Accurate Line Intensities Research Articles

Cavity ring-down spectroscopy of CO2 near λ = 2.06 µm: Accurate transition intensities for the Orbiting Carbon Observatory-2 (OCO-2) “strong band”

The λ = 2.06 µm absorption band of CO2 is widely used for the remote sensing of atmospheric carbon dioxide, making it relevant to many important top-down measurements of carbon flux. The forward models used in the retrieval algorithms employed in these measurements require increasingly accurate line intensity and line shape data from which absorption cross-sections can be computed. To overcome accuracy limitations of existing line lists, we used frequency-stabilized cavity ring-down spectroscopy to measure 39 transitions in the 12C16O2 absorption band. The line intensities were measured with an estimated relative combined standard uncertainty of ur = 0.08 %. We predicted the J-dependence of the measured intensities using two theoretical models: a one-dimensional spectroscopic model with Herman-Wallis rotation-vibration corrections, and a line-by-line ab initio dipole moment surface model [Zak et al. JQSRT 2016;177:31-42]. For the second approach, we fit only a single factor to rescale the theoretical integrated band intensity to be consistent with the measured intensities. We find that the latter approach yields an equally adequate representation of the fitted J-dependent intensity data and provides the most physically general representation of the results. Our recommended value for the integrated band intensity equal to 7.183 × 10−21 cm molecule−1 ± 6 × 10−24 cm molecule−1 is based on the rescaled ab initio model and corresponds to a fitted scale factor of 1.0069 ± 0.0002. Comparisons of literature intensity values to our results reveal systematic deviations ranging from −1.16 % to +0.33 %.

X-ray diffraction at the National Ignition Facility.

We report details of an experimental platform implemented at the National Ignition Facility to obtain in situ powder diffraction data from solids dynamically compressed to extreme pressures. Thin samples are sandwiched between tamper layers and ramp compressed using a gradual increase in the drive-laser irradiance. Pressure history in the sample is determined using high-precision velocimetry measurements. Up to two independently timed pulses of x rays are produced at or near the time of peak pressure by laser illumination of thin metal foils. The quasi-monochromatic x-ray pulses have a mean wavelength selectable between 0.6 Å and 1.9 Å depending on the foil material. The diffracted signal is recorded on image plates with a typical 2θ x-ray scattering angle uncertainty of about 0.2° and resolution of about 1°. Analytic expressions are reported for systematic corrections to 2θ due to finite pinhole size and sample offset. A new variant of a nonlinear background subtraction algorithm is described, which has been used to observe diffraction lines at signal-to-background ratios as low as a few percent. Variations in system response over the detector area are compensated in order to obtain accurate line intensities; this system response calculation includes a new analytic approximation for image-plate sensitivity as a function of photon energy and incident angle. This experimental platform has been used up to 2 TPa (20 Mbar) to determine the crystal structure, measure the density, and evaluate the strain-induced texturing of a variety of compressed samples spanning periods 2-7 on the periodic table.

ExoMol molecular line lists – XXXIII. The spectrum of Titanium Oxide

ABSTRACT Accurate line lists are crucial for correctly modelling a variety of astrophysical phenomena, including stellar photospheres and the atmospheres of extrasolar planets. This paper presents a new line database Toto for the main isotopologues of titanium oxide (TiO): $^{46}\text{Ti}^{16}\text{O}$, $^{47}\text{Ti}^{16}\text{O}$, $^{48}\text{Ti}^{16}\text{O}$, $^{49}\text{Ti}^{16}\text{O}$, and $^{50}\text{Ti}^{16}\text{O}$. The $^{48}\text{Ti}^{16}\text{O}$ line list contains transitions with wave-numbers up to 30 000 cm−1, i.e. longwards of 0.33 μm. The Toto line list includes all dipole-allowed transitions between 13 low-lying electronic states (X 3Δ, a1Δ, d 1Σ+, E 3Π, A 3Φ, B3Π, C 3Δ, b 1Π, c1Φ, f1Δ, e 1Σ+). Ab initio potential energy curves (PECs) are computed at the icMRCI level and combined with spin–orbit and other coupling curves. These PECs and couplings are iteratively refined to match known empirical energy levels. Accurate line intensities are generated using ab initio dipole moment curves. The Toto line lists are appropriate for temperatures below 5000 K and contain 30 million transitions for $^{48}\text{Ti}^{16}\text{O}$; it is made available in electronic form via the CDS data centre and via www.exomol.com. Tests of the line lists show greatly improved agreement with observed spectra for objects such as M-dwarfs GJ876 and GL581.

Accurate line intensities of methane from first-principles calculations

In this work, we report first-principle theoretical predictions of methane spectral line intensities that are competitive with (and complementary to) the best laboratory measurements. A detailed comparison with the most accurate data shows that discrepancies in integrated polyad intensities are in the range of 0.4%–2.3%. This corresponds to estimations of the best available accuracy in laboratory Fourier Transform spectra measurements for this quantity. For relatively isolated strong lines the individual intensity deviations are in the same range. A comparison with the most precise laser measurements of the multiplet intensities in the 2ν3 band gives an agreement within the experimental error margins (about 1%). This is achieved for the first time for five-atomic molecules. In the Supplementary Material we provide the lists of theoretical intensities at 269K for over 5000 strongest transitions in the range below 6166cm−1. The advantage of the described method is that this offers a possibility to generate fully assigned exhaustive line lists at various temperature conditions. Extensive calculations up to 12,000cm−1 including high-T predictions will be made freely available through the TheoReTS information system (http://theorets.univ-reims.fr, http://theorets.tsu.ru) that contains ab initio born line lists and provides a user-friendly graphical interface for a fast simulation of the absorption cross-sections and radiance.

Accurate line intensities for water transitions in the infrared: Comparison of theory and experiment

Ab initio calculations of water intensities are becoming mature and are claimed to have 1% accuracy in many cases. Experimental intensities with 1% accuracy can be achieved with some care. An intercomparison of ab initio against experimental water intensities is presented for a variety of infrared bands for H216O and some for H218O and H217O. A new calculated H216O line list is presented for which uncertainties in the ab initio line intensities are evaluated. Much of the data show agreement within 2% between ab initio and experiment, however, for some bands, notably those involving excitation of some stretching modes, there are larger offsets of up to 8% attributed to ab initio calculation errors but still within the uncertainty of the ab initio calculation. In the ν1 fundamental band differences of between +5% and −13% are found which show systematic dependence on wavenumber, ΔKa, and ΔJ, again attributable to ab initio calculation errors. In the ν2 band, intensity-dependent differences up to 2% originate from the analysis of the experimental data. At present experiments are important to validate ab initio calculations but ab initiopredictions can be very useful in validating the experiment. As the two procedures display significantly different systematic errors, it is suggested that combining both gives the best results; this study will also facilitate further improvements of the theoretical methodology.

ExoMol line lists - I. The rovibrational spectrum of BeH, MgH and CaH in theX 2Σ+state

Accurate line lists for three molecules, BeH, MgH and CaH, in their ground electronic states are presented. These line lists are suitable for temperatures relevant to exoplanetary atmospheres and cool stars (up to 2000 K). A combination of empirical and ab initio methods is used. The rovibrational energy levels of BeH, MgH and CaH are computed using the programs level and dpotfit in conjunction with ‘spectroscopic’ potential energy curves (PECs). The PEC of BeH is taken from the literature, while the PECs of CaH and MgH are generated by fitting to the experimental transition energy levels. Both spin-rotation interactions (except for BeH, for which it is negligible) and non-adiabatic corrections are explicitly taken into account. Accurate line intensities are generated using newly computed ab initio dipole moment curves for each molecule using high levels of theory. Full line lists of rotation–vibration transitions for 9BeH, 24MgH, 25MgH, 26MgH and 40CaH are made available in an electronic form as supporting information to this paper and at www.exomol.com.

NUMERICAL SIMULATIONS OF RADIATIVE MAGNETIZED HERBIG–HARO JETS: THE INFLUENCE OF PRE-IONIZATION FROM X-RAYS ON EMISSION LINES

We investigate supersonic, axisymmetric magnetohydrodynamic (MHD) jets with a time-dependent injection velocity by numerical simulations with the PLUTO code. Using a comprehensive set of parameters, we explore different jet configurations in the attempt to construct models that can be directly compared to observational data of microjets. In particular, we focus our attention on the emitting properties of traveling knots and construct, at the same time, accurate line intensity ratios and surface brightness maps. Direct comparison of the resulting brightness and line intensity ratios distributions with observational data of microjets shows that a closer match can be obtained only when the jet material is pre-ionized to some degree. A very likely source for a pre-ionized medium is photoionization by X-ray flux coming from the central object.

Line positions and strengths of 41 bands including 10 OCS isotopologues in the 3850–4200 cm −1 region

The present analysis substantially improves the spectroscopic characterization of near infrared OCS in a window region (3850–4200 cm −1) important for atmospheric studies of Venus. Previous studies in this spectral region cataloged numerous OCS line positions, but accurate line intensities were measured for only three strong bands. In this paper, the corresponding line intensities are obtained for 41 OCS bands, including weak isotopic bands reported for the first time. The 2ν 3 (0002–0000) band is analyzed for 10 OCS isotopologues (adding 16O 13C 34S, 17O 12C 32S, 16O 12C 36S, 18O 12C 34S, and 16O 13C 33S). In addition, observations of 0332–0330 of the main isotope, 16O 12C 32S, provides accurate vibration–rotation parameters for the upper state (and the lower state, 0330 of 16O 12C 32S). Finally, one unidentified band is seen at 3969.3 cm −1; its lower state is clearly the ground state of 16O 12C 32S. The line strengths of these seven previously unanalyzed bands plus 34 other bands of the OCS isotopologues, 16O 12C 32S, 16O 12C 34S, 16O 13C 32S, 16O 12C 33S, and 18O 12C 32S, were least-squares fitted to determine strength parameters, S v and Herman-Wallis coefficients. Finally, the intensities of 17 additional very weak bands were estimated to provide an extensive new database of OCS line parameters to support remote sensing of Venus. The integrated intensity in cm −1/(molecule cm −2) at 296 K is 8.1×10 −19 for the 3800–4200 cm −1 region.

Global modeling of 13C 16O 2 absolute line intensities from CW-CRDS and FTS measurements in the 1.6 and 2.0 μm regions

Line intensities of 13C 16O 2 have been measured between 5851 and 6580 cm −1 using CW-cavity ring down spectroscopy (CRDS) and in the 4700–5050 and 6050–6850 cm −1 regions using Fourier transform spectroscopy. As a result of the high sensitivity (noise equivalent absorption α min∼3×10 −10 cm −1) and high dynamics allowed by CW-CRDS, accurate line intensities of 2039 transitions ranging between 1.1×10 −28 and 1.3×10 −23 cm −1/(molecule cm −2) were measured with an average accuracy of 4%. These transitions belong to a total of 48 bands corresponding to the Δ P=9 series of transitions. Additionally, unapodized absorption spectra of 13C-enriched samples have been recorded using a high-resolution Bruker IFS125HR Fourier transform spectrometer. Spectral resolutions of 0.004 cm −1 (maximum optical path difference (MOPD)=225 cm) and 0.007 cm −1 (MOPD=128.6 cm), and pressure×path length products in the ranges 5.2–12 and 69–450 hPa×m have been used for the lower and higher energy spectral regions, respectively. Absolute line intensities have been measured in the 2001 i−00001, 3001 i−00001 ( i=1, 2, 3) and 00031−00001 bands. An excellent agreement was achieved for the line intensities of the 3001 i−00001 ( i=1, 2, 3) bands measured by both FTS and CW-CRDS. The CW-CRDS and FTS experimental intensity data together with selected intensity information from the literature have been fitted simultaneously using the effective operators approach. Two sets of effective dipole moment parameters have thus been obtained, which reproduce the observed line intensities in the 2.0 and 1.6 μm regions within experimental uncertainties.

A continuous-wave difference-frequency generation laser operating in the mid-infrared (3–5 μm) region for accurate line intensity measurements

In this paper we present a compact tuneable continuous-wave infrared laser, based on difference-frequency generation (DFG) using quasi phase matching in periodically poled LiNbO3 (PPLN), for accurate line intensity measurements. Several 0.1μW of infrared radiation (tuneable in the 3–5μm range) are obtained using a diode-pumped Nd:YAG laser (output power about 800mW at 1064nm, linewidth 1kHz) together with a tuneable external-cavity diode laser (output power about 500μW–50mW in the 805–885nm region, linewidth 1MHz). Using this infrared DFG laser for absorption experiments of gas-phase molecules, both very high-resolution (1MHz) and a high signal-to-noise ratio (up to several 1000) can be achieved in measurement time of only a few minutes, as demonstrated using absorption spectra of N2O in different wavelength regions. The present paper focuses in particular on the accuracy of the wavelength calibration and on the determination of absorption coefficients with experimental uncertainties of less than 1%.

Determination of nuclear Overhauser enhancement factors from NMR spin-lattice relaxation rates

Valuable information on molecular structure and motion can often be derived from the spin-lattice relaxation times in high resolution NMR spectra,t provided the relevant relaxation mechanisms can be elucidated. For example the relative contribution from dipole--dipole interaction may be deduced from the nuclear Overhauser enhancement2 (1/1) although there are practical difficulties in securing accurate line intensities. This note proposes an alternative method of estimating the dipolar contribution based on rates of spin-lattice relaxation. Consider a system of two spins, ! and S, with four energy levels. The deviations of the spin populations from their equilibrium values are governed by four coupled differential equations of the form,

Spectrographic Observations of the Flare of July 20, 1961

view Abstract Citations (9) References (19) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Spectrographic Observations of the Flare of July 20, 1961 Zirin, Harold ; Acton, Loren W. Abstract Spectroscopic observations of the late stages of a limb flare which occurred on July 20,1961, are presented. Because these observations were made with an achromatic coronagraph, accurate line intensity ratios are possible. The He I and He ii line ratios indicate a recombination-cascade spectrum. Measurements of cbntinuum densities give values around 1010 in the post-flare loop region and 1011 or more in the flare. Ca xv line widths correspond to 4.3 X 100 K. A magnetic loop structure which guides successive surges is studied. Publication: The Astrophysical Journal Pub Date: May 1967 DOI: 10.1086/149172 Bibcode: 1967ApJ...148..501Z full text sources ADS |

Spectrophotometry of 14 southern planetary nebulae

The theory of radiative and collisional processes in gaseous nebulae makes possible precise predictions of line and continuum intensities as functions of density, electron temperature, and degree of ionization of the emitting gases. It predicts, for example, the relative intensities of successive members of the Balmer series (Balmer decrement), and of corresponding transitions in the Balmer and Paschen series. Hence accurate line intensity measurements are not only of astrophysical interest; they also serve to assess the amount of space absorption.