconvolution procedure described by Henke, we made accurate measurements for the X-ray transition intensities in N 7+, O 8+, Ne 8+ -He, H 2 collision cases.

Emission spectroscopy of SiH in a silane glow-discharge

A diagnostic feature of silane-hydrogen glow-discharges is obtained by analysing the A2 Delta -X2II band system of the SiH radical. The measurement is performed with a moderate resolution spectrometer and, if combined with numerical calculation of the spectrum, is shown to lead to rotational and vibrational effective temperatures. Abnormal excitation of SiH is observed; however both results are relevant for comparing the microscopic state of the glow-discharge in various thin-film deposition devices.