Pressure-broadened Linewidths Research Articles

Simple semiclassical model of pressure-broadened infrared/microwave linewidths in the temperature range 200–3000 K

Abstract There is a need for line-broadening parameters for molecules found in exoplanetary atmospheres for a variety of broadeners and a range of temperatures. The use of an easily handled semiclassical theoretical expression is suggested for the calculation of pressure-broadened linewidths for (vib)rotational transitions over a large temperature range (200–3000 K) starting from a minimal set of input parameters: kinetic molecular properties and the character of the leading term in the intermolecular interaction potential. Applications to NO and OH colliding with rare-gas atoms and non-polar molecules demonstrate good consistency with available measurements over the full temperature range indicated. The procedure therefore can be expected to provide realistic estimates for line broadening of ‘exotic’ molecules and molecular ions present in hot planetary atmospheres.

Re-evaluation of ortho-para-dependence of self pressure-broadening in the ν1 + ν3 band of acetylene.

Optical frequency comb-referenced measurements of self pressure-broadened line profiles of the R(8) to R(13) lines in the ν1 + ν3 combination band of acetylene near 1.52 µm are reported. The analysis of the data found no evidence for a previously reported [Iwakuni et al., Phys. Rev. Lett. 117(14), 143902 (2016)] systematic alternation in self pressure-broadened line widths with the nuclear spin state of the molecule. The present work brought out the need for the use of an accurate line profile model and careful accounting for weak background absorptions due to hot band and lower abundance isotopomer lines. The data were adequately fit using the quadratic speed-dependent Voigt profile model, neglecting the small speed-dependent shift. Parameters describing the most probable and speed-dependent pressure-broadening, most probable shift, and the line strength were determined for each line. Detailed modeling of the results of Iwakuni et al. showed that their neglect of collisional narrowing due to the speed-dependent broadening term combined with the strongly absorbing data recorded and analyzed in transmission mode were the reasons for their results.

A path to scaling helium-3 spin-exchange optical pumping

Polarized 3He has contributed to breakthroughs in particle physics, neutron scattering, and lung imaging, mostly by optical pumping of liter quantities in valved cells or in sealed cells pumped in situ. Only metastability exchange optical pumping currently meets the demand for delivering large quantities of highly polarized 3He to external volumes. Our development program is under way to achieve medium to large-scale spin-exchange optical pumping of 3He. Thin-walled aluminosilicate cells placed in contact with thermal reservoirs inside a pressure vessel were optically pumped with kilowatt-scale lasers spectrally narrowed to match pressure-broadened absorption linewidths. We report here the key elements of this program and significant milestones: multi-liter volume valved cells that provide long longitudinal lifetimes that reached polarization as high as 85% inside a thermally stable and pressurized environment.

Accuracy tests of radiation schemes used in hot Jupiter global circulation models

The treatment of radiation transport in global circulation models (GCMs) is crucial to correctly describe Earth and exoplanet atmospheric dynamics processes. The two-stream approximation and correlated-k method are currently state-of-the-art approximations applied in both Earth and hot Jupiter GCM radiation schemes to facilitate rapid calculation of fluxes and heating rates. Their accuracy have been tested extensively for Earth-like conditions, but verification of the methods' applicability to hot Jupiter-like conditions is lacking in the literature. We are adapting the UK Met Office GCM, the Unified Model (UM), for the study of hot Jupiters, and present in this work the adaptation of the Edwards-Slingo radiation scheme based on the two-stream approximation and the correlated-k method. We discuss the calculation of absorption coefficients from high temperature line lists and highlight the large uncertainty in the pressure-broadened line widths. We compare fluxes and heating rates obtained with our adapted scheme to more accurate discrete ordinate (DO) line-by-line (LbL) calculations ignoring scattering effects. We find that, in most cases, errors stay below 10 % for both heating rates and fluxes using ~ 10 k-coefficients in each band and a diffusivity factor D = 1.66. The two-stream approximation and the correlated-k method both contribute non-negligibly to the total error. We also find that using band-averaged absorption coefficients, which have previously been used in radiative-hydrodynamical simulations of a hot Jupiter, may yield errors of ~ 100 %, and should thus be used with caution.

Pressure-broadened line widths and pressure-induced line shifts coefficients of the (1-0) and (2-0) bands of [formula omitted

This work presents, for the 1-0 band of CO, the measurements of air and H 2 -broadened line widths and H 2 -induced line shifts coefficients. These values were retrieved using a multispectrum fitting procedure on spectra recorded with the Fourier transform spectrometer at Reims. These results provide confirmation of other similar measurements previously published. In addition, we present the results for self and H 2 broadened line widths for the 2-0 band.

Trace detection of methane using continuous wave cavity ring-down spectroscopy at 1.65 μm

The use of continuous wave cavity ring-down spectroscopy (cw CRDS) is demonstrated for the detection of tropospheric methane. Spectra of the 2ν3 overtone band of methane at wavelengths near 1.65 μm have been recorded in both laboratory air and under reduced pressures, and show quantitative agreement with absorption intensities and pressure-broadened line widths reported in the HITRAN 96 database. Experiments demonstrate a minimum detectable absorption coefficient that is currently αmin = 1.5 × 10−8 cm−1, equivalent to a number density of CH4 at ambient temperature of 2.9 × 1011 molecules cm−3 in a low pressure environment where Doppler broadening dominates the spectral lineshapes. The effects of pressure broadening reduce the detection limit of CH4 in 1 atm of air to 1.3 × 1012 molecules cm−3, corresponding to 52 parts per billion by volume. This is well below the 1.72 parts per million by volume average methane concentration in the lower troposphere.

Semiclassical modeling of infrared pressure-broadened linewidths: A comparative analysis in CO2–Ar at various temperatures

A novel semiclassical approach, which makes use of the exact trajectory implemented within the Robert–Bonamy formalism, is employed for modeling infrared pressure-broadened linewidths. As a prototype, the carbon dioxide molecule perturbed by argon is examined in the temperature range 160–760 K, for which various measurements and computations are available. For a meaningful comparison with previous theoretical works done with both semiclassical and quantum approaches, the ab initio intermolecular potential surface of Parker et al. [J. Chem. Phys. 64, 1668 (1976)] is used. Our values are found to be in agreement with up-to-date experimental data at all temperatures studied.

Measurement of the radiative lifetime of O 2( a1Δ g) using cavity ring down spectroscopy

Cavity ring down spectroscopy has been used to detect the weak a 1Δ g− X 3Σ g − electronic transition in oxygen and to obtain a measurement of the radiative lifetime of the a 1Δ g state. (0,0) band transitions near 1274 nm were scanned using the frequency-doubled output of a tunable OPO system. Pressure-broadened linewidths were measured, and a FWHM-broadening coefficient of 5.5(5) MHz/Torr was obtained. Analysis of the ring down traces provided a measurement of the line strength of an individual rovibronic transition. The Einstein A-coefficient of the a 1Δ g state was calculated using branching ratios determined from the Honl–London factors. The A-coefficient obtained is 2.3(3)×10 −4 s −1 , corresponding to a radiative lifetime of 4300(500) s ( ∼72 min ). This value is in agreement with those reported by Badger et al. (2.58×10 −4 s −1) and Špalek et al. (2.24(40)×10 −4 s −1 ). In addition, a value of 2.77(8)×10 −45 cm 5/ molecule was obtained for the background collisional absorption coefficient for oxygen at 1274 nm.

Pressure broadening of millimeter-wave ozone lines by atmospheric gases

Pressure-broadened linewidths of the 4 13-4 04, 6 15-6 06, 14 3 11-15 2 14, and 28 5 23-29 4 26 rotational transitions of ozone have been measured for the foreign gases N 2 and O 2 over a pressure range of 200–1000 m Torr. The temperature dependence of the broadening has been measured for the 4 13-4 04 and 6 15-6 06 transitions over the range 195–320 K. The air-broadened linewidth of the 6 15-6 06 transition was measured at 294 K and is readily calculated for other temperatures and transitions from the N 2 and O 2 results. The results are compared with recent theoretical calculations, and also with previous measurements, where these are available. The measurements have direct application to the interpretation of observations of microwave emission of middle atmosphere ozone. The results also serve as reference points for theoretical linewidth calculations applicable from the microwave to the infrared spectral regions.

CHARACTERISTICS OF WATER VAPOR OVER KITT-PEAK AS DETERMINED FROM FTS DATA

ABSTRACT Spectra of three water vapor lines obtained with the Fourier transform spectrometer at the McMath telescope have been analyzed to obtain the atmospheric water-vapor content. The results for 52 days between 1978 and 1983 show a maximum of between 20 and 25 mm precipitable water vapor in July and August and minima between February and June and between October and December. Outside of a secondary maximum due to two days in January 1983 the results are in generally good agreement with those obtained with a grating spectrograph. The pressure-broadened linewidths have been used to make corrections not exceeding 11% to the results obtained with the spectrograph.

On the calculation of pressure-broadened linewidths for symmetric-top molecules using the modified Anderson theory

An approximation, accurate to better than 0.5%, to the Anderson-Tsao-Curnutte theory of linewidth is presented. This approximation is shown to be suitable for calculating linewidths of symmetric top molecules where dipole-dipole forces are dominant, but higher-order multipole terms are not negligible. Some insight is gained as to the relative importance of various types of collisions between interacting molecules from this approach to the calculation of the spectral linewidths for rotational states of molecules. Experimental results for CH 3 35Cl and CH 3 127I are presented for comparison with the calculated results.

Accurate measurements of pressure-broadened linewidths in a transversely excited CO2discharge

A novel technique is used to measure He-dominated pressure-broadening coefficients in the 10μm band of CO <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> . Gain measurements in a pulsed transversely excited CO <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> discharge are made at line center using a CW CO <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> laser as a probe, and at a known offset frequency using a CW N <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> O laser. By measuring this gain ratio as a function of discharge pressure, we determine the linewidth with an accuracy of ∼ 2 percent. Linewidths are measured for nine different transitions in the 10 μm <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">P</tex> -branch, and the <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">J</tex> -dependence of the CO <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> -He pressure-broadening coefficient is determined and compared to theory. In addition, we examine the temperature dependence of the linewidth under conditions of constant number density and find that the linewidth increases as <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T^{0.42 \pm 0.06}</tex> . This agrees well with a recent theoretical prediction of T <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.38</sup> (R. T. Pack, "Pressure broadening of the dipole and Raman lines of CO <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> by He and Ar. Temperature dependence," J. Chem. Phys., vol. 70, pp. 3424-3433, 1979). To our knowledge, these experiments represent the first direct linewidth measurements in a transversely excited CO <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> discharge.

Application of the rotational diffusion model to gaseous N 2 CARS spectra

A theoretical model of CARS intensities based on Gordon's rotational diffusion model has been found to give good agreement with 300 K N 2 spectra over the pressure range one to 100 atmospheres. The model requires only that the isolated pressure-broadened linewidths be known and greatly simplifies the task of calculating CARS spectra for overlapping transitions.

Pressure-broadened linewidths in the 25-μ band of HF and the influence of polymer formation

Pressure-broadened linewidths for the fundamental band of HF were determined in laser absorption experiments for the lines P4 through P13. Measurements were made in the temperature range 23–150 °C for the lines P9–P13 and the absorption cross section was found to be independent of temperature. HF polymer formation was observed and the average polymer chain length was determined to be 4.6 with an enthalpy of formation of 26.5 Kcal.

Pressure-broadened linewidths of ethylene

Self-Broadened and foreign-gas (N 2 and O 2) broadened linewidths of C 2H 4 at 300°K have been calculated using the Anderson-Tsao-Curnutte theory of line broadening for a wide range of quantum numbers J and K α, for all three types of bands. The molecular quadrupole moment tensor of C 2H 4 is taken from Mulder and Huiszoon. Air-broadened linewidths of ethylene at 200°K have also been computed, so that the temperature dependence can be estimated.

Pressure-broadened linewidths of the R(95)_3/2 NO transition

Pressure-broadened ir linewidths of nitric oxide are currently used in measurement of its concentration for studies on combustion, chemical kinetics, and air pollution.(1) We have measured the self-broadened width of the R(9.5)(3/2) transition in the NO fundamental band by CO laser transmittance and found a halfwidth of gamma = 0.0783 +/- 0.0067 cm(-1) atm(-1). The analysis is shown to depend on the treatment of A-doubling, even though the doublet spacing is less than the Doppler width. The foreign gas broadening by N(2) of this transition is similarly measured, and the ratio of self-broadening ability to the broadening ability of N(2) is found to be 1.30 +/- 0.31.

Tunable Laser Measurements of Line Intensities and Pressure-Broadened Linewidths of HBr: Ar on the HBr Infrared Fundamental Region

Tunable diode laser spectroscopy has made measurements of line intensities, pressure-broadened linewidths, and pressure-induced line shifts more or less routine because of the very high resolving power available with these devices and the relatively large tuning range possible with one laser.

Scaling theoretic deconvolution of bulk relaxation data: State-to-state rates from pressure-broadened linewidths

Inversion of certain types of relaxation data is shown to be possible by application of two new developments. First, a recently developed scaling theory of rotationally inelastic cross sections is utilized in deriving the corresponding formulas for the state-to-state rate constants kjj′. Second, practical means are presented for assessing the number of independent pieces of information contained in the experimental data. Application of this work to simple relaxation times (i.e., T1 or T2) provides an essential reduction in the number of unknowns and allows for the determination of the rates k0Δ by inversion of a set of algebraic equations subject to the constraints k0Δ?0. The dimensionality of this set of equations is related to the number of independent pieces of information contained in the experimental data. A stable and fast method for solving the equations is given. The inversion of pressure broadening data to yield state-to-state rate constants is illustrated for the CO–, O2–, N2–rare gas systems.

Pressure-broadened linewidths of HNO3

N2 and O2 broadened linewidths of NHO3 at 300°K have been computed for about 1500 transitions of a- and c-type and for about 2500 transitions of b-type. (AIP)

Two-Photon Coherent Transient Measurements of Doppler-Free Linewidths with Broadband Excitation

We report the first measurements of Doppler-free two-photon linewidths obtained with a simple coherent transient technique using a wide-bandwidth laser. Measurements of two-photon optical-free-induction decay are used to determine pressure-broadening coefficients for the $3S\ensuremath{-}4D$ transition of sodium for collision with helium, neon, and argon. Pressure-broadened linewidths of several tens of MHz are measured with laser linewidths of several GHz.