Absolute Line Intensities Research Articles

Full-Energy Peak Efficiency Calibration of a Metallic Magnetic Calorimeter Detector for Photon Spectrometry Below 100 keV

Knowledge of absolute emission intensities of X-ray lines below 100 keV is limited or missing. An absolute determination of these intensities necessitates a high-resolution spectrometer with an accurate efficiency calibration. In this work, we will show and discuss the FEP efficiency calibration of a MMC detector combining a measurement of a standard source with Monte Carlo simulations using the code PENELOPE-2014. The simulated efficiency detection successfully interpolated the experimental efficiency values.

An experimental water line list at 1950 K in the 6250–6670 cm[formula omitted] region

An absorption spectrum of H216O at 1950 K is recorded in a premixed methane/air flat flame using a cavity-enhanced optical frequency comb-based Fourier transform spectrometer. 2417 absorption lines are identified in the 6250–6670 cm−1 region with an accuracy of about 0.01 cm−1. Absolute line intensities are retrieved using temperature and concentration values obtained by tunable diode laser absorption spectroscopy. Line assignments are made using a combination of empirically known energy levels and predictions from the new POKAZATEL variational line list. 2030 of the observed lines are assigned to 2937 transitions, once blends are taken into account. 126 new energy levels of H216O are identified. The assigned transitions belong to 136 bands and span rotational states up to J=27.

High Precision Cavity Ring Down Spectroscopy of 6ν3 Overtone Band of 14N216O near 775 nm

Transitions of the 6ν3 overtone band of 14N216O near 775 nm have been studied by continuous-wave cavity ring-down spectroscopy. Line positions and intensities were derived from a fit of the line shape using a hard-collisional profile. The line positions determined with absolute accuracy of 5×10−4 cm−1 allowed us to reveal finer ro-vibrational couplings taking place after J>14 except a strong anharmonic interaction identified by the effective Hamiltonian model. The absolute line intensities have also been retrieved with an estimated accuracy of 2% for a majority of the unblended lines. A new set of ro-vibrational and dipole moment parameters were derived from the experimental values. A comparison between the line positions and intensities of the 6ν3 band obtained in this work and those from previous studies is given.

ERO modeling of beryllium erosion by helium plasma in experiments at PISCES-B

The beryllium erosion by helium plasma irradiation is studied at the PISCES-B linear plasma device and interpreted using the accompanying simulations by the ERO code. The influence of plasma conditions and varying negative biasing of the Be plasma target on BeI and BeII absolute line intensities are reproduced in detail by the simulations. The synthetic axial line intensity shapes and line ratios match with experiment. This indicates that atomic data are quite accurate. The initial population state of quasi-metastable 3P level in BeI is found to be MS:GS= 0.33:1 for all conditions. The yields determined by the modeling interpretation are compared to the SDTrimSP code simulations in the binary collision approximation.

Speed-dependent Voigt profile parameters for oxygen B-band measured by cavity ring-down spectrometer referenced to the optical frequency comb

We present the results of the line-shape analysis using the speed-dependent Voigt profile for the self-perturbed oxygen B-band resulting from the transitions. The line-shape parameters data set contains the absolute positions, line intensities, pressure broadening, self-shift and speed-dependent line-shape coefficients.

Ab initio calculations for the first-positive bands of N2

The absolute absorption spectral line intensities of the 0-0, 1-1, 2-2, 3-3, and 4-4 bands of N2(A3Σu+−B3Пg) are computed for the first time with the accurate potential energy curves (PECs) and transition dipole moment curves (TDMCs) using the ab initio calculation. During the calculation process, the precise multireference configuration interaction (MRCI) approach with the augmented correlation consistent polarized valence quadruple-zeta set (aug-cc-pVQZ) and active space 3110311 are used. The calculated PECs of the two states (A3Σu+, B3Пg) agree well with Rydberg-Klein-Rees (RKR) potential, and the calculated TDMCs are more reasonable than the results of other studies. Our calculated Einstein A coefficients are also well consistent with the recent experiment results. The intensities of five bands are obtained from the summation of their respective absolute line intensities, for instance, the 0-0 band intensities are 3.25×10−16cm−1/(moleculecm−2) at 300K and 1.65×10−16cm−1/(moleculecm−2) at 3000K. The theoretical results are credible. They are also beneficial for astrophysics where high temperature dominates.

A global ab initio dipole moment surface for methyl chloride

A new dipole moment surface (DMS) for methyl chloride has been generated at the CCSD(T)/aug-cc-pVQZ(+d for Cl) level of theory. To represent the DMS, a symmetry-adapted analytic representation in terms of nine vibrational coordinates has been developed and implemented. Variational calculations of the infrared spectrum of CH3Cl show good agreement with a range of experimental results. This includes vibrational transition moments, absolute line intensities of the ν1, ν4, ν5 and 3ν6 bands, and a rotation–vibration line list for both CH335Cl and CH337Cl including states up to J=85 and vibrational band origins up to 4400cm−1. Across the spectrum band shape and structure are well reproduced and computed absolute line intensities are comparable with highly accurate experimental measurements for certain fundamental bands. We thus recommend the DMS for future use.

Quantum Cascade Laser Measurements of Line Intensities, N2-, O2- and Ar- Collisional Broadening Coefficients of N2O in the ν3 Band Near 4.5 µm.

This study deals with precise measurements of absolute line intensities, N2-, O2- and Ar- collisional broadening coefficients of N2O in the P-branch of the ν3 vibrational band near 4.5 µm. Collisional broadening coefficients of N2O-air are derived from the N2- and O2- broadening contributions by considering an ideal atmospheric composition. Studies are performed at room temperature for 10 rotational transitions over 2190-2202 cm(-1) spectral range using a distributed-feedback quantum cascade laser. To retrieve spectroscopic parameters for each individual transition, measured absorption line shape is simulated within Voigt and Galatry profiles. The obtained results compare well with previous experimental data available in the literature: the discrepancies being less than 4% for most of the probed transitions. The spectroscopic data reported here are very useful for the design of sensors used to monitor the abundance of N2O in earth's atmosphere.

Line parameter study of ozone at 5 and 10 μm using atmospheric FTIR spectra from the ground: A spectroscopic database and wavelength region comparison

Atmospheric ozone concentration measurements mostly depend on spectroscopic methods that cover different spectral regions. Despite long years of measurement efforts, the uncertainty goal of 1% in absolute line intensities has not yet been reached. Multispectral inter-comparisons using both laboratory and atmospheric studies reveal that important discrepancies exist when ozone columns are retrieved from different spectral regions. Here, we use ground based FTIR to study the sensitivity of ozone columns on different spectroscopic parameters as a function of individual bands for identifying necessary improvements of the spectroscopic databases. In particular, we examine the degree of consistency that can be reached in ozone retrievals using spectral windows in the 5 and 10μm bands of ozone. Based on the atmospheric spectra, a detailed database inter-comparison between HITRAN (version 2012), GEISA (version 2011) and S&MPO (as retrieved from the website at the end of 2015) is made. Data from the 10μm window are consistent to better than 1%, but there are larger differences when the windows at 5μm are included. The 5μm results agree with the results from 10μm within ±2% for all databases. Recent S&MPO data are even more consistent with the desired level of 1%, but spectroscopic data from HITRAN give about 4% higher ozone columns than those from GEISA. If four sub-windows in the 5μm band are checked for consistency, retrievals using GEISA or S&MPO parameters show less dispersion than those using HITRAN, where one window in the P-branch of the ν1+ν3 band gives about 2% lower results than the other three. The atmospheric observations are corroborated by a direct comparison of the spectroscopic databases, using a simple statistical analysis based on intensity weighted spectroscopic parameters. The bias introduced by the weighted average approach is investigated and it is negligible if relative differences between databases do not correlate with line intensities. This is the case for the comparison of HITRAN with GEISA in the 10μm region and the agreement between the simple analysis and the full retrieval is better than 0.1%. At 5μm biases might be as high as 1.4%, and the proposed method is thus limited to the same level of accuracy. Implications of the new data for database improvements and further studies, in particular in the 5μm region, are discussed.

A global potential energy surface and dipole moment surface for silane.

A new nine-dimensional potential energy surface (PES) and dipole moment surface (DMS) for silane have been generated using high-level ab initio theory. The PES, CBS-F12(HL), reproduces all four fundamental term values for (28)SiH4 with sub-wavenumber accuracy, resulting in an overall root-mean-square error of 0.63 cm(-1). The PES is based on explicitly correlated coupled cluster calculations with extrapolation to the complete basis set limit, and incorporates a range of higher-level additive energy corrections to account for core-valence electron correlation, higher-order coupled cluster terms, and scalar relativistic effects. Systematic errors in computed intra-band rotational energy levels are reduced by empirically refining the equilibrium geometry. The resultant Si-H bond length is in excellent agreement with previous experimental and theoretical values. Vibrational transition moments, absolute line intensities of the ν3 band, and the infrared spectrum for (28)SiH4 including states up to J = 20 and vibrational band origins up to 5000 cm(-1) are calculated and compared with available experimental results. The DMS tends to marginally overestimate the strength of line intensities. Despite this, band shape and structure across the spectrum are well reproduced and show good agreement with experiment. We thus recommend the PES and DMS for future use.

Chemical Composition, Physical Properties and Populating Mechanism of Some O(I) States for a DC Discharge in Oxygen with Water Cathode

This paper reports the results of the experimental study of parameters for a DC oxygen discharge with water cathode in the pressure range of 0.1–1 bar and the discharge current of 40 mA. The radius of positive column, the cathode voltage drop, the cathode current density and the electric field strength were measured. Rotational temperatures of N2 (C3Πu, V = 0) and OH (A2Σ, V = 0) and absolute line intensities of atomic oxygen with wave length of 845 and 777 nm were determined as well. Plasma composition modeling was carried out by the combined solution of the Boltzmann equation for electrons, the equations of vibrational kinetics for ground states of N2, O2, H2O molecules, and the equations of chemical kinetics, and the plasma conductivity equation. Calculations were carried out taking into consideration the discharge radial heterogeneity and using experimental values of E/N and gas temperatures. The main particles being formed in plasma were shown to be ·OH, H2O2, O(3P), O2(a1Δg), O2(b1Σ g + ), H(1S). On the basis of this calculation and experimental values of line intensities, the populating mechanism of (3p 3P) level of atomic oxygen was discussed. The comparison of some properties of discharges in O2, N2 and air was done.

Measurements of the number density of water molecules in plasma by using a combined spectral−probe method

A novel method for measuring the number density of water molecules in low-temperature plasma is developed. The absolute intensities of rotational lines of the (0,0) band of the OH(A2Σ–X2П) transition are used. Lines with sufficiently large rotational quantum numbers referring to the so-called “hot” group of molecules produced by electron-impact dissociative excitation of water molecules are chosen for measurements. The excitation rate of a process with a known cross section is determined by measuring the parameters of plasma electrons by means of the probe method. The measured number densities of molecules are compared with those in the initial plasma-forming mixture. The time evolution of the particle densities in plasma is investigated. The problems of the sensitivity and applicability of the absolute spectral method are considered.

Theoretical rovibrational spectroscopy beyond fc-CCSD(T): the cation CNC+

An accurate near-equilibrium potential energy surface (PES) for CNC+ is constructed based on a high-level composite ab initio method. By combining explicitly correlated all-electron CCSD(T)-F12b with scalar relativistic effects and higher order correlation up to coupled cluster theory with singles, doubles, triples and quadruples (CCSDTQ) we achieve convergence in the wavenumbers of the fundamentals to ca. 1 cm−1. Rovibrational energies are calculated in a variational approach and vibrational term energies and rotational constants are in excellent agreement with available experimental data. Accurate values for centrifugal distortion constants of CNC+ in different vibrational states are predicted. Especially the centrifugal distortion constants in the vibrational ground state of D0 = 0.563 · 10−6 cm−1 and H0 = 0.188 · 10−10 cm−1 should be superior to experimentally derived values. Reassignments of some experimentally observed transitions are suggested based on a comparison of experimental and calculated term differences. The bending part of the PES appears to be almost quartic and the band origin of the bending vibration is predicted at 94.2 cm−1. Absolute line intensities are calculated for various transitions in CNC+. For the bending vibration, an intensity is predicted that is three orders of magnitude smaller than for the antisymmetric stretching vibration.

Surprisingly weak fine-structure line emission from S 140

GREAT observations of the [Oi] and [Cii] fine-structure lines in S 140 show very low intensities towards the embedded sources. We measure line-to-continuum cooling ratios of 10 −5 − 10 −4 , i.e. values matching the far-IR line deficit seen in ULIRGs. Normal PDR models fail to explain the combination of source size, line ratios, and absolute line intensities. A match is, however, possible when modelling the sources in terms of inside-out PDRs. The small beam-filling of PDRs in a dense homogeneous medium can lead to a line deficit compared to the infrared continuum, emitted from a much larger region.

Detection of atmospheric 15NO2 in the ν3 spectral region (6.3 μm)

A reinvestigation of the ν3 band of 15NO2 has been performed using experimental data acquired in 2000 [Orphal J, Perrin A, Flaud JM, Smirnov M, Himmelmann, Voigt S, Burrows JP. J Mol Spec 2000;204:72–9]. The linelist generated in this way during this study was used to detect this isotopologue in the atmosphere for the first time, using balloon-borne solar occultation spectra measured by the JPL MkIV Fourier transform spectrometer. It is shown that over the 15–35km altitude range where 15NO2 can be detected, the retrieved 15/14 NO2 ratio is within 5% of the expected value (0.00364), implying that the absolute line intensities in the new linelist are accurate. Over the same altitude range the RMS spectral fitting residuals reduce significantly as a result of including the new 15NO2 linelist, improving the accuracy of retrievals of all gases that absorb in the 1550–1650cm−1 region (e.g., 14NO2, H2O, HDO, O2).

Intensities, broadening and narrowing parameters in the ν3 band of methane

The P-branch of methane׳s ν3 band is probed to carry out an extensive study of the 2905–2908cm−1 infrared spectral region. Absolute line intensities as well as N2-, O2-, H2-, He-, Ar- and CO2-broadening coefficients are determined for nine transitions at room temperature. Narrowing parameters due to the Dicke effect have also been investigated. A narrow emission line-width (~0.0001cm−1) difference-frequency-generation (DFG) laser system is used as the tunable light source. To retrieve the CH4 spectroscopic parameters, Voigt and Galatry profiles were used to simulate the measured line shape of the individual transitions.

Empirical determination of low J values of 13CH4 transitions from jet cooled and 80 K cell spectra in the icosad region (7170–7367 cm−1)

The absorption spectrum of 13CH4 was recorded at two low temperatures in the icosad region near 1.38µm, using direct absorption tunable diode lasers. Spectra were obtained using a cryogenic cell cooled at liquid nitrogen temperature (80K) and a supersonic jet providing a 32K rotational temperature in the 7173–7367cm−1 and 7200–7354cm−1 spectral intervals, respectively. Two lists of 4498 and 339 lines, including absolute line intensities, were constructed from the 80K and jet spectra, respectively.All the transitions observed in jet conditions were observed at 80K. From the temperature variation of their line intensities, the corresponding lower state energy values were determined. The 339 derived empirical values of the J rotational quantum number are found close to integer values and are all smaller than 4, as a consequence of the efficient rotational cooling. Six R(0) transitions have been identified providing key information on the origins of the vibrational bands which contribute to the very congested and not yet assigned 13CH4 spectrum in the considered region of the icosad.

Determination of the Electron Temperature of Atmospheric Pressure Argon Plasmas by Absolute Line Intensities and a Collisional Radiative Model

The electron temperature in atmospheric argon plasmas created by a DBD jet is determined using a combination of Absolute Line Intensity (ALI) measurements and a collisional radiative model (CRM). The ALI measurements have been performed to determine the densities of the states in the 4p level. In addition, the ground state density is taken into account, which is found via the pressure and the gas temperature. The density ratio of the ground state and 4p state gives a value of the excitation temperature, which by means of the CRM is transformed into the electron temperature. With this method, the electron temperature in the active zone of a pure argon plasma is found to be 1.27 eV which is significantly higher than the experimental value reported by others. An error analysis shows that the relative error in the electron temperature obtained in this way is about 5%. In the afterglow, the temperature decreases gradually to about 0.88 eV. The addition of 2% O2 leads to a decrease in electron temperature of about 5%.

Line positions and intensities for the ν12 band of 13C12CH6

High-resolution, high signal-to-noise spectra of a high-purity (99%) mono-substituted 13C-enriched ethane (13C12CH6) in the ν12 fundamental band near 12.2μm region were recorded with a Bruker IFS 125HR Fourier transform spectrometer. The data were obtained for four sample pressures at three different temperatures (130–208K) using a 20.38-cm long coolable absorption cell. The spectra were fitted simultaneously to retrieve individual line positions and absolute line intensities for 1660 absorption features. A multispectrum nonlinear least squares spectrum fitting technique was employed in the analysis. Constraints were used to fit each pair of doublet components arising from torsional Coriolis interaction of the excited ν12=1 state with the nearby excited torsional ν6=3 state. Line positions and absolute line intensities were retrieved by simultaneously fitting the four experimental spectra recorded at three low sample temperatures (130K, 178K and 208K). The measured positions and intensities were determined for the standard reference temperature of 296K and compared with the predicted values listed in the HITRAN database. Calculated line intensities at each of the measured spectrum temperatures are also provided as supplemental data. Integrated intensities are also reported.

High-sensitivity CRDS absorption spectroscopy of acetylene between 5851 and 6341 cm−1

The absorption spectrum of acetylene has been recorded at room temperature (297 K) using high-sensitivity cavity ring-down spectroscopy (αmin ∼ 5×10−11 cm−1) in the 5851 and 6341 cm−1 interval corresponding to a region of very weak absorption. A list of about 10,700 absorption features with estimated absolute line intensities was constructed. The smallest intensities are of the order of 5×10−29 cm molecule−1. The line list includes about 2500 absorption lines of ethylene present at the ppm level in the acetylene sample and identified on the basis of a high-resolution Fourier transform spectrum specifically recorded. A total of more than 2700 lines of 12C2H2 were rovibrationally assigned in comparison with accurate predictions provided by a global effective operator model. Overall, the present effort adds about 2260 new assignments to the set of about 500 assigned transitions available in the literature. The new assignments correspond to 45 new bands and 17 already-known bands, for which additional J lines were assigned. Spectroscopic parameters were derived for the upper vibrational levels from a band by band fit of the line positions (typical root mean square deviation values are of the order of 0.001 cm−1). A few of the analysed bands were found to be affected by rovibrational perturbations, which are discussed. The new data will be valuable to refine the parameters of the global effective Hamiltonian and dipole moments of 12C2H2.