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Abstract: A new spectroscopic experiment has been constructed at the Jet Propulsion Laboratory. This experiment employs the collisional cooling technique to study gases at temperatures below their condensation points. Specifically, the experiment has been designed to mimic the temperatures of the interstellar medium for the study of water under these conditions. The ultimate purpose is to measure state-to-state excitation rates under equilibrium conditions to provide insights into the interpretation of interstellar water spectra obtained from space-borne platforms such as the Herschel Space Observatory. This paper presents an overview of the new experiment including a discussion of its design and a detailed explanation of the system. In addition, we report a pressure broadening study of the J=5←4 transition of CO at 576GHz used as a system validation. The new results are compared with measurements from the literature as a validation of the new experiment. In addition, the non-classical behavior of the pressure broadening of CO at low temperature observed in the measurements will be discussed. [Copyright &y& Elsevier]

Pressure-broadened line shapes in the 30013←00001 (ν 1 +4 ν 2 0 +ν 3 ) band of 12 C 16 O 2 at 6228 cm −1 are reanalyzed using new spectra recorded with sample temperatures down to 170 K. High resolution, high signal-to-noise (S/N) laboratory measurements of line shapes (Lorentz air- and self-broadened half-width coefficients, pressure-shift coefficients and off-diagonal relaxation matrix element coefficients) as a function of gas sample temperatures for various pressures and volume mixing ratios are presented. The spectra were recorded using two different Fourier transform spectrometers (FTS): (1) the McMath-Pierce FTS located at the National Solar Observatory on Kitt Peak, Arizona (and reported in Devi et al., J Mol Spectrosc 2007;245:52-80) and, (2) the Bruker IFS-125HR FTS at the Jet Propulsion Laboratory in Pasadena, California. The 19 spectra taken at Kitt Peak were all recorded near room temperature while the 27 Bruker spectra were acquired both at room temperature and colder temperatures (170-296 K). Various spectral resolutions (0.004–0.011 cm −1 ), absorption path lengths (2.46–121 m) and CO 2 samples (natural and 12 C-enriched) were included in the dataset. To maximize the accuracies of the various retrieved line parameters, a multispectrum nonlinear least squares spectrum fitting software program was used to adjust the ro-vibrational constants ( G , B , D etc.) and intensity parameters (including Herman-Wallis terms) instead of directly measuring the individual line positions and intensities. To minimize systematic residuals, line mixing (via off-diagonal relaxation matrix elements) and quadratic speed dependence parameters were included in the analysis. Contributions from other weakly absorbing bands: the 30013←00001 and 30012←00001 bands of 13 C 16 O 2 , the 30013←00001 band of 12 C 16 O 18 O, hot bands 31113←01101 and 32212←02201 of 12 C 16 O 2 , as well as the 40013←10001 and the 40014←10002 bands of 12 C 16 O 2, present within the fitted interval were also measured. Results from previous works and new calculations are compared to present measurements, where appropriate. [ABSTRACT FROM AUTHOR]

In this paper we report line shape parameters of 12 CH 4 for several hundred 2ν 3 transitions in the spectral regions 5891–5996 cm −1 (P branch) and 6015–6115 cm −1 (R branch). Air- and self-broadening coefficients were measured as a function of temperature; line mixing via off-diagonal relaxation matrix element coefficients was also obtained for 47 transition pairs. In total, nearly 1517 positions and intensities were retrieved, but many transitions were too weak for the line shape study. For this analysis, we used 25 high-resolution (0.0056 and 0.0067 cm −1 ) and high signal-to-noise (S/N) spectra of high-purity 12 CH 4 and the same high-purity 12 CH 4 broadened by dry air recorded at different sample temperatures between 130 K and 295 K with the Bruker IFS 125HR Fourier transform spectrometer at JPL. Three different absorption cells were used (1) a White cell set to a path length of 13.09 m for room temperature data, (2) a single-pass 0.2038 m long coolable cell (for self-broadening) and (3) a multipass cell with 20.941 m total path coolable Herriott cell (for air-broadening). In total there were 13 spectra with pure 12 CH 4 (0.27–599 Torr) and 12 air-broadened spectra with total sample pressures of 80–805 Torr and volume mixing ratios (VMR) of methane between 0.18 and 1.0. An interactive multispectrum nonlinear least-squares technique was employed to fit the individual P10–P1 and R0–R10 manifolds in all the spectra simultaneously. Results obtained from the present analysis are compared to other recent measurements. [ABSTRACT FROM AUTHOR]

Accurate knowledge of spectral line shape parameters is important for infrared transmission and radiance calculations in the terrestrial atmosphere. In this paper, we report the self- and air-broadened Lorentz half-widths, pressure-induced shifts and line mixing coefficients (via off-diagonal relaxation matrix elements) along with their temperature dependences for methane ν 2 +ν 3 absorption lines in the 4500–4630 cm −1 region of the Octad. For this, we recorded 14 high-resolution, high signal to noise ratio (S/N) spectra of high-purity (99.95% 12 C-enriched) samples of pure methane and its dilute mixtures in dry air between 298 K and 148 K. A Bruker IFS 125HR Fourier transform spectrometer (FTS) at the Jet Propulsion Laboratory, Pasadena, California, was used to obtain the experimental data. The absorption cell used for this study was a specially built 20.38 cm long coolable cell installed in its sample compartment. The sample pressures for the pure 12 CH 4 spectra were 4.5−385 Torr; for the air-broadened spectra the total pressures ranged between 95 and 300 Torr with the methane volume mixing ratios between 0.04 and 0.097. All 14 spectra were fitted simultaneously using an interactive multispectrum nonlinear least-squares curve fitting technique. The results are compared to values reported in the literature. [ABSTRACT FROM AUTHOR]

A new cryogenic Herriott cell and associated transfer optics have been designed and fabricated at Connecticut College under contract with NASA Langley Research Center to operate for the first time with the broad-band Bruker IFS-125HR Fourier transform spectrometer at the Jet Propulsion Laboratory (JPL). This 0.375 m base-length optical cavity produces an absorption path length, at 293 K, of 20.941 (±0.006) m. The Herriott cell, constructed from oxygen-free high conductivity copper, is placed inside its own vacuum enclosure, which is isolated from the transfer optics chamber by one CaF 2 window and separately evacuated. The transfer optics chamber is in turn coupled to the sample compartment of the Bruker IFS-125HR holding another set of transfer optics. The entire spectrometer, including the transfer optics chamber can be evacuated to ∼10 mTorr; the cell vacuum enclosure is cryogenically evacuated to pressures below 10 −6 Torr. A closed-cycle helium refrigerator cools the Herriott cell. Initially tested at Connecticut College for temperatures between 250 and 50 K, the system has successfully been in operation for over two years at JPL. The cell has been used for recording spectra between 75 and 250 K, achieving excellent temperature uniformity (± 0.15 K) and long term stability (< 0.05 K/day). Configured with a single indium-sealed CaF 2 window, it has provided Doppler-limited infrared absorption spectra of a number of molecular species above 2000 cm −1 to investigate detailed spectroscopic properties (e.g. molecular line parameters at temperatures and pressures relevant to planetary atmospheres). The design, performance and detailed characterization of the Herriott cell system are discussed. [ABSTRACT FROM AUTHOR]

• Obtained H 2 -broadened CH 4 spectra in the v 2 + v 3 band (2.2 µm) at 80 – 370 K. • Fit 28 spectra simultaneously and achieved spectrum fitting residuals better than 1%. • Retrieved H 2 -broadened widths, H 2 -pressue shifts, and their temperature dependences. • Determined collisional line mixing coefficients through full line mixing operation. • Showed J and A/E/F dependence of H 2 -widths for CH4 in the v2+v3 band. We have made the measurements of H 2 -broadened line shape parameters of CH 4 transitions in the v 2 + v 3 band located at 2.2 µm. These measurements support atmospheric opacity calculations of cold giant planets, hot Jupiters, and low mass brown dwarfs, for which H 2 is the dominant atmospheric constituent. For this reason, a series of spectra of pure CH 4 and CH 4 H 2 mixtures were obtained in the Octad (4100–4600 cm−1) region for a wide temperature range between 80 and 370 K using a Bruker 125HR high-resolution Fourier transform spectrometer (FTS) at Jet Propulsion Laboratory (JPL). Three custom-designed gas absorption cells vacuum-coupled to the FTS were used to obtain the spectra. All spectra were fit simultaneously using a multispectrum non-linear least-squares fitting software, which adopts a speed-dependent Voigt line shape using full line mixing taken into account through a relaxation matrix operation. H 2 pressure broadened half-widths, and H 2 pressure-induced shift coefficients were determined in 11 J-manifolds covering P(4) – R(6) plus the Q-branch. The temperature dependences of the width and shift coefficients were determined based upon the power law and the linear models. Collisional line mixing coefficients for CH 4 H 2 were retrieved for ten transition pairs. For the same transitions, speed dependence parameters for the CH 4 H 2 transitions were found to be smaller than for CH 4 -air-broadening. We have compiled the retrieved line parameters in an electronic format and reported as supplemental document. This will facilitate the analyses of planetary and exoplanetary atmospheres using ground-, air- and space-based observations (e.g., Keck I and II, SOFIA, JWST). Image, graphical abstract Figure. Spectrum fitting residuals in the R(6) manifold. (a) with no line mixing, (b) with line mixing, In the bottom panel the observed and calculated spectra where two pairs of transitions mix together, e.g., F 1 and F 2 , and A 1 and A 2 are labeled. The vertical bars (in black at the top of the bottom panel) represent the locations of all transitions included in the calculations. The vertical bars in red, blue, and green indicate the line position of the 1st, 2nd and 3rd sets of five strongest lines each group, respectively. [ Colors available in the online version ]. [ABSTRACT FROM AUTHOR]