Infrared absorption cross sections for methanol

Infrared absorption cross sections for methanol, CH₃OH, have been determined near 3.4 and 10 μm from spectra recorded using a high-resolution FTIR spectrometer (Bruker IFS 125HR) and a multipass cell with a maximum optical path length of 19.3 m. Methanol/dry synthetic air mixtures were prepared and spectra were recorded at 0.015 cm^?1 resolution (calculated as 0.9/MOPD) at a number of temperatures and pressures (50–760 Torr and 204–296 K) appropriate for atmospheric conditions. Intensities were calibrated using composite methanol spectra taken from the Pacific Northwest National Laboratory (PNNL) IR database. The new measurements in the 10 μm region indicate problems with the existing methanol spectroscopic line parameters in the HITRAN database, which will impact the accuracy of satellite retrievals.     

UV Fourier transform absorption cross sections of benzene,toluene, meta-, ortho-, and para-xylene

New measurements of the absorption cross sections of gaseous benzene, toluene, meta-, ortho-, and para-xylene have been performed with a Fourier transform spectrometer Bruker IFS120 M at the resolution of 1 cm^?1 over the 30 000–42 000 cm^?1 spectral range. The recordings were carried out under different pressure and temperature conditions with pure samples. The effect of the temperature on the absorption cross sections is investigated. Comparison with the literature shows large differences, largely attributed to the experimental difficulties encountered during these previous measurements and to a resolution effect. To our knowledge, it is the first time that such a dataset of UV absorption cross sections with temperature dependence is reported in the literature. Such data should be useful for upcoming remote sensing applications, such as atmospheric studies both on Earth and on other planets.     

Fourier transform measurements of SO2 absorption cross sections:: I. Temperature dependence in the 24 000–29 000 cm−1 (345–420 nm) region

Absorption cross sections of SO₂ have been obtained in the 24 000–29 000 cm^?1 spectral range (345–420 nm) with a Fourier transform spectrometer at a resolution of 2 cm^?1. Pure SO₂ samples were used and measurements were performed at room temperature (298 K) as well as at 318, 338 and 358 K. This is the first time that temperature effects in this spectral region are reported and investigated. This paper is the first of a series that will report on measurements of the absorption cross section of SO₂ in the UV/visible region at a higher than previously reported resolution and that will investigate temperature effects in support of tropospheric, stratospheric and astrophysical or planetary applications.     

Acetonitrile (CH3CN) infrared absorption cross sections in the 3 μm region

High resolution infrared absorption cross sections of acetonitrile have been determined from spectra recorded in the 3 μm spectral region using a Bruker IFS 125 HR Fourier transform spectrometer (FTS) and a multipass White cell. The eleven synthetic air-broadened acetonitrile spectra were recorded at a resolution of 0.015 cm⁻¹ (calculated as 0.9/MOPD (Maximum Optical Path Difference), the Bruker definition of resolution) over a range of different temperatures and pressures that are representative of conditions in the Earth's atmosphere (50-760 Torr and 207-296 K). Intensities were calibrated using infrared spectra recorded at the Pacific Northwest National Laboratory (PNNL). These new cross sections will enable satellite retrievals of acetonitrile in the 3 μm region from atmospheric spectra recorded by satellite instruments, such as the ACE (Atmospheric Chemistry Experiment)-FTS.     

Ground-based solar absorption studies for the Carbon Cycle science by Fourier Transform Spectroscopy (CC-FTS) mission

Carbon cycle science by Fourier transform spectroscopy (CC-FTS) is an advanced study for a future satellite mission. The goal of the mission is to obtain a better understanding of the carbon cycle in the Earth's atmosphere by monitoring total and partial columns of CO₂, CH₄, N₂O, and CO in the near infrared. CO₂, CH₄, and N₂O are important greenhouse gases, and CO is produced by incomplete combustion. The molecular O₂ column is also needed to obtain the effective optical path of the reflected sunlight and is used to normalize the column densities of the other gases. As part of this advanced study, ground-based Fourier transform spectra are used to evaluate the spectral region and resolution needed. Spectra in the 3950–7140 cm^?1 region with a spectral resolution of 0.0042 cm^?1 recorded at Kiruna (67.84°N, 20.41°E, and 419 m above sea level), Sweden, on 1 April 1998, were degraded to the resolutions of 0.01, 0.1, and 0.3 cm^?1. The effect of spectral resolution on the retrievals has been investigated with these four Kiruna spectra. To obtain further information on the spectral resolution, optical components and spectroscopic parameters required by the future mission, high-resolution solar absorption spectra between 2000 and 15000 cm^?1 were recorded using Fourier transform spectrometers at Kitt Peak (31.9°N, 111.6°W, and 2.1 km above sea level), Arizona, on 25 July 2005 and Waterloo (43.5°N, 80.6°W, and 0.3 km above sea level), Ontario, on 22 November 2006 with spectral resolutions of 0.01 and 0.1 cm^?1, respectively. Dry air volume mixing ratios (VMRs) of CO₂ and CH₄ were retrieved from these ground-based observations. The HITRAN 2004 spectroscopic parameters are used with the SFIT2 package for the spectral analysis. The measurement precisions for CO₂ and CH₄ total columns are better than 1.07% and 1.13%, respectively, for our observations. Based on these results, a Fourier transform spectrometer (maximum spectral resolution of 0.1 cm^?1 or 5 cm maximum optical path difference (MOPD)) operating between 2000 and 15000 cm^?1 is suggested as the primary instrument for the mission. Further progress in improving the atmospheric retrievals for CO₂, CH₄, and O₂ requires new laboratory measurements of the spectroscopic line parameters.     

Mid-infrared absorption cross sections for acetone (propanone)

Infrared absorption cross sections for acetone (propanone) have been determined in the 830-1950 cm⁻¹ spectral region from spectra recorded using a high-resolution FTIR spectrometer (Bruker IFS 125HR) and a multipass cell with a maximum optical path length of 19.3 m. The spectra of mixtures of acetone with dry synthetic air were recorded at 0.015 cm⁻¹ resolution (calculated as 0.9/MOPD using the Bruker definition of resolution) at a number of temperatures between 194 and 251 K and pressures appropriate for atmospheric conditions. Intensities were calibrated using three acetone spectra (recorded at 278, 293 and 323 K) taken from the Pacific Northwest National Laboratory (PNNL) IR database. The new absorption cross sections have been combined with previous high spectral resolution results to create a more complete set of acetone absorption cross sections appropriate for atmospheric remote sensing. These cross sections will provide an accurate basis for upper tropospheric/lower stratospheric retrievals of acetone in the mid-infrared spectral region from ACE and MIPAS satellite data.     

Water-vapor continuum absorption in the 800–1250 cm−1 spectral region at temperatures from 311 to 363 K

About 200 pure water-vapor spectra covering the region from 800 to 3500 cm^?1 were recorded with resolution of 0.1 cm^?1 at temperatures 311, 318, 325, 339, 352, and 363 K using a 2 m base White cell coupled to the BOMEM DA3.002 FTIR spectrometer. The water-vapor pressure varied from 28 to 151 mbar (21–113 Torr). Under these conditions, the continuum absorbance is quite measurable with the available path lengths up to 116 m. A program was developed for spectral processing that calculates, fits, and removes ro-vibrational structure from the spectrum. The spectra obtained were used to retrieve averaged and smoothed binary absorption coefficients over the region from 800 to 1250 cm^?1. Our continuum data extrapolated to room temperature are in reasonable agreement with the MT_CKD continuum model. But at higher temperatures the MT_CKD model provides very low values, which are up to 50% less than those experimentally measured.     

Validation of HNO3 spectroscopic parameters using atmospheric absorption and emission measurements

The improved database of HNO₃ spectroscopic parameters in the 600–950 cm^?1 spectral region presented in [Gomez L, Tran H, Perrin A, Gamache RR, Laraia A, Orphal J, et al. Some improvements of the HNO₃ spectroscopic parameters in the spectral region from 600 to 950 cm^?1. JQSRT 2008, in press] is tested by comparisons between calculations and atmospheric remotely sensed absorption and emission spectra. The line parameters in the 11.3 μm region are validated using ground-based Fourier transform solar absorption measurements, whereas those in the 13.1 μm region are successfully tested using balloon-borne atmospheric emission spectra. In both regions, the quality of the line parameters and the consistency between band intensities is confirmed through comparisons with emission spectra collected by the satellite-borne MIPAS instrument.     

The near infrared cavity-enhanced absorption spectrum of methyl cyanide

The absorption spectrum of methyl cyanide (CH₃CN) has been measured in the near IR between 6000 and 8000 cm^?1 with a resolution of 0.12 cm^?1 using Fourier transform incoherent broadband cavity-enhanced absorption spectroscopy. The spectrum contains several weakly perturbed spectral regions; potential vibrational combination bands contributing to the spectrum are outlined. Line positions and cross-sections of CH₃CN between 6814 and 7067 cm^?1 have been measured at high-resolution of 0.001 cm^?1 using diode laser based off-axis cavity-enhanced absorption spectroscopy. A total of 4630 new absorption lines of CH₃CN are identified in this region. A value for the self-broadening coefficient has determined to be (3.3±0.2)×10^?3 cm^?1 mbar^?1 for one isolated line at 7034.171 cm^?1. Several line series have been identified in these regions and an autocorrelation analysis performed with a view to aiding future assignments of the rotational-vibrational transitions.     

Photo-absorption studies on carbonyl sulphide in 30,000–91,000 cm−1 region using synchrotron radiation

Photo-absorption spectrum of carbonyl sulphide (OCS) is recorded in 30,000–91,000 cm^?1 (3300–1050 Å) region at an average resolution of 1.2 Å using Photo-physics beamline on the 450 MeV Indus-1 synchrotron radiation source at RRCAT Indore, India. Owing to significant absorption cross section dependence, spectra of OCS are recorded at various pressures (0.001–5 mbar) to optimize the S/N ratio for band systems appearing at different energy regions. The spectral region below 70,000 cm^?1 has contributions from dissociation mechanism of the ground state of OCS and three valence band systems arising from promotion of a 3π electron to 4π and 10σ orbital. Improved S/N ratio helped in unambiguous assignment of the valence band progressions at 42,000–48,000 cm^?1, 53,000–62,000 cm^?1 and 63,500–70,000 cm^?1 regions to the ¹Δ←X¹Σ⁺ transition, the relatively intense and sharp bands of ¹Π←X¹Σ⁺ transition and intense but broad bands of ¹Σ⁺←X¹Σ⁺ transition, respectively, and obtain the vibrational frequencies. Above 70,000 cm^?1 Rydberg series arising from s, p, d and f orbitals converging to the ionic ground state X²Π of OCS⁺ (90,121 cm^?1) are identified. Long progression in the first few members of the Rydberg series is suggestive of mixed valence character. Quantum defects are evaluated and used to discuss the nature of the molecular orbital. The present study provides a unifying picture of the VUV photo-absorption spectrum of OCS up to its first ionization limit.     

CO2 pressure broadening and shift coefficients for the 1–0 band of HCl and DCl

CO₂ broadened spectra of the 1–0 band of H³⁵Cl and H³⁷Cl, observed near 2886 cm^?1, and the 1–0 band of D³⁵Cl and D³⁷Cl, located near 2089 cm^?1, have been recorded at room temperature and five total pressures between 150 and 700 Torr, using a Bruker IFS125HR Fourier transform spectrometer. Spectra of pure HCl were also recorded. CO₂ broadening and shift coefficients of HCl and DCl have been measured using multi-spectrum non-linear least squares fitting of Voigt profiles. The analysis of the 1–0 band of DCl was complicated by the presence of overlapping CO₂ bands, which were included in the treatment as absorption coefficients calculated taking line-mixing effects into account.     

Measurements of N2- and O2-broadening and shifting parameters of methane spectral lines in the 5550–6236 cm−1 region

The absorption spectra of mixtures of methane (CH₄) with N₂ and O₂ at different partial pressures of both CH₄ and buffer gases for three temperatures 240, 267, and 296 K have been recorded using the Bruker IFS 125 HR FTIR spectrometer in the 5550–6236 cm^?1 region. The multispectrum fitting procedure has been applied to these spectra to recover the spectral line parameters. The main goal of this procedure was the determination of the N₂- and O₂-broadening and shifting coefficients and the exponents of their temperature dependences. These parameters have been derived for 452 assigned lines with good values of the signal to noise ratio. The rotational dependence of the mean values of these parameters is discussed. The temperature dependence exponents were observed for both N₂ and O₂ buffer gases.     

High-resolution spectrum of the ν9 band and reinvestigation of the ν8 band of cis-CH3ONO

The infrared spectrum of methyl nitrite CH₃ONO has been recorded at a spectral resolution of 0.003 cm^?1 using a Fourier-transform spectrometer Bruker IFS125HR. The ν₈ band of the cis isomer has been reinvestigated in the 780–880 cm^?1 spectral range to complete the study made by Goss et al. (2004) [3] and to fit the internal rotor splittings. The BELGI-IR program, which enables us to treat an isolated infrared band for asymmetric molecules containing one internal methyl rotor has been used for the analysis and predictions of spectra. Finally 1036 lines (913 A-type and 123 E-type lines for J≤50 and K_a≤28) have been assigned for the cis isomer and fitted with a standard deviation of 0.00047 cm^?1.Furthermore, for the first time, the ν₉ band of cis-CH₃ONO was investigated in the 540–660 cm^?1 spectral range and rather large internal rotation splittings were also observed at higher J values. For the ν₉ band, the effective approach performed with the BELGI-IR program allowed us to analyze and reproduce 682 lines up to J=50 and K_a=18 with a standard deviation of 0.00051 cm^?1. The multiple vibration–rotation–torsion interactions, which are likely to occur between the excited v₉=1 and v₈=1 states and the torsional manifolds are discussed.     

Infrared absorption cross sections for ethane (C2H6) in the 3 μm region

Infrared absorption cross sections for ethane have been measured in the 3 μm spectral region from spectra recorded using a high-resolution FTIR spectrometer (Bruker IFS 125/HR). Results are presented for pure ethane gas from spectra recorded at 0.004 cm⁻¹ resolution and for mixtures with dry synthetic air from spectra obtained at 0.015 cm⁻¹ resolution (calculated as 0.9/MOPD using the Bruker definition of resolution), at a number of temperatures and pressures appropriate for atmospheric conditions. Intensities were calibrated using three ethane spectra (recorded at 278, 293, and 323 K) taken from the Pacific Northwest National Laboratory (PNNL) IR database.     

ICLAS of water in the 770 nm transparency window (12 746–13 558 cm−1). Comparison with current experimental and calculated databases

The absorption spectrum of water vapor has been investigated by intracavity laser spectroscopy (ICLAS) in the 12 746–13 558 cm^?1 spectral region corresponding to an interesting transparency window of the atmosphere, partly obscured by the A band of molecular oxygen.The achieved sensitivity—in the order of α_min～10^?9 cm^?1—has allowed one to measure 1062 water lines with intensities ranging from 1.6×10^?28 to 2.35×10^?24 cm/molecule at 296 K. A total of 169 new and improved energy levels belonging to 21 vibrational states could be determined from 374 newly measured transitions. The retrieved experimental line list is compared with the spectra calculated by Schwenke and Partridge, and Barber and Tennyson. Comparison with the available experimental databases shows that the obtained results represent a significant improvement of the knowledge of the water absorption in the considered region, in particular in the region of the oxygen A band.     

Investigation of new band parameters with temperature dependence for self-broadened methane gas in the range 9000 to 14,000 cm−1 (0.71 to 1.1 μm)

This paper describes new measurements and modelling of the absorption of methane gas, one of the most important gases observed in the atmospheres of the outer planets and Titan, between 9000 and 14,000 cm^?1 (0.7 to 1.1 μm) and compares them with current best available spectral models.A series of methane spectra were measured at the UK's Natural Environment Research Council (NERC) Molecular Spectroscopy Facility (based at the Rutherford Appleton Laboratory, Oxfordshire, UK) using a Brüker 125HR Fourier transform spectrometer. To approximate the conditions found in outer planet atmospheres, the spectra were measured over a wide range of pressures (5 bar to 38 mbar) and temperatures (290–100 K) with path lengths of 19.3, 17.6, 16.0 and 14.4 m. The spectra were recorded at a moderate resolution of 0.12 cm^?1 and then averaged to 10 cm^?1 resolution prior to fitting a series of increasingly complex band-models including temperature dependence. Using the most complex model, a Goody line distribution with a Voigt line shape and two lower energy state levels, the typical rms residual error in the fit is between 0.01 and 0.02 in the wings of the main absorption bands.The new spectral parameters were then compared with the measured spectra and spectra calculated using existing data and shown to be able to accurately reproduce the measured absorption. The improvement in the temperature dependence included in the model is demonstrated by comparison with existing cold methane spectral data for a typical Jovian path.     

Infrared absorption cross sections for propane (C3H8) in the 3 μm region

Infrared absorption cross sections for propane have been measured in the 3 μm spectral region from spectra recorded using a high-resolution FTIR spectrometer (Bruker IFS 125 HR). The spectra of mixtures of propane with dry synthetic air were recorded at 0.015 cm⁻¹ resolution (calculated as 0.9/MOPD using the Bruker definition of resolution), at a number of temperatures and pressures appropriate for atmospheric conditions. Intensities were calibrated using two propane spectra (recorded at 278 and 293 K) taken from the Pacific Northwest National Laboratory (PNNL) IR database.     

Critical evaluation of measured rotation–vibration transitions and an experimental dataset of energy levels of HD18O

All available transitions from microwave to visible region (0.2–12 105 cm^?1) of the HD¹⁸O molecule were collected and tested using the RITZ computer code. Literature data were completed by transitions assigned to HD¹⁸O in long path Fourier transform absorption spectra of the H₂O, HDO and D₂O gas mixtures with natural abundance of oxygen-18. In addition about 40 unassigned lines between 4200 and 6600 cm^?1 of our previous water study associated with the HD¹⁸O molecule have been found and assigned. The new long path absorption spectra of the HDO and D₂O mixtures allow us to observe about 1000 transitions of HD¹⁸O in the 6125–10 720 cm^?1 spectral region. These data have been critically analyzed and used to obtain the most complete and precise set of the experimental energy levels of this molecule.     

Extended line positions,intensities, empirical lower state energies and quantum assignments of NH3 from 6300 to 7000 cm−1

Nearly 4800 features of ammonia between 6300 and 7000 cm^?1 with intensities ≥4×10^?24 cm^?1/(molecule·cm^?2) at 296 K were measured using 16 pure NH₃ spectra recorded at various temperatures (296–185 K) with the McMath–Pierce Fourier Transform Spectrometer at Kitt Peak National Observatory, AZ. The line positions and intensities were retrieved by fitting individual spectra based on a Voigt line shape profile and then averaging the values to form the experimental linelist. The integrated intensity of the region was 4.68×10^?19 cm^?1/(molecule·cm^?2) at 296 K. Empirical lower state energies were also estimated for 3567 absorption line features using line intensities retrieved from 10 spectra recorded at gas temperature between 185 and 233 K. Finally, using Ground State Combination Differences (GSCDs) and the empirical lower state energy estimates, the quantum assignments were determined for 1096 transitions in the room temperature linelist, along with empirical upper state energies for 434 levels. The assignments correspond to seven vibrational states, as confirmed from recent ab initio calculations. The resulting composite database of ¹⁴NH₃ line parameters will provide experimental constraints to ab initio calculations and support remote sensing of gaseous bodies including the atmospheres of Earth, (exo)planets, brown dwarfs, and other astrophysical environments.     

Infrared absorption cross sections for acetone (propanone) in the 3 μm region

Infrared absorption cross sections for acetone (propanone), CH₃C(O)CH₃, have been determined in the 3 μm spectral region from spectra recorded using a high-resolution FTIR spectrometer (Bruker IFS 125 HR) and a multipass cell with a maximum optical path length of 19.3 m. The spectra of mixtures of acetone with dry synthetic air were recorded at 0.015 cm⁻¹ resolution (calculated as 0.9/MOPD using the Bruker definition of resolution) at a number of temperatures and pressures (50-760 Torr and 195-296 K) appropriate for atmospheric conditions. Intensities were calibrated using three acetone spectra (recorded at 278, 293 and 323 K) taken from the Pacific Northwest National Laboratory (PNNL) IR database.