Showing papers by "S.A. Tashkun published in 2016"

A room temperature CO2 line list with ab initio computed intensities

Abstract: Atmospheric carbon dioxide concentrations are being closely monitored by remote sensing experiments which rely on knowing line intensities with an uncertainty of 0.5% or better. We report a theoretical study providing rotation–vibration line intensities substantially within the required accuracy based on the use of a highly accurate ab initio dipole moment surface (DMS). The theoretical model developed is used to compute CO2 intensities with uncertainty estimates informed by cross comparing line lists calculated using pairs of potential energy surfaces (PES) and DMS׳s of similar high quality. This yields lines sensitivities which are utilized in reliability analysis of our results. The final outcome is compared to recent accurate measurements as well as the HITRAN2012 database. Transition frequencies are obtained from effective Hamiltonian calculations to produce a comprehensive line list covering all 12C16O2 transitions below 8000 cm − 1 and stronger than 10 − 30 cm /molecule at T = 296 K .

NOSD-1000, the high-temperature nitrous oxide spectroscopic databank

Abstract: We present a high-temperature version, NOSD-1000, of the nitrous oxide spectroscopic databank. The databank contains the line parameters (positions, intensities, air- and self-broadened half-widths and coefficients of temperature dependence of air- and self-broadened half-widths) of the most abundant isotopologue 14N216O of the nitrous oxide molecule. The reference temperature is Tref=1000 K and the intensity cutoff is Icut=10−25 cm−1/(molecule cm−2). More than 1.4 million lines covering the 260–8310 cm−1 spectral range are included in NOSD-1000. The databank has been generated within the framework of the method of effective operators and based on the global fittings of spectroscopic parameters (parameters of the effective Hamiltonian and effective dipole moment operators) to observed data collected from the literature. Line-by-line simulation of a medium-resolution high-temperature (T=873 K) spectrum has been performed in order to validate the databank. NOSD-1000 is freely accessible via the Internet.

Analyses and modeling of the 12CH4 spectrum at 80 K between 6539 and 6800 cm−1

Abstract: Accurate rovibrational analysis of the lower part of the Icosad 12 CH 4 spectrum in the 6539–6800 cm −1 region is reported. The analysis uses the WKLMC experimental line list previously obtained in Grenoble by Cavity Ring Down Spectroscopy and Differential Absorption Spectroscopy at T =80 K. Based on an ab initio potential energy surface, the full Hamiltonian of methane nuclear motion was reduced to an effective Hamiltonian using high-order Contact Transformations method with a subsequent empirical optimization of parameters. About 2445 experimental line positions were fitted with RMS standard deviations of 0.005 cm −1 and 1644 line intensities were modeled using effective dipole moment with the RMS deviation of 11.5%. Twenty five vibrational energy levels of the lower part of the Icosad were determined.

High sensitivity Cavity Ring Down Spectroscopy of N2O near 1.22 μm: (I) Rovibrational assignments and band-by-band analysis

Abstract: The absorption spectrum of nitrous oxide (N 2 O) in natural isotopic abundance has been recorded near 1.22 µm by Cavity Ring Down Spectroscopy using an External Cavity Diode Laser (ECDL) as light source. The room temperature recordings were performed at a pressure of 10.0 Torr in the 7915–8334 cm −1 spectral range (1.26–1.19 μm). The typical noise equivalent absorption of the spectra, on the order of α min ~ 2×10 −11 cm −1 , allowed for the detection of lines with intensities on the order of 5×10 −29 cm/molecule. More than 3300 transitions belonging to 64 bands of five nitrous oxide isotopologues ( 14 N 2 16 O, 14 N 15 N 16 O, 15 N 14 N 16 O, 14 N 2 18 O and 14 N 2 17 O) have been rovibrationally assigned on the basis of the predictions of the effective Hamiltonian models developed for each isotopologue. For comparison, only 13 bands were previously measured by Fourier Transform spectroscopy in the studied region. All identified bands belong to the Δ P =13 and 14 series of transitions, where P= 2 V 1 + V 2 +4 V 3 is the polyad number ( V i are vibrational quantum numbers). The line positions and intensities are provided for all assigned lines. The maximum deviations between the measured position values and those predicted by the effective Hamiltonian models are about 0.2 cm −1 for the main isotopologue but reach values larger than 1 cm −1 for the less abundant minor isotopologues. The band-by-band analysis led to the determination of the rovibrational parameters of a total of 62 bands. The typical rms value of the ( ν obs − ν fit ) differences is 0.7×10 −3 cm −1 . Among the 62 bands, 49 are newly measured, for 13 others the rotational analysis is significantly improved and extended. A few resonance perturbations due to intra- and inter-polyad couplings are identified and discussed.

The CO2 absorption spectrum in the 2.3 µm transparency window by high sensitivity CRDS: (I) Rovibrational lines

Abstract: The absorption of carbon dioxide is very weak near 2.3 µm which makes this transparency window of particular interest for the study of Venus’ lower atmosphere. As a consequence of the weakness of the transitions located in this region, previous experimental data are very scarce and spectroscopic databases provide calculated line lists which should be tested and validated by experiment. In this work, we use the Cavity Ring Down Spectroscopy (CRDS) technique for a high sensitivity characterization of the CO 2 absorption spectrum in two spectral intervals of the 2.3 µm window: 4248–4257 and 4295–4380 cm −1 which were accessed using a Distributed Feed Back (DFB) diode laser and a Vertical External Cavity Surface Emitting Laser (VECSEL) as light sources, respectively. The achieved sensitivity (noise equivalent absorption, α min , on the order of 5×10 −10 cm −1 ) allowed detecting numerous new transitions with intensity values down to 5×10 −30 cm/molecule. The rovibrational assignments were performed by comparison with available theoretical line lists in particular those obtained at IAO Tomsk using the global effective operator approach. Hot bands of the main isotopologue and 16 O 12 C 18 O bands were found to be missing in the HITRAN database while they contribute importantly to the absorption in the region. Additional CRDS spectra of a CO 2 sample highly enriched in 18 O were recorded in order to improve the spectroscopy of this isotopologue. As a result about 700 lines of 16 O 12 C 18 O, 16 O 12 C 17 O, 17 O 12 C 18 O, 12 C 18 O 2 and 13 C 18 O 2 were newly measured. The status of the different databases (HITRAN, CDSD, variational calculations) in the important 2.3 µm transparency window is discussed. Possible improvements to correct evidenced deficiencies are suggested.

High sensitivity cavity ring down spectroscopy of N2O near 1.22 μm: (II) 14N216O line intensity modeling and global fit of 14N218O line positions

Abstract: In a recent work (Karlovets et al., 2016 [1] ), we reported the measurement and rovibrational assignments of more than 3300 transitions belonging to 64 bands of five nitrous oxide isotopologues ( 14 N 2 16 O, 14 N 15 N 16 O, 15 N 14 N 16 O, 14 N 2 18 O and 14 N 2 17 O) in the high sensitivity CRDS spectrum recorded in the 7915–8334 cm −1 spectral range. The assignments were performed by comparison with predictions of the effective Hamiltonian models developed for each isotopologue. In the present paper, the large amount of measurements from our previous work mentioned above and literature are gathered to refine the modeling of the nitrous oxide spectrum in two ways: (i) improvement of the intensity modeling for the principal isotopologue, 14 N 2 16 O, near 8000 cm −1 from a new fit of the relevant effective dipole moment parameters, (ii) global modeling of 14 N 2 18 O line positions from a new fit of the parameters of the global effective Hamiltonian using an exhaustive input dataset collected in the literature in the 12–8231 cm −1 region. The fitted set of 81 parameters allowed reproducing near 5800 measured line positions with an RMS deviation of 0.0016 cm −1 . The dimensionless weighted standard deviation of the fit is 1.22. As an illustration of the improvement of the predictive capabilities of the obtained effective Hamiltonian, two new 14 N 2 18 O bands could be assigned in the CRDS spectrum in the 7915–8334 cm −1 spectral range. A line list at 296 K has been generated in the 0–10,700 cm −1 range for 14 N 2 18 O in natural abundance with a 10 −30 cm/molecule intensity cutoff.

Global modeling of the 15N216O line positions within the framework of the polyad model of effective Hamiltonian and a room temperature 15N216O line list

Abstract: The global modeling of 15N216O line positions in the 4–12,516 cm−1 region has been performed using the polyad model of effective Hamiltonian. The effective Hamiltonian parameters were fitted to the line positions collected from an exhaustive review of the literature. The dimensionless weighted standard deviation of the fit is 1.31. The fitted set of 109 parameters allowed reproducing more than 18,000 measured line positions with an RMS value of 0.001 cm−1. A line list was calculated for a reference temperature 296 K, natural abundance (1.32×10−5), and an intensity cutoff 10−30 cm/molecule. The line list is based on the fitted set of the effective Hamiltonian parameters for 15N216O obtained in this work and the effective dipole moment parameters of the 15N216O and 14N216O isotopologues. Accurate values of the 15N216O total partition function are also given.

LED-based Fourier transform spectroscopy of 16O12C18O and 12C18O2 in the 11,260–11,430 cm−1 range

Abstract: The absorption spectrum of the 16 O 12 C 18 O and 12 C 18 O 2 carbon dioxide isotopologues has been recorded in the 11,260– 11,430 cm −1 spectral range using Bruker IFS 125 HR Fourier transform spectrometer with resolution 0.05 cm −1 at temperature 297 K and path length 24 m. The 18 O enriched sample of carbon dioxide at total pressure 96.5 mbar was used for these purposes. The spectrometer used LED emitter as a light source. This gave possibility to reach the minimal detectable absorption coefficient α min ~1.4×10 −7 cm −1 using 23,328 scans. In the recorded spectrum we have assigned the 00051–00001 band for both 16 O 12 C 18 O and 12 C 18 O 2 isotopologues using the predictions performed within the framework of the method of effective operators. The line positions and intensities of the observed bands are found. The comparison of the observed and predicted line positions and intensities is performed confirming good accuracy of the predictions. The spectroscopic parameters for the observed bands are determined.