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The correct interpretation of infrared (IR) observations of planetary atmospheres requires an accurate knowledge of temperature and partial and global pressures. Precise laboratory measurements of absorption intensities and line profiles, in the 200-350 K temperature range, are, therefore, critical. However, for gases only existing in complex chemical equilibria, such as nitrous or hypobromous acids, it is not possible to rely on absolute pressure measurements to measure absolute integrated optical absorption cross sections or IR line intensities. To overcome this difficulty, a novel dual-beam terahertz (THz)/mid-IR experimental setup has been developed, relying on the simultaneous use of two instruments. The setup involves a newly constructed temperature-controlled (200-350 K) cross-shaped absorption cell made of inert materials. The cell is traversed by the mid-IR beam from a high-resolution Fourier transform spectrometer using along a White-cell optical configuration providing absorption path lengths from 2.8 to 42 m and by a THz radiation beam (82.5 GHz to 1.1 THz), probing simultaneously the same gaseous sample. The THz channel records pure rotational lines of molecules for which the dipole moment was previously measured with high precision using Stark spectroscopy. This allows for a determination of the partial pressure in the gaseous mixture and enables absolute line intensities to be retrieved for the mid-IR range. This new instrument opens a new possibility for the retrieval of spectroscopic parameters for unstable molecules of atmospheric interest. The design and performance of the equipment are presented and illustrated by an example of simultaneous THz and mid-IR measurement on nitrous acid (HONO) equilibrium.
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The far infrared spectrum of cis-methyl formate has been recorded on the AILES beamline of the synchrotron SOLEIL using a Fourier transform infrared spectrometer coupled to a long path cell. The very weak fundamental band associated with the methyl-top torsion mode (ν(18)) was observed. The frequency analysis was performed using the "rho axis method", and the microwave and millimeter-wave data from the literature. A precise determination of the band origins (ν(18)(A) = 132.4303 cm(-1) and ν(18)(E) = 131.8445 cm(-1)) and of the barrier height [V(3) = 370.7398 (58) cm(-1)] have been obtained. The intensity of the ν(18) fundamental band was determined to be 3.4 × 10(-21) cm(-1)∕(molecule cm(-2)) at 297 K, equally shared among A-A and E-E transitions, thus leading to a dipole moment component µ(c)(3) equal to 0.0483 D. The results were compared with the ab initio calcula-tions of Senent et al. [Astrophys. J. 627, 567 (2005)].
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The High Enthalpy Source (HES) is a novel high temperature source developed to measure infrared line-by-line integrated absorption cross sections of flowing gases up to 2000 K. The HES relies on a porous graphite furnace designed to uniformly heat a constant flow of gas. The flow compensates thermal dissociation by renewing continuously the gas sample and eliminating dissociation products. The flowing characteristics have been investigated using computational fluid dynamics simulation confirming good temperature uniformity. The HES has been coupled to a high-resolution Fourier transform spectrometer to record emission spectra of methane at temperatures ranging between 700 and 1400 K. A radiative model has been developed to extract absolute line intensities from the recorded spectra.
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Absolute line intensities in the nu(6) and nu(8) interacting bands of trans-HCOOH, observed near 1105.4 and 1033.5 cm(-1), respectively, and the dissociation constant of the formic acid dimer (HCOOH)(2) have been measured using Fourier transform spectroscopy at a resolution of 0.002 cm(-1). Eleven spectra of formic acid, at 296.0(5) K and pressures ranging from 14.28(25) to 314.0(24) Pa, have been recorded between 600 and 1900 cm(-1) with an absorption path length of 19.7(2) cm. 437 integrated absorption coefficients have been measured for 72 lines in the nu(6) band. Analysis of the pressure dependence yielded the dissociation constant of the formic acid dimer, K(p)=361(45) Pa, and the absolute intensity of the 72 lines of HCOOH. The accuracy of these results was carefully estimated. The absolute intensities of four lines of the weak nu(8) band were also measured. Using an appropriate theory, the integrated intensity of the nu(6) and nu(8) bands was determined to be 3.47 x 10(-17) and 4.68 x 10(-19) cm(-1)(molecule cm(-2)) respectively, at 296 K. Both the dissociation constant and integrated intensities were compared to earlier measurements.
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We measured absolute line intensities in two bands of (12)C(2)H(2) near 7.5 µm, namely the nu(4) + nu(5)(Sigma(+)(u))-0(Sigma(+)(g)) and nu(4) + nu(5)(Delta(u))-0(Sigma(+)(g)) bands, using Fourier transform spectroscopy with an accuracy estimated to be better than 2%. Using theoretical predictions from Watson [J. K. G. Watson, J. Mol. Spectrosc. 188, 78 (1998)], the observation of the forbidden nu(4) + nu(5)(Delta(u))-0(Sigma(+)(g)) band and the Herman-Wallis behavior exhibited by its rotational lines were studied quantitatively in terms of two types of interactions affecting the levels involved by the band: l-type resonance and Coriolis interaction. In the case of the nu(4) + nu(5)(Sigma(+)(u))-0(Sigma(+)(g)) band, the influence of l-type resonance is also confirmed. We also attributed the intensity asymmetry observed between the R and P branches of that latter band to a Coriolis interaction with l = 1 levels. We did not observe the nu(4) + nu(5)(Sigma(-)(u))-0(Sigma(+)(g)) band, consisting only of a Q branch, in agreement with Watson's prediction. Copyright 2000 Academic Press.
RESUMO
We have recorded the infrared absorption spectrum of pyrrole at 0.005 cm-1 spectral resolution using a Fourier transform interferometer. The rotational analysis of the symmetric out-of-plane C-H bend 22(1)0 fundamental band at 722.132993(5) cm-1 was performed, allowing 6760 lines to be assigned. These lines were fitted simultaneously to literature data on nu1 [A. Mellouki, R. Georges, M. Herman, D. L. Snavely, and S. Leytner, Chem. Phys. 220, 311-322 (1997)] and microwave lines [G. Wlodarczak, L. Martinache, J. Demaison, and B. P. Van Eijck, J. Mol. Spectrosc. 127, 200-208 (1988)]. A set of rotation parameters was determined for the ground state in Ir and IIIr representations, together with vibration-rotation constants for the v1 = 1 and v22 = 1 vibrational states. The fine structure in the strongest of the hot bands in that range was highlighted by division, from the experimental data, of the spectrum of the 22(1)0 band, computed using the vibration-rotation parameters. The rotational assignment of 930 lines in the strongest hot band was performed. The 22(1)024(1)1 vibrational assignment is proposed, leading to x22,24 = 1.90 cm-1. The transition dipole matrix element for the 22(1)0 band is estimated to ||
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We have measured the FT spectrum of natural OCS from 4800 to 8000 cm-1 with a near Doppler resolution and a line-position accuracy between 2 and 8 x 10(-4) cm-1. For the normal isotopic species 16O12C32S, 37 vibrational transitions have been analyzed for both frequencies and intensities. We also report six bands of 16O12C34S, five bands of 16O13C32S, two bands of 16O12C33S, and two bands of 18O12C32S. Important effective Herman-Wallis terms are explained by the anharmonic resonances between closely spaced states. As those results complete the study of the Fourier transform spectra of natural carbonyl sulfide from 1800 to 8000 cm-1, a new global rovibrational analysis of 16O12C32S has been performed. We have determined a set of 148 molecular parameters, and a statistical agreement is obtained with all the available experimental data. Copyright 1998 Academic Press.
RESUMO
We have measured absolute line intensities in the nu(2) fundamental band at 1238 cm(-1) of both isotopomers of hypochlorous acid, HOCl. To obtain the partial pressure of the species in the sample mixture, unavailable through direct measurement since HOCl exists only in equilibrium with H(2)O and Cl(2)O and may decay by secondary reactions, we relied on known absolute line intensities in the pure rotational far-infrared (FIR) spectrum determined from Stark effect measurements. We have thus recorded simultaneously the FIR pure rotation spectrum of HOCl using a Bruker IFS120HR interferometer and the spectrum of a few vibration-rotation lines in the infrared (IR) nu(2) band using a tunable diode laser spectrometer. The absolute intensities of these IR lines thus determined allowed us to "calibrate" the intensities of vibration-rotation lines in the whole nu(2) band, measured previously using Fourier transform spectroscopy. The treatment of the data took into account the blackbody emission contribution in the FIR and the evolution of the HOCl amount during the recording of the spectra. The latter was found to be almost constant over hours after conditioning of the cell. The square of the nu(2) band vibrational transition dipole moment was determined to be 0.013947(23) D(2) and 0.013870(51) D(2) for HO(35)Cl and HO(37)Cl, respectively, that is, 29 to 73% lower than previous measurements. A linear Herman-Wallis factor was also determined for both isotopomers. Finally, the line intensities were least-squares fitted using a model that takes into account a weak resonance between the (010) and (002) levels. Copyright 2000 Academic Press.