Variational Line List Research Articles

The CH3D Absorption Spectrum Near 1.58 μm: Extended Line Lists and Rovibrational Assignments.

Monodeuterated methane (CH3D) contributes greatly to absorption in the 1.58 μm methane transparency window. The spectrum is dominated by the 3ν2 band near 6430 cm-1, which is observed in natural methane and used for a number of planetary applications, such as the determination of the D/H ratio. In this work, we analyze the CH3D spectrum recorded by high-sensitivity differential absorption spectroscopy in the 6099-6530 cm-1 region, both at room temperature and at 81 K. Following a first contribution to this topic by Lu et al., the room-temperature line list is elaborated (11,189 lines) and combined with the previous 81 K list (8962 lines) in order to derive about 4800 empirical lower-state energy values from the ratio of the line intensities measured at 81 K and 294 K (2T-method). Relying on the position and intensity agreements with the TheoReTS variational line list, about 2890 transitions are rovibrationally assigned to twenty bands, with fifteen of them being newly reported. Variational positions deviate from measurements by up to 2 cm-1, and the band intensities are found to be in good agreement with measurements. All the reported assignments are confirmed by Ground-State Combination Difference (GSCD) relations; i.e., all the upper-state energies (about 1370 in total) have coinciding determinations through several transitions (up to 8). The energy values, determined with a typical uncertainty of 10-3 cm-1, are compared to their empirical and variational counterparts. The intensity sum of the transitions assigned between 6190 and 6530 cm-1 represents 76.9 and 90.0% of the total experimental intensities at 294 K and 81 K, respectively.

Jet-cooled ethylene cavity ring-down spectroscopy between 5880 and 6200 cm−1

Absorption spectra of jet-cooled ethylene (ethene, C2H4) are recorded at three different rotational temperatures (6/8 K, 12 K, 38 K) using cavity ring-down spectroscopy (CRDS) in the 5880–6200 cm−1 spectral region. Rotational cooling is used to determine the various vibrational band centers by simplifying drastically the rotational band structure. A line-by-line assignment, based on a direct comparison with the TheoReTS variational line list and a systematic use of lower state combination difference (LSCD), is performed. Experimental line lists including line position and line integrated absorption cross sections are drawn up. The 6/8 K, 12 K and 38 K spectra contain 668, 1553 and 1679 absorption lines respectively. Overall, 320 rovibrational lines are assigned across 20 interacting vibrational bands. Among the 20 vibrational cold bands identified in this work, 14 had never been observed before. Line intensities are in the range of 10−24 - 10−20 cm/molecule. A direct comparison between our work and the TheoReTS and ExoMol theoretical line lists, as well as with the recent experimental work of Ben Fathallah et al. (2024) is provided.

High resolution spectroscopy and a theoretical line list of ethylene between 5000 and 9000 cm−1

The absorption spectrum of ethylene (C2H4) in the range 5000–9000 cm−1 has been recorded by high-resolution Fourier transform spectroscopy (FTS) at four temperatures (130, 201, 240 and 297 K) and two pressures for each temperature value. The recorded spectra show a very high spectral congestion even at the lowest temperature. Here we present the analysis of a subset of the data limited to a weak absorption interval (6700–7260 cm−1) at 130 and 297 K. An empirical list of 12,243 lines was retrieved from the 130 K spectrum. Line intensities range between about 10−25 and 10−22 cm/molecule. In the 7120–7260 cm−1 region, a list of 2276 lines was combined to the 130 K list to derive the empirical value of the lower state energy of 1249 transitions from the variation of the line intensity between 130 and 297 K (2T-method). The line list of 12C2H4 transitions calculated by the variational method is provided in the 5200–9000 cm−1 range (241,921 and 179,927 transitions at 296 K and 130 K, respectively, above an intensity cutoff of 1x10−25 cm/molecule). This list is an extension of the Theoretical Reims-Tomsk Spectral (TheoReTS) list available at lower energy. In spite of the considerable spectral congestion and of significant deviations between the variational and experimental line lists, 647 transitions could be rovibrationally assigned to seven vibrational bands in the FTS spectrum at 130 K between 6700 and 7260 cm−1. The assignments rely on the position and intensity agreement combined with a systematic use of Lower State Combination Difference (LSCD) relations. The comparison between the empirical values of the lower state energy and their exact value provided by the rovibrational assignment is discussed.

Full spectroscopic model and trihybrid experimental-perturbative-variational line list for ZrO

ABSTRACT Zirconium monoxide (ZrO) absorption lines define rare S-type stars and are currently being sought on exoplanets. Successful detection is dependent on an accurate and comprehensive line list, with existing data not ideal for many applications. Specifically, the Plez et al. line list is near-complete but has insufficient accuracy for high-resolution cross-correlation, while the Sorensen & Bernath data have high accuracy but only consider a small number of spectral bands. This article presents a novel spectroscopic model, variational line list, and trihybrid line list for the main 90Zr16O isotopologue, as well as isotopologue-extrapolated hybrid line lists for the 91Zr16O, 92Zr16O, 93Zr16O, 94Zr16O, and 96Zr16O isotopologues. These were constructed using Duo based on icMRCI-SD/CASSCF ab initio electronic data calculated using molpro, experimental energies obtained from a previous MARVEL data compilation, and perturbative energies from Sorensen & Bernath. The new 90Zr16O ExoMol-style trihybrid line list, ZorrO, comprises 227 118 energies (13 075 experimental) and 47 662 773 transitions up to 30 000 cm −1 (333 nm) between 10 low-lying electronic states (X 1Σ+, a 3Δ, A 1Δ, b 3Π, B 1Π, C 1Σ+, d 3Φ, e 3Π, f 3Δ, and F 1Δ). The inclusion of experimental energy levels in ZorrO means ZrO will be much easier to detect using high-resolution ground-based telescopes in the 12 500–17 500 cm−1 (571–800 nm) spectral region. The inclusion of variational energy levels means that the ZorrO line list has very high completeness and can accurately model molecular absorption cross-sections even at high temperatures. The ZorrO data will hopefully facilitate the first detection of ZrO in the atmosphere of a hot Jupiter exoplanet, or alternatively more conclusively exclude its presence.

Absorption in exoplanet atmospheres: Combining experimental and theoretical databases to facilitate calculations of the molecular opacities of water

We developed a method to reduce large theoretical molecular rovibrational line lists to significantly shorter lists that capture both the morphology of the spectrum and the total opacity. Water is used as a test case: the routines are demonstrated using the state-of-the-art POKAZATEL (H2O, 5.7 billion transitions) and VTT (HDO, 700 million transitions) databases. We validate the methods, and, using only 2.3 million transitions, we reproduce spectra from the full databases over the entire wavelength domain, at all resolutions, and in a fraction of the computational time that would be required were the full line lists used. As the variational line lists are applicable to much higher temperatures than commonly used experimentally-based databases, realistic simulations of high-temperature exoplanet atmospheres are possible. The approach is specifically developed to be applied in radiative transfer studies where larger line lists would result in prohibitively large calculation times. In addition to the light line lists, we collected collisional-broadening parameters of water in H2, He, CO2, and self-broadened, from several literature sources, and produced rotational quantum number dependent broadening tables, which can be used in combination with the line list to produce pressure broadened spectra for different atmospheres. The light line lists, in combination with the broadening parameters, allows the calculation of full Voigt line shapes in line-by-line, layer-by-layer calculations of radiative transfer models on personal computers. We demonstrate the new line lists in: 1) James Webb Space Telescope and Large UV/Optical/IR Surveyor simulations to detect HDO in an oxygen dominated atmosphere on Trappist-1 b and 2) simulated high-resolution emission spectra from HD 189733 b, constraining the water detectability using the cross-correlation method.

Synthesis of ab initio and effective Hamiltonian line lists for ozone

A synthetic line list for the ozone molecule is presented. Variational calculation using the semi-empirical potential energy surface (PES) and ab initio dipole moment surface (DMS) produce very accurate values of line intensities, but give line positions far away from their experimental values. Furthermore assignment of approximate rotational and vibrational quantum numbers are missing from variationally calculated line list. Effective Hamiltonian calculations are complimentary to ab initio line lists in these properties giving excellent value of line positions, close to experimental ones, and a full set of quantum numbers assignment. The synthesis of both these qualities in one line list is highly desirable. Here a synthetic line list for 16O3 is presented. The method of corrections of distorted intensities in variational linelist with ab initio DMS due to the artificial intensity stealing is developed and applied. Comparison of the synthetic line list with all major published line lists and available experimental data is given. The calculated line intensities agree to within experimental error for most bands for which accurate measurements are available.

The absorption spectrum of ammonia between 5650 and 6350 cm−1

A room temperature Fourier Transform absorption spectrum of ammonia measured in 1991 and available from the Kitt Peak data center is analyzed in the 5650 - 6350 cm−1 region. This spectral interval is part of the important 1.6 µm atmospheric transparency window where ammonia could be detected as trace species. The centers and intensities of 2779 ammonia lines were retrieved using a multiline fitting procedure. The spectrum was assigned by the joint use of a recent variational line list and of lower state combination difference relations. Overall, 1762 lines were assigned to 29 bands providing 914 new experimental energies. The obtained assignments and upper state experimental energies are the first for ammonia in the studied region. The comparison of our line list to the variational line list, to the HITRAN2016 list and to a very recent experimental line list is presented and discussed.

CRDS absorption spectrum of 17O enriched water vapor in the 12,277–12,894 cm−1 range

The absorption spectrum of water vapor enriched in oxygen-17 has been recorded by a continuous-wave cavity ring-down spectrometer (CW-CRDS) at room temperature in the 12,277–12,894 cm−1 region. The typical sensitivity of 2 × 10−10 cm−1 allowed the detection of water vapor transitions with intensities larger than 1 × 10−28 cm−1/(molecule cm−2). In total, more than 3400 lines were observed in the recorded spectra with a typical frequency accuracy of 0.002 cm−1. In which more than 3400 transitions were ro-vibrationally assigned to five water isotopic species (H217O, H216O, H218O, HD16O, and HD17O) using the known empirical energy levels combined with the calculated variational line lists based on the results of Partridge and Schwenke. About 1300 transitions of H217O were assigned, leading to the determination of new energy levels of 17 vibration states. Those assigned H217O transitions are involved with the upper vibration states of (013), (023), (032), (051), (061), (070), (080), (090), (112), (131), (150), (160), (211), (230), (301), (310), and (320). The maximum values of the rotational numbers J and Ka were 16 and 9, respectively. The 5ν1 band of HD17O was assigned for the first time as well. The line positions and energy levels determined in this work were compared with literature, and it shows a significant improvement of the line positions of H217O and HD17O in the 12,277–12,894 cm−1 region.

Marvel analysis of the measured high-resolution rovibrational spectra of H232S

44 325 measured and assigned transitions of H232S, the parent isotopologue of the hydrogen sulfide molecule, are collated from 33 publications into a single database and reviewed critically. Based on this information, rotation-vibration energy levels are determined for the ground electronic state using the Measured Active Rotational-Vibrational Energy Levels (Marvel) technique. The ortho and para principal components of the measured spectroscopic network of H232S are considered separately. The verified set of 25 293 ortho- and 18 778 para- H232S transitions determine 3969 ortho and 3467 para energy levels. The Marvel results are compared with alternative data compilations, including a theoretical variational linelist.

Analysis of the red and green optical absorption spectrum of gas phase ammonia

Room temperature NH3 absorption spectra recorded at the Kitt Peak National Solar Observatory in 1980 are analyzed. The spectra cover two regions in the visible: 15,200 – 15,700 cm−1 and 17,950 – 18,250 cm−1. These high overtone rotation-vibration spectra are analyzed using both combination differences and variational line lists. Two variational line lists were computed using the TROVE nuclear motion program: one is based on an ab initio potential energy surface (PES) while the other used a semi-empirical PES. Ab initio dipole moment surfaces are used in both cases. 95 energy levels with J=1−7 are determined from analysis of the experimental spectrum in the 5νNH (red) region and 46 for 6νNH (green) region. These levels span four vibrational bands in each of the two regions, associated with stretching overtones.

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.

Study of HDO absorption in the 11,200–12,400 cm−1 range using LED-based Fourier transform spectroscopy

A high resolution Fourier transform spectrum of mono-deuterated water vapor was studied between 11,200 and 12,400 cm−1 using a Fourier transform spectrometer with the spectral resolution of 0.05 cm−1, coupled to a multi-pass White-type cell that provided an optical path length of 24 m. A light-emitting diode was used as the radiation source to obtain a measurement signal-to-noise ratio of about 104. The rotational-vibrational assignment of more than 1560 lines was carried out using the VTT variational line list. The parameters of the spectral lines (line centers, intensities and half-widths) were determined by least-squares fitting of the Voigt contour parameters to the experimental data. A total of 584 rotational–vibrational energy levels with maximum rotational quantum numbers of J = 16 and Ka = 9 were determined. For the first time, 64 new energy levels belonging to the nine vibrational states: (032), (112), (301), (141), (410), (221), (330), (061) and (170) were derived from the experiment.

Absorption spectra of ammonia near 1 μm

An ammonia absorption spectrum recorded at room temperature in the region 8800–10,400cm−1 is analysed using a variational line list, BYTe, and ground state energies determined using the MARVEL procedure. BYTe is used as a starting point to initialise assignments by combination differences and the method of branches. Assignments are presented for the region 9400–9850cm−1. 642 lines are assigned to 6 previously unobserved vibrational bands, (2v1+2v42)±, (2v1+v31)± and (v1+v31+2v42)±, leading to 428 new energy levels with 208 confirmed by combination differences. A recently calculated purely ab initio NH3 PES is also used to calculate rovibrational energy levels. Comparison with assigned levels shows better agreement between observed and calculated levels than for BYTe for higher vibrational bands.

High-resolution absorption measurements of NH3 at high temperatures: 2100–5500 cm−1

High-resolution absorption spectra of NH3 in the region 2100–5500cm−1 at 1027°C and approximately atmospheric pressure (1045±3mbar) are measured. An NH3 concentration of 10% in volume fraction is used in the measurements. Spectra are recorded in a high-temperature gas-flow cell using a Fourier Transform Infrared (FTIR) spectrometer at a nominal resolution of 0.09cm−1. The spectra are analysed by comparison to a variational line list, BYTe, and experimental energy levels determined using the MARVEL procedure. 2308 lines have been assigned to 45 different bands, of which 1755 and 15 have been assigned or observed for the first time in this work.

ExoMol molecular line lists – XVII. The rotation–vibration spectrum of hot SO3

Sulphur trioxide (SO$_3$) is a trace species in the atmospheres of the Earth and Venus, as well as well as being an industrial product and an environmental pollutant. A variational line list for $^{32}$S$^{16}$O$_{3}$, named UYT2, is presented containing 21 billion vibration-rotation transitions. UYT2 can be used to model infrared spectra of SO$_3$ at wavelengths longwards of 2 $\mu$m ($\nu < 5000$ cm$^{-1}$) for temperatures up to 800 K. Infrared absorption cross sections are also recorded at 300 and 500 C are used to validate the UYT2 line list. The intensities in UYT2 are scaled to match the measured cross sections. The line list is made available in electronic form as supplementary data to this article and at \url{www.exomol.com}.

A near infrared line list for NH3: Analysis of a Kitt Peak spectrum after 35 years

A Fourier Transform (FT) absorption spectrum of room temperature NH3 in the region 7400–8640cm−1 is analysed using a variational line list and ground state energies determined using the MARVEL procedure. The spectrum was measured by Dr. Catherine de Bergh in 1980 and is available from the Kitt Peak data center. The centers and intensities of 8468 ammonia lines were retrieved using a multiline fitting procedure. 2474 lines are assigned to 21 bands providing 1692 experimental energies in the range 7500–9200cm−1. The spectrum was assigned by the joint use of the BYTe variational line list and combination differences. The assignments and experimental energies presented in this work are the first for ammonia in the region 7400–8640cm−1, considerably extending the range of known vibrational-excited states.

High-resolution absorption measurements of NH3 at high temperatures: 500–2100 cm−1

High-resolution absorption spectra of NH3 in the region 500–2100cm−1 at temperatures up to 1027°C and approximately atmospheric pressure (1013±20 mbar) are measured. NH3 concentrations of 1000ppm, 0.5% and 1% in volume fraction were used in the measurements. Spectra are recorded in high temperature gas flow cells using a Fourier Transform Infrared (FTIR) spectrometer at a nominal resolution of 0.09cm−1. Measurements at 22.7°C are compared to high-resolution cross sections available from the Pacific Northwest National Laboratory (PNNL). The higher temperature spectra are analysed by comparison to a variational line list, BYTe, and experimental energy levels determined using the MARVEL procedure. Approximately 2000 lines have been assigned, of which 851 are newly assigned to mainly hot bands involving vibrational states as high as v2=5.

Analysis of an 18O and D enhanced water spectrum and new assignments for HD18O and D218O in the near-infrared region (6000–7000 cm−1) using newly calculated variational line lists

An experimental infrared spectrum due to Orphal and Ruth (2008) [10] recorded using isotopically enriched water in the 6000–7000cm−1 region is analysed and assigned. The assignment procedure is based on the use of known transition frequencies for H216O and H218O, existing variational line lists for HD16O and D216O, and newly calculated variational line lists for HD18O and D218O. These new variational line lists are presented herein. The main absorption comes from HD16O and HD18O, for which there are few previous assignments in the region. Assignments to 426 new HD18O lines are presented. In all 3254 of the 4768 lines observed in the spectrum are assigned, resulting in a number of newly determined energy levels. These assignments are in agreement with the recent work of Mikhailenko et al. (2012) [41].

Analysis of high temperature ammonia spectra from 780 to 2100 cm −1

A recently-recorded set [Hargreaves et al., Astrophys. J., in press] of Fourier transform emission spectra of hot ammonia is analyzed using a variational line list. Approximately 3350 lines are newly assigned to mainly hot bands from vibrational states as high as v 2 = 2. 431 new energy levels of these states are experimentally determined, considerably extending the range of known rotationally-excited states. Comparisons with a recent study of high J levels in the ground and first vibrational states [Yu et al., J. Chem. Phys., 133 (2010) 174317] suggests that while the line assignments presented in that work are correct, their energy level predictions suffer from problems associated with the use of very high-order perturbation series in the effective Hamiltonian. It is suggested that variational calculations provide a more stable method for analyzing spectra involving highly-excited states of ammonia.

Spectrum of hot water in the 2000–4750 cm −1 frequency range

An emission spectrum recorded in an oxyacetylene torch [P.-F. Coheur, P.F. Bernath, M. Carleer, R. Colin, O.L. Polyansky, N.F. Zobov, S.V. Shirin, R.J. Barber, J. Tennyson, J. Chem. Phys. 122 (2005) 074307] is analyzed for the region covering stretching fundamentals and associated hot bands of water. Many lines could be assigned on the basis of previously determined energy levels. New assignments made with a new variational linelist allow a further 800 energy levels covering 15 vibrational states and rotations up to J = 32 to be assigned. A simultaneous re-analysis of previously reported sunspot absorption spectra leads to the assignment of 581 further lines in the L-band spectrum and 67 in the N-band spectrum.