Overlapping Absorption Lines Research Articles

Manipulation of spin-1 solid-state targets

The spin-1 nuclear magnetic resonance (NMR) lineshape for polycrystalline materials can be manipulated with selective radio frequency (RF) to alter the overlapping absorption lines in a dynamically polarized target. This process can be performed in solid-state targets either in a frozen spin state or during continuous microwave pumping to manipulate the vector and tensor polarizations of the spin-1 target. In this paper, we present an analytical description of the spin energy levels responsible for the dynamics in the continuous wave NMR spectrum under RF, as well as a new simplified analysis of the lineshape itself that relies on three basic conditions. These conditions can be used to measure the vector and tensor polarization in an RF-manipulated signal in real-time or to simulate the spin-1 NMR lineshape’s response to locally applied RF irradiation. The analytical description of the RF-manipulated lineshape as well as the resulting simulations can be used to optimize the figure of merit in high-energy and nuclear scattering experiments using spin-1 solid-state targets.

Pure NH3 Spectrum Measurements and Analysis of Overlapping Absorption Lines in the 6611.6–6613.5 cm−1 Region

Pure NH3 Spectrum Measurements and Analysis of Overlapping Absorption Lines in the 6611.6–6613.5 cm−1 Region

Theoretical and experimental study of inversion of spectral absorption function using experimental data of laser absorption spectrum with slow scanning and fast modulation of wavelength

Based on the tested data of laser absorption spectra, a matrix slicing method is proposed to invert the absorption function of spectral lines by using the two parameters of laser modulation frequency and laser scanning range as well as transmitted wave signal and reference wave signal under the condition of slow uniform scanning wavelength and fast periodic modulation wavelength. When the modulation is single frequency sinusoidal modulation, an accurate contour of the spectral line absorption function can be obtained by using the matrix data consisting of the values of the transmitted wave signal by the reference wave signal through the minimum value of two slice integrals with the interval of half modulation period, and the amplitude of modulation can be estimated. When the fast modulation of the wavelength is distorted to the multi-frequency superposition modulation, the absorption function is also formed by using the complementarity of multiple slices. The method above is utilized for investigating a real absorption function inversion process involving multiple overlapping absorption lines in the range of the scanning wavelengths. Moreover, the scanning range of laser wavelength can be calibrated by the interval parameters of several spectral lines in the scanning wavelength range. The absorption function of CO at 4300.700 cm<sup>–1</sup> and CO<sub>2</sub> at 6336 cm<sup>–1</sup> are successfully obtained by using this matrix slice method for experimental verification.

A Method for Correcting the Interference of Overlapping Absorption Lines Using Second Harmonic Spectral Reconstruction

The measurement accuracy of trace gas detection based on infrared absorption spectroscopy is influenced by the overlap of absorption lines. A method for correcting the interference of overlapping absorption lines using second harmonic spectral reconstruction (2f-SR) is proposed to improve the measurement accuracy. 2f-SR includes three parts: measurement of gas temperature and use of the differences in temperature characteristics of absorption lines to correct the temperature error, 2f signal restoration based on laser characteristics to eliminate the influence of waveform change on overlapping absorption lines, and fast multi-peak fitting for the separation of interference from overlapping absorption lines. The CH4 measurement accuracy based on overlapping absorption lines is better than 0.8% using 2f-SR. 2f-SR has a lower minimum detection limit (MDL) and a higher detection accuracy than the separation of overlapping absorption lines based on the direct absorption method. The MDL is reduced by two to three orders of magnitude and reaches the part per million by volume level. 2f-SR has clear advantages for correcting the interference of overlapping absorption lines in terms of both MDL and measurement accuracy.

A method for separation of overlapping absorption lines in intracavity gas detection

We propose a novel method to separate overlapping absorption lines in gas detection based on fiber laser intracavity absorption spectroscopy. The method combines continuous wavelet transform, linear regression analysis and chaos particle swarm optimization together to retrieve the respective absorbance distributions of overlapping lines. The method’s principle is outlined and the rules for parameter selection are given. The performance of the method is demonstrated by both the simulated and experimental data. When the method is applied to the mixed gas concentration calibration of CO and CO2, high linearity is obtained with linear correlation coefficients of 0.9996 and 0.9997 respectively. The relative errors of CO and CO2 concentration retrieval are no more than 0.93% and 1.13% respectively.

External-cavity diode laser spectrometer for measuring the concentration ratio by absorption in the range of

The method for determining the carbon isotopic ratio from absorption spectra in the optical cavity containing atmospheric air with the natural contents of carbon dioxide is described. The measurements are performed at atmospheric pressure in the conditions of overlapping absorption lines. The measurement accuracy is 0.3 ‰.

PROSPECTING IN LATE-TYPE DWARFS: A CALIBRATION OF INFRARED AND VISIBLE SPECTROSCOPIC METALLICITIES OF LATE K AND M DWARFS SPANNING 1.5 dex

Knowledge of late K and M dwarf metallicities can be used to guide planet searches and constrain planet formation models. However, the determination of metallicities of late-type stars is difficult because visible wavelength spectra of their cool atmospheres contain many overlapping absorption lines, preventing the measurement of equivalent widths. We present new methods, and improved calibrations of existing methods, to determine metallicities of late-K and M dwarfs from moderate resolution (1300 < R < 2000) visible and infrared spectra. We select a sample of 112 wide binary systems that contain a late-type companion to a solar-type primary star. Our sample includes 62 primary stars with previously published metallicities, as well as 50 stars with metallicities determined from our own observations. We use our sample to empirically determine which features in the spectrum of the companion are best correlated with the metallicity of the primary. We derive metallicity calibrations for different wavelength ranges, and show that it is possible to get metallicities reliable to < 0.10 dex using either visible, J, H, or K band spectra. Our calibrations are applicable to dwarfs with metallicities of -1.04 < [Fe/H]< +0.56 and spectral types from K7 to M5. Lastly, we use our sample of wide binaries to test and refine existing calibrations to determine M dwarf metallicities. We find that the zeta parameter, which measures the ratio of TiO can CaH bands, is correlated with [Fe/H] for super-solar metallicities, and zeta does not always correctly identify metal-poor M dwarfs. We also find that existing calibrations in the K and H band are quite reliable for stars with [Fe/H] > -0.5, but are less useful for more metal-poor stars.

Error analysis for CO and CH&amp;lt;sub&amp;gt;4&amp;lt;/sub&amp;gt; total column retrievals from SCIAMACHY 2.3 μm spectra

Abstract. A detailed sensitivity analysis of the Iterative Maximum Likelihood Method (IMLM) algorithm and its application to the SCIAMACHY 2.3 μm spectra is presented. The sensitivity analysis includes a detailed assessment of the impact of aerosols in the 2.3 μm range. Results show that near strong aerosol sources mineral dust and biomass aerosols can have an effect of ~7–10% on the CH4 total columns retrieved from this wavelength range if aerosol scattering is neglected in the retrieval algorithm. Similar but somewhat larger effects are found for CO, but due to the larger variability of CO these errors are less important. Away from strong sources much smaller effects of a few percent are found. Using CH4 as a proxy for CO and/or including aerosol information in the retrieval algorithm significantly reduces these errors for both CO and CH4. Spectroscopic uncertainties are mostly negligible except for uncertainties in the CH4 intrinsic line intensities, which can be important. Application of the IMLM algorithm to the SCIAMACHY 2.3 μm spectra shows that the quality of the retrieved CO and CH4 total columns is good, except for a bias for large instrument-noise errors which is partly due to remaining calibration issues. Polarization sensitivity of the SCIAMACHY instrument has a negligible effect on the retrieved CO and CH4 total columns. The H2O total columns, which have to be retrieved simultaneously with CO and CH4 due to overlapping absorption lines, agree well with H2O total columns from ECMWF data. This ensures that the fit to the H2O absorptions is of sufficient quality not to hamper the retrieved CO and CH4 total columns from SCIAMACHY spectra.

Level mixing induced transparency

We discuss level mixing induced transparency (LMIT), which is experimentally found in a thick absorber of FeCO3 containing the 57Fe Mossbauer nuclei. LMIT is observed if two hyperfine levels of the excited state nucleus cross. Absorption of gamma radiation drops by about 25% with respect to the sum of two overlapping absorption lines. The absorption deficit is explained by the mixing of the crossing levels with a symmetry breaking interaction. The level mixing induces a polarization change of the scattered radiation. In such a way, the radiation develops in “normal modes,” which are less absorbed in a thick absorber. We show that, in spite of the absorption drop, the area of the absorption line does not change. This confirms the general knowledge that interference phenomena (destructive or constructive) do not change the lifetime of an excited state particle (atom, nucleus, etc).

Determination of molecular line parameters for acrolein (C 3H 4O) using infrared tunable diode laser absorption spectroscopy

Acrolein (C 3H 4O) molecular line parameters, including infrared (IR) absorption positions, strengths, and nitrogen broadened half-widths, must be determined since they are not included in the high resolution transmission (HITRAN) molecular absorption database of spectral lines. These parameters are required for developing a quantitative analytical method for measuring acrolein in a single puff of cigarette smoke using tunable diode laser absorption spectroscopy (TDLAS). The task is complex since acrolein has many highly overlapping infrared absorption lines in the room temperature spectrum and the cigarette smoke matrix contains thousands of compounds. This work describes the procedure for estimating the molecular line parameters for these overlapping absorption lines in the wavenumber range (958.7–958.9 cm −1) using quantitative reference spectra taken with the infrared lead-salt TDLAS instrument at different pressures and concentrations. The nitrogen broadened half-width for acrolein is 0.0937 cm −1 atm −1 and to our knowledge, is the first time it has been reported in the literature.

Determination of molecular parameters for 1,3-butadiene and propylene using infrared tunable diode laser absorption spectroscopy

A technique has been developed for the determination of molecular parameters, including infrared absorption line positions, strengths, and nitrogen-broadened half-widths for 1,3-butadiene (C 4H 6) and propylene (C 3H 6). The parameters for these two molecules are required for quantitation using Tunable Diode Laser Absorption Spectroscopy (TDLAS). These molecules have populations of highly overlapping infrared absorption lines in their room temperature spectra. The technique reported here provides a procedure for estimating the molecular parameters for these overlapping absorption lines from quantitative reference spectra taken with the TDLAS instrument at different pressures and concentrations. The system was developed for the quantitation of gaseous constituents in a single puff of cigarette smoke and this paper will describe the procedure and some of the factors that influence the accuracy of quantitation for 1,3-butadiene, including the approach taken to minimize the adverse effects of the absorption due to propylene in the same spectral region.

The Extreme Ultraviolet Spectrum of the Quasi-coherent Oscillations of the Dwarf Nova SS Cygni

Data obtained by the Extreme Ultraviolet Explorer satellite are used to determine the EUV spectrum of the quasi-coherent oscillations of the dwarf nova SS Cygni. It is found that the spectrum of the oscillations is neither blue nor red nor gray relative to the net (oscillation-phase integrated) spectrum and hence that the oscillations cannot be explained by variations in the effective temperature, absorbing column density, or effective area, respectively. Instead, it is found that the amplitude of the oscillations is high at the relative maxima of the net spectrum and low to zero at the relative minima of the net spectrum. This behavior can be explained either by variations in the emission line flux atop a constant underlying continuum or by variations in the optical depth of a haze of overlapping absorption lines, in which case the optical depths must be τ 1 at the relative maxima of the net spectrum and τ 1 at the relative minima.

Non-LTE model atmosphere analysis of the early ultraviolet spectra of Nova OS Andromedae 1986

We have analyzed the early optically thick ultraviolet spectra of Nova OS And 1986 using a grid of spherically symmetric, non-LTE, line-blanketed, expanding model atmospheres and synthetic spectra with the following set of parameters: $5,000\le$ T$_{model}$ $\le 60,000$K, solar abundances, $\rho \propto r^{-3}$, $\v_{max} = 2000\kms$, $L=6 \times 10^{4}\Lsun$, and a statistical or microturbulent velocity of 50 $\kms$. We used the synthetic spectra to estimate the model parameters corresponding to the observed {\it IUE} spectra. The fits to the observations were then iteratively improved by changing the parameters of the model atmospheres, in particular T$_{model}$ and the abundances, to arrive at the best fits to the optically thick pseudo-continuum and the features found in the {\it IUE} spectra. The {\it IUE} spectra show two different optically thick subphases. The earliest spectra, taken a few days after maximum optical light, show a pseudo-continuum created by overlapping absorption lines. The later observations, taken approximately 3 weeks after maximum light, show the simultaneous presence of allowed, semi-forbidden, and forbidden lines in the observed spectra. Analysis of these phases indicate that OS And 86 had solar metallicities except for Mg which showed evidence of being underabundant by as much as a factor of 10. We determine a distance of 5.1 kpc to OS And 86 and derive a peak bolometric luminosity of $\sim$ 5 $\times$ 10$^4$ L$_{\odot}$. The computed nova parameters provide insights into the physics of the early outburst and explain the spectra seen by {\it IUE}. Lastly, we find evidence in the later observations for large non-LTE effects of Fe{\sc ii} which, when included, lead to much better agreement with the observations.

A Sensitive Isotope Selective Nondispersive Infrared Spectrometer for 13CO2 and 12CO2 Concentration Measurements in Breath Samples

Abstract We present a nondispersive infrared spectrometer (NDIRS) for the measurement of the 13CO2/12CO2-ratio in breath samples. A commercial NDIR spectrometer for CO2 concentration measurements in industrial process control was modified using two separate optical channels for the 13CO2 and 12CO2 detection. Cross interference due to overlapping absorption lines of both isotopic gases was successfully eliminated. The sensitivity of this device is ± 0.4‰ of the 13CO2/12CO2-ratio in a range of 2.5 to 5% of total CO2. This is sufficient for biomedical applications. Our spectrometer is small in size, cheap and simple to operate and thus a true alternative to isotope ratio mass spectrometers (IRMS). Several biomedical applications with breath samples were demonstrated and were compared in very good agreement with IRMS.

Magnetic surface anisotropy and ferromagnetic resonance in single-crystal GdAl2

Ferromagnetic resonance in bulk single-crystal specimens of GdAl2 shows two overlapping absorption lines. The variation of this structure with the temperature of the specimen and the crystallographic orientation of the magnetisation M0 is described. The line with the lower resonance field is attributed to the normal bulk mode and that at higher field to a surface-induced mode. The latter is shifted to a higher field by magnetic surface anisotropy. At higher temperatures (T>80K) the bulk mode is more intense but as T is reduced the relative strength changes and below about 30K the surface-induced mode is entirely dominant. For specimens with (110) surfaces the mode separation is greatest when M0 is oriented between (111) and (111) directions and least when M0 is along (001) and (001). The model employed to interpret these phenomena is based upon a classical equation of motion for M0, Maxwell's equations and boundary conditions for the magnetisation at the sample surface which involve the magnetic surface anisotropy. By interpreting the separation of the resonance lines as a function of the orientation of M0 the expression for the magnetic surface anisotropy energy as a function of this orientation is deduced and values for the necessary three magnetic surface anisotropy constants are deduced as a function of temperature. The main surface anisotropy gives an easy direction of magnetisation perpendicular to the surface. The easiest directions within the surface are (001) and (001). Interpretation of the resonance linewidth and relative intensity of the two modes is not satisfactory. It is suggested that this is because the measured lines are influenced by inhomogeneous broadening which is absent from the theory.

Selective two-quantum laser excitation of molecules with overlapping absorption lines

A method of two-quantum excitation of molecules at Doppler-broadened transitions in a standing-wave radiation field is considered. It is shown that it is possible to increase the excitation selectivity of molecules with overlapping absorption lines when using spectra of two-photon absorption. Estimations of two-quantum transition probabilities for molecular gases are given.

Overlap Emissivity of CO2 and H2O in the 15-µm Spectral Region

Radiative transfer in model atmospheres including CO2, water vapor, ozone, and clouds can be explored by computer simulation for the purpose of predicting the mean temperature at the surface of the earth. Where two or more gases have overlapping absorption lines, the overlap emissivity determined from the band-averaged expressions is a possible source of uncertainty. This method has been tested in a 250-cm-1 region around 15 µm where the CO2 absorption overlaps the tail of the water vapor rotation band. Published high resolution spectra of these gases enable the overlap effects to be calculated for small spectral intervals and summed over the band. This calculation was made and compared with the value given by the approximate treatment for the same gas concentrations, pressure, and temperature. Agreement was within 10 percent, indicating that the more detailed spectral calculation is unnecessary.

Equivalent Width Due to Two Overlapping Lines*

The equivalent width, or integrated absorptance, of two unequal, overlapping absorption lines is equal to the sum of the individual equivalent widths minus an overlap correction term Δ. A method is given for rigorous evaluation of the correction. The rigorous result is unwieldy in the general case, but for the special case of two lines widely separated, the correction term reduces to tractable form. However, an approximate solution is given that is shown to introduce relatively small error for all cases considered, and is quite convenient to use.

Errors in Spectral Absorption Measurements Due to Absorbing Species in the Atmosphere*

The accuracy of spectroscopic absorptance measurements can be seriously affected by the presence of absorbing species in the atmosphere anywhere in the optical train. The error is due to the band-pass nature of a monochromator, and therefore is just as severe when a double-beam spectrometer is used as when a single-beam spectrometer is used.Using a method recently developed by Sakai for the calculation of the integrated absorptance of a pair of overlapping absorption lines, we have calculated the error in the measured integrated absorptance of a sample gas which contains H2O or CO2. The method used places no restrictions on the half-width or strength of either line. Factors have been derived for correcting the measured absorptance in a number of experimentally important cases. The calculated correction factors give results which are in good agreement with experiments made using a spectrometer in which the atmospheric H2O and CO2 were removed from the optical train.

Thermodynamics of Spin Systems in Solids

Quasiequilibrium states of the spin system in a solid are described in terms of one "Zeeman" temperature for each spin species plus one "dipole-dipole" temperature, ${T}_{D}$. Energy and entropy are calculated and used to predict the steady state of processes such as cross relaxation. It is predicted and demonstrated by an experiment on the nuclear spins in LiF that the state of the "dipole-dipole" system has a strong influence on such steady states. Continuous wave (cw) and pulse spectroscopy are discussed for systems with low ${T}_{D}$. Techniques are proposed (and have been used) to measure ${T}_{D}$ and one Zeeman temperature simultaneously, using coherent pulse instrumentation, and for preparing a state of low ${T}_{D}$ in a large magnetic field by complete adiabatic demagnetization followed by sudden magnetization. A density matrix formalism is proposed for the description of quasiequilibrium situations in the case of "spins" with unequally spaced energy levels. Finally the influence of the "nonsecular" part of the spin-spin Hamiltonian on the quasi-equilibrium states is estimated by a perturbation calculation, and the resulting description includes the cases of low- or zero-magnetic field and partly or completely overlapping absorption lines.