Search

Abstract: New absorption cells designed specifically to achieve stable temperatures down to 66K inside the sample compartment of an evacuated Bruker IFS-125HR Fourier transform spectrometer (FTS) were developed at Connecticut College and tested at the Jet Propulsion Laboratory (JPL). The temperature stabilized cryogenic cells with path lengths of 24.29 and 20.38cm were constructed of oxygen free high conductivity (OFHC) copper and fitted with wedged ZnSe windows using vacuum tight indium seals. In operation, the temperature-controlled cooling by a closed-cycle helium refrigerator achieved stability of ±0.01K. The unwanted absorption features arising from cryodeposits on the cell windows at low temperatures were eliminated by building an internal vacuum shroud box around the cell which significantly minimized the growth of cryodeposits. The effects of vibrations from the closed-cycle helium refrigerator on the FTS spectra were characterized. Using this set up, several high-resolution spectra of methane isotopologues broadened with nitrogen were recorded in the 1200–1800cm−1 spectral region at various sample temperatures between 79.5 and 296K. Such data are needed to characterize the temperature dependence of spectral line shapes at low temperatures for remote sensing of outer planets and their moons. Initial analysis of a limited number of spectra in the region of the R(2) manifold of the ν 4 fundamental band of 13CH4 indicated that an empirical power law used for the temperature dependence of the N2-broadened line widths would fail to fit the observed data in the entire temperature range from 80 to 296K; instead, it follows a temperature-dependence similar to that reported by Mondelain et al. . The initial test was very successful proving that a high precision Fourier transform spectrometer with a completely evacuated optical path can be configured for spectroscopic studies at low temperatures relevant to the planetary atmospheres. [Copyright &y& Elsevier]

Highlights • 5819 line positions and intensities were retrieved from measured spectra in the 5550- 5825 cm−1 region. • More than 2800 new assignments were made in the region. • More than 3445 experimental line positions were modeled to 0.0024 cm−1. Abstract A new study of 12CH 4 line positions and intensities was performed for the Tetradecad regions 5550,000–5695.250, 5718.8–5724.250 and 5792.36–5814.290 cm−1 using long path (202 m, 602 m, 1604 m and 1804 m) spectra of normal samples of CH 4 at different pressures recorded with a Fourier transform spectrometer in Reims, France. Line positions and intensities were retrieved by least squares curve-fitting procedures and analyzed using the effective Hamiltonian and the effective dipole moment expressed in terms of irreducible tensor operators adapted to spherical top molecules. An 80 K spectrum recorded in Jet Propulsion Laboratory (JPL), Pasadena, of enriched 12CH 4 was used for low-J line positions. Another 80 K spectrum of enriched 13CH 4 from JPL was used to discern the isotopic lines. A new measured linelist contains positions and intensities for 5819 features. Quantum assignments were made for more than 3400 transitions, which represent ∼95% of the integrated line intensity observed in this region. All assigned 3445 line positions were fitted with RMS standard deviations of 0.0024 cm−1. The sum of observed intensities between 5550 and 5695 cm−1 fell within 2% of the predicted value from ab initio variational calculations reported in the TheoReTS database (http://theorets.univ-reims.fr ; http://theorets.tsu.ru). [ABSTRACT FROM AUTHOR]

A new study of 12 CH 4 line positions and intensities was performed for part of the Tetradecad region between 5300 and 5550 cm −1 using four long path (202 m, 602 m, 1604 m and 1804 m) spectra of normal samples of CH 4 at different pressures recorded with a Fourier transform spectrometer in Reims, France. Line positions and intensities were retrieved by least square curve-fitting procedures and analyzed using the effective Hamiltonian and the effective dipole moment expressed in terms of irreducible tensor operators adapted to spherical top molecules. An 80 K spectrum from JPL of enriched 12 CH 4 was used for low-J line positions. One other 80 K spectrum from JPL of enriched 13 CH 4 was used to discern the isotopic lines. A new measured linelist contains positions and intensities for 5934 features. Quantum assignments were made for 2847 transitions, which represent ∼90% of the integrated line intensity observed in this region. All assigned line positions and 2227 selected line intensities were fitted with RMS standard deviations of 0.0025 cm −1 and 8.6%, respectively. The sum of observed intensities between 5300 and 5550 cm −1 fell within 2% of the predicted value from ab initio variational calculations. [ABSTRACT FROM AUTHOR]

This paper presents the results of a spectroscopic line shape study of self- and nitrogen-broadened 12 CH 3 D transitions in the ν 3 and ν 5 bands in the Triad region. We combined five pure gas spectra with eighteen spectra of lean mixtures of 12 CH 3 D and nitrogen, all recorded with a Bruker IFS-125 HR Fourier transform spectrometer. The spectra have been analyzed simultaneously using a multispectrum nonlinear least squares fitting technique. N 2 -broadened line parameters for 184 transitions in the ν 3 band and 205 transitions in the ν 5 band were measured. In addition, line positions and line intensities were measured for 168 transitions in the ν 3 band and 214 transitions in the ν 5 band. We have observed 10 instances of weak line mixing corresponding to K ″=3 A1 or A2 transitions. Comparisons were made for the N 2 -broadening coefficients and associated temperature exponents with corresponding values calculated using a semi-classical Robert Bonamy type formalism that involved an inter-molecular potential with terms corresponding to short- and long-range interactions, and exact classical molecular trajectories. The theoretical N 2 -broadened coefficients are overestimated for high J values, but are in good agreement with the experimental values for small and middle range J values. [ABSTRACT FROM AUTHOR]

Abstract: A new study of 12CH3D line positions and intensities was performed for the upper portion of the Enneadecad polyad between 4000 and 4550cm−1. For this, FTIR spectra were recorded with D-enriched methane samples (at 80K with a Bruker 125 IFS at 0.005cm−1 resolution and at 291K with the McMath-Pierce FTS at 0.011cm−1 resolution, respectively). Line positions and intensities were retrieved by least square curve-fitting procedures and analyzed using the effective Hamiltonian and the effective dipole moment expressed in terms of irreducible tensor operators adapted to symmetric top molecules. Initially, only the cold spectrum was used to identify quantum assignments and predict 12CH3D relative intensities in this region. To assign higher quanta up to J equal 14, additional line positions and intensities were obtained from two room temperature spectra. In the final stage, measured intensities from both the cold and the room temperature data were normalized to corresponding values at 296K and then averaged. Combining the two temperature datasets confirmed the assumed quantum assignments and also demonstrated the relative accuracies to be better than ±0.0002cm−1 for line positions and at least ±6% for intensities so that ∼1160 features were selected. Including additional assignments from the room temperature spectra alone permitted 1362 line intensities of 11 bands (involving 23 vibrational symmetry components) to be reproduced with an RMS of 9%. Over 4085 selected positions for 12 bands were modeled to 0.008cm−1. Nevertheless a number of known assignments could not be modeled to within our experimental precisions. More work is needed to obtain a complete characterization of this complex polyad. [Copyright &y& Elsevier]

We studied the R(0) line profile in the CH4 ν4 band from room temperature to 188 K with N2 as a perturber, to 100 K with O2 as perturber, and from room temperature to 15 K using He as a perturber. The N2 and O2 measurements were performed over a total pressure range of 15–110 mbar, and for the He measurements the maximum sample pressure at 15 K was 1.10 mbar. Broadening parameters were determined, taking into account the confinement narrowing (Dicke effect), and narrowing parameters, deduced from the soft or hard collision model, are compared with the dynamic friction coefficient calculated values. We also obtained preliminary values for the temperature dependence of the N2, O2 and He line broadening parameters for this transition. [ABSTRACT FROM AUTHOR]