Off-diagonal Relaxation Matrix Element Research Articles

Positions, intensities and line shape parameters for the 1←0 bands of CO isotopologues

Positions and intensities for transitions in the 1←0 bands of the four CO isotopologues (12C16O, 13C16O, 12C18O and 13C18O) were retrieved from analyses of spectra of different low-pressure pure samples and high-pressure air-broadened mixtures (1940–2260 cm−1) in two different multispectrum fittings involving selected groups of 21 (for 13C16O) and 20 (for 12C18O) spectra. Air-broadened half-width and air-shift coefficients, their temperature dependences, line mixing via off-diagonal relaxation matrix element coefficients for CO-air collision systems, and quadratic speed dependence parameters were measured for 13C16O and 12C18O transitions. In both the fittings, two low-pressure room-temperature spectra of a high purity natural sample of CO recorded with the Kitt Peak FTS were included to retrieve the positions and intensities of 12C16O transitions. All other spectra in the fittings were obtained with either 13C-enriched or 18O-enriched samples using the JPL Bruker IFS-125HR FTS. Four absorption cells with path lengths between 0.5131(5) and 20.38(2) cm were used to record the data. The cells are temperature controlled, except for the shortest cell. Positions and intensities for the 1←0 band transitions were determined from the retrieved ro-vibrational constants (G, B, D and H), band intensity and Herman-Wallis parameters by applying the theoretical quantum mechanical expressions. The band strengths of 12C16O, 12C18O and 13C18O are very close (∼0.3%) to HITRAN2012 values but for 13C16O the band strength is ∼4.8% larger than the HITRAN2012 and 2.6% larger than the HITRAN2016 value.

Line shape parameters of air-broadened water vapor transitions in the ν1 and ν3 spectral region

A Bruker IFS-120HR Fourier transform spectrometer located at the Pacific Northwest National Laboratory (PNNL) in Richland, Washington was used to record a series of spectra of pure H2O and air-broadened H2O in the regions of the ν1 and ν3 bands (3450–4000 cm−1) at different pressures, temperatures and volume mixing ratios of H2O in air. Eighteen high-resolution, high signal-to-noise (S/N) ratio absorption spectra were recorded at T = 268, 296 and 353 K using two temperature-controlled absorption cells with path lengths of 9.906(1) and 19.95(1) cm. The resolution of the spectra recorded with the 9.906 cm and 19.95 cm absorption cells was 0.006 and 0.008 cm−1, respectively. A multispectrum nonlinear least squares fitting technique was employed to fit all the eighteen spectra simultaneously to retrieve 313 accurate line positions, 315 intensities, 229 Lorentz air-broadened half-width and 213 air-shift coefficients and their temperature dependences (136 for air-broadened width and 128 for air-shift coefficients, respectively). Room temperature self-broadened half-width coefficients for 209 transitions and self-shift coefficients for 106 transitions were also measured. Line mixing coefficients were experimentally determined for isolated sets of 10 transition pairs for H2O-air and 8 transition pairs for H2O-H2O using the off-diagonal relaxation matrix element formalism, and 85 quadratic speed dependence parameters were measured. Modified Complex Robert-Bonamy (MCRB) calculations of self-, and air-broadened (from N2- and O2-broadening) half-width and air-shift coefficients, and temperature dependence exponents of air-broadened half-width coefficients are made. The measurements and calculations are compared with each other and with similar parameters reported in the literature.

Line parameters for CO2- and self-broadening in the ν3 band of HD16O

Pressure-broadened line shape parameters of transitions in the ν3 band of HDO (ν0=3707.4cm-1) were measured from spectra of HDO and mixtures of HDO and CO2 for application to accurate retrievals of HDO abundances and D/H ratios for CO2-rich planetary atmospheres of Mars and Venus. A few calculated line lists have recently been published on HDO-CO2 line shapes and their temperature dependences, but the present study represents the first laboratory measurements of those parameters in the ν3 band; Measurements for nearly 100 transitions in the ν3 band have been made. Room temperature measurements of self-broadened width and shift coefficients for all of these transitions, line mixing via off-diagonal relaxation matrix element coefficients and quadratic speed dependence parameter were measured for the majority of these transitions. All these measurements were made by simultaneously fitting eleven high-resolution spectra of HDO and HDO-CO2 mixtures at various temperatures and pressures recorded with the Bruker Fourier transform spectrometer at the Jet Propulsion Laboratory. Two specially built coolable absorption cells with path lengths of 20.38cm and 20.941m were used to contain the sample mixtures. Multispectrum nonlinear least squares fitting algorithm was employed in the analysis.Calculations using the Modified Complex Robert-Bonamy formalism (MCRB) were made for the half-width coefficients, their temperature dependences and pressure shift coefficients for the HDO–CO2 and HDO–HDO collision systems. The calculations were made for all ν3 band transitions in the 1100–4100cm-1 region on the HITRAN2012 database. Present measurements are compared with the MCRB calculations and other literature values.

Spectroscopic line parameters of 12CH4 for atmospheric composition retrievals in the 4300–4500 cm−1 region

Due to the importance of methane as a trace atmospheric gas and a greenhouse gas, we have carried out a precise line-shape study to obtain the CH4–CH4 and CH4–air half-width coefficients, CH4–CH4 and CH4–air shift coefficients and off-diagonal relaxation matrix element coefficients for methane transitions in the spectral range known as the “methane Octad”. In addition, the associated temperature dependences of these coefficients have been measured in the 4300–4500cm−1 region of the Octad. The high signal to noise ratio spectra of pure methane and of dilute mixtures of methane in dry air with high resolution have been recorded at temperatures from 148K to room temperature using the Bruker IFS 125 HR Fourier transform spectrometer (FTS) at the Jet Propulsion Laboratory, Pasadena, California. The analysis of spectra was done using a multispectrum non-linear least-squares curve fitting technique. Theoretical calculations have been performed and the results are compared with the previously published line positions, intensities and with the line parameters available in the GEISA and HITRAN2012 databases.

Line parameters including temperature dependences of self- and air-broadened line shapes of 12C16O2: 1.6-μm region

Pressure-broadened line shapes in the 30013←00001 (ν1+4ν20+ν3) band of 12C16O2 at 6228cm−1 are reanalyzed using new spectra recorded with sample temperatures down to 170K. 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 (170-296K). Various spectral resolutions (0.004–0.011cm−1), absorption path lengths (2.46–121m) and CO2 samples (natural and 12C-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 13C16O2, the 30013←00001 band of 12C16O18O, hot bands 31113←01101 and 32212←02201 of 12C16O2, as well as the 40013←10001 and the 40014←10002 bands of 12C16O2, present within the fitted interval were also measured. Results from previous works and new calculations are compared to present measurements, where appropriate.

Spectral line parameters including line shapes in the 2ν3 Q branch of 12CH4

In this study, we report the first experimental measurements of spectral line shape parameters (self- and air-broadened Lorentz half-widths, pressure-shifts, and line mixing (via off-diagonal relaxation matrix elements) coefficients and their temperature dependences, where appropriate) for transitions in the 2ν3 Q branch manifolds, Q(11)–Q(1) of methane (12CH4), in the 5996.5–6007-cm−1 region. The analysis included 23 high-resolution, high signal-to-noise laboratory absorption spectra recorded with the Bruker IFS-125HR Fourier transform spectrometer (FTS) at JPL. The experimental data were obtained using 12C-enriched 12CH4 and dilute mixtures of 12CH4 in dry air in the 130–296K range using a room-temperature long path absorption cell and, two custom-built coolable cells. In the analysis, an interactive multispectrum fitting software was employed where all the 23 spectra (11 self-broadened and 12 air-broadened) were fit simultaneously. By carefully applying reasonable constraints to the parameters for severely blended lines, we were able to determine a self-consistent set of broadening, shift and line mixing (relaxation matrix coefficients) parameters for CH4–CH4 and CH4–air collisions. In the majority of cases, a quadratic speed dependence parameter common for all transitions in each Q(J) manifold was determined. However, temperature dependences of the Q branch line mixing parameter could not be determined from the present data. Since no other experimental line shape measurements have been reported for this Q-branch, the present results are compared to available values in the HITRAN2012 database.

Self- and air-broadened line shapes in the 2ν3 P and R branches of 12CH4

In this paper we report line shape parameters of 12CH4 for several hundred 2ν3 transitions in the spectral regions 5891–5996cm−1 (P branch) and 6015–6115cm−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 0.0067cm−1) and high signal-to-noise (S/N) spectra of high-purity 12CH4 and the same high-purity 12CH4 broadened by dry air recorded at different sample temperatures between 130K and 295K 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 13.09m for room temperature data, (2) a single-pass 0.2038m long coolable cell (for self-broadening) and (3) a multipass cell with 20.941m total path coolable Herriott cell (for air-broadening). In total there were 13 spectra with pure 12CH4 (0.27–599Torr) and 12 air-broadened spectra with total sample pressures of 80–805Torr 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.

Self- and air-broadened line shape parameters in the ν2+ν3 band of 12CH4: 4500–4630 cm−1

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 ν2+ν3 absorption lines in the 4500–4630cm−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% 12C-enriched) samples of pure methane and its dilute mixtures in dry air between 298K and 148K. 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 20.38cm long coolable cell installed in its sample compartment. The sample pressures for the pure 12CH4 spectra were 4.5−385Torr; for the air-broadened spectra the total pressures ranged between 95 and 300Torr 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.

Line mixing in some water vapor transitions perturbed by N2, Ar and He pressure

Line mixing effect has been studied for five pairs of H2O lines perturbed by N2, Ar and He. Two pairs of lines (in terms of the rotational quantum numbers J, Ka, and Kc) [101]←[212], [212]←[303] and [303]←[212], [212]←[101] belong to the ν2 band, two pairs of lines [312]←[413], [321]←[422] and [431]←[330], [422]←[321], belong to the ν1+ν2+ν3 band and fifth pair of the lines [541]←[652], [541]←[634] belong to the 3ν1+ν2 and 8ν2 band, respectively. Pressure broadening coefficients of H2O (half-widths, pressure shifts and off-diagonal relaxation matrix elements) have been calculated for all lines and for two pair of lines from the ν1+ν2+ν3 band perturbed by N2 and Ar they have been obtained experimentally.

Line shape parameters of PH3 transitions in the Pentad near 4–5 μm: Self-broadened widths, shifts, line mixing and speed dependence

Accurate knowledge of spectroscopic line parameters of PH3 is important for remote sensing of the outer planets, especially Jupiter and Saturn. In a recent study, line positions and intensities for the Pentad bands of PH3 have been reported from analysis of high-resolution, high signal-to noise room-temperature spectra recorded with two Fourier transform spectrometers (2014) [1]. The results presented in this study were obtained during the analysis of positions and intensities, but here we focus on the measurements of spectral line shapes (e.g. widths, shifts, line mixing) for the 2ν4, ν2+ν4, ν1 and ν3 bands. A multispectrum nonlinear least squares curve fitting technique employing a non-Voigt line shape to include line mixing and speed dependence of the Lorentz width was employed to fit the spectra simultaneously. The least squares fittings were performed on five room-temperature spectra recorded at various PH3 pressures (∼2–50Torr) with the Bruker IFS-125HR Fourier transform spectrometer (FTS) located at the Pacific Northwest National Laboratory (PNNL), in Richland, Washington. Over 840 Lorentz self-broadened half-width coefficients, 620 self-shift coefficients and 185 speed dependence parameters were measured. Line mixing was detected for transitions in the 2ν4, ν1 and ν3 bands, and their values were quantified for 10 A+A− pairs of transitions via off-diagonal relaxation matrix element formalism. The dependences of the measured half-width coefficients on the J and K rotational quanta of the transitions are discussed. The self-width coefficients for the ν1 and ν3 bands from this study are compared to the self-width coefficients for transitions with the same rotational quanta (J, K) reported for the Dyad (ν2 and ν4) bands. The measurements from present study should be useful for the development of a reliable theoretical modeling of pressure-broadened widths, shifts and line mixing in symmetric top molecules with C3v symmetry in general, and of PH3 in particular.

Spectral line parameters including temperature dependences of air-broadening for the 2 ← 0 bands of 13C16O and 12C18O at 2.3 μm

The first air broadening line shape parameters were determined for the 2←0 bands of 13C16O near 4166.8cm−1 and 12C18O near 4159.0cm−1. Air-broadened Lorentz half-width coefficients, their temperature dependence exponents; air-induced pressure shift coefficients, their temperature dependences; and air line mixing coefficients were measured. Additionally, speed-dependent line shapes with line mixing employing the off-diagonal relaxation matrix element coefficients were applied to minimize the fit residuals. Finally, individual line positions and line intensities of the two isotopologues were constrained to the well-known theoretical quantum mechanical expressions in order to obtain the rovibrational (G, B, D and H) and band intensity parameters (including Herman–Wallis coefficients). For this, laboratory spectra were recorded at 0.005cm−1 resolution using a temperature-controlled coolable absorption cell configured inside a Bruker IFS 125HR Fourier transform spectrometer. Gas temperatures and pressures for the spectra varied from 150 to 298K and 20 to 700Torr, respectively. Results were obtained from broad-band multispectrum least-squares fitting of the 4000–4360cm−1 spectral region. Four isotope-enriched pure sample spectra and twelve spectra with air+CO samples (13C16O or 12C18O, as appropriate) were fitted simultaneously. The results obtained for 13C16O and 12C18O are compared and discussed.

Spectral line parameters including temperature dependences of self- and air-broadening in the 2←0 band of CO at 2.3 μm

Temperature dependences of pressure-broadened half-width and pressure-induced shift coefficients along with accurate positions and intensities have been determined for transitions in the 2←0 band of 12C16O from analyzing high-resolution and high signal-to-noise spectra recorded with two different Fourier transform spectrometers. A total of 28 spectra, 16 self-broadened and 12 air-broadened, recorded using high-purity (≥99.5% 12C-enriched) CO samples and CO diluted with dry air (research grade) at different temperatures and pressures, were analyzed simultaneously to maximize the accuracy of the retrieved parameters. The sample temperatures ranged from 150 to 298K and the total pressures varied between 5 and 700Torr. A multispectrum nonlinear least squares spectrum fitting technique was used to adjust the rovibrational constants (G, B, D, etc.) and intensity parameters (including Herman–Wallis coefficients), rather than determining individual line positions and intensities. Self- and air-broadened Lorentz half-width coefficients, their temperature dependence exponents, self- and air-pressure-induced shift coefficients, their temperature dependences, self- and air- line mixing coefficients, their temperature dependences and speed dependence have been retrieved from the analysis. Speed-dependent line shapes with line mixing employing off-diagonal relaxation matrix element formalism were needed to minimize the fit residuals. This study presents a precise and complete set of spectral line parameters that consistently reproduce the spectrum of carbon monoxide over terrestrial atmospheric conditions.

A multispectrum analysis of the ν 4 band of 13CH 4: Widths, shifts, and line mixing coefficients

Line positions, Lorentz air-broadened half width and air pressure-induced shift coefficients have been measured for nearly 200 transitions in the ν 4 band of 13CH 4 from high-resolution spectra recorded with the McMath-Pierce Fourier transform spectrometer. Three room temperature spectra of 13CH 4 used in the previous study of Malathy Devi et al. (Air-broadened Lorentz halfwidths and pressure-induced line shifts in the ν 4 band of 13CH 4. Appl. Opt. 1988; 27: 2296–2308) were analyzed together with a large number of additional spectra of self- and air-broadened CH 4 recorded at 210–314 K and one room-temperature spectrum of self-broadened 13CH 4. Analyses applying the multispectrum nonlinear least squares fitting technique were performed to retrieve the spectral line parameters. In addition to air-broadened half width and shift coefficients, self-broadened half width and shift coefficients were determined for at least 56 13CH 4 ν 4 transitions. Off-diagonal relaxation matrix element coefficients for air-broadened line mixing were also determined for 28 pairs of P and R transitions in a number of J manifolds, and mixing parameters for self-broadening were also determined for some of these pairs. Temperature-dependences of the pressure-induced shift and mixing parameters for the 13CH 4 lines could not be determined from the spectra used in the present analysis, but temperature dependences of the half width coefficients were determined for the strongest transitions. The results of this study are compared with other studies of air- and self-broadened 13CH 4 and 12CH 4.

Multispectrum analysis of 12CH 4 in the ν 4 spectral region: II. Self-broadened half widths, pressure-induced shifts, temperature dependences and line mixing

Accurate values for line positions, absolute line intensities, self-broadened half width and self-pressure-induced shift coefficients have been measured for over 400 allowed and forbidden transitions in the ν 4 band of methane ( 12CH 4). Temperature dependences of half width and pressure-induced shift coefficients were also determined for many of these transitions. The spectra used in this study were recorded at temperatures between 210 and 314 K using the National Solar Observatory's 1 m Fourier transform spectrometer at the McMath-Pierce solar telescope. The complete data set included 60 high-resolution (0.006–0.01 cm −1) absorption spectra of pure methane and methane mixed with dry air. The analysis was performed using a multispectrum nonlinear least squares curve fitting technique where a number of spectra (20 or more) were fit simultaneously in spectral intervals 5–15 cm −1 wide. In addition to the line broadening and shift parameters, line mixing coefficients (using the off-diagonal relaxation matrix element formalism) were determined for more than 50 A-, E-, and F-species transition pairs in J manifolds of the P- and R-branches. The measured self-broadened half width and self-shift coefficients, their temperature dependences and the line mixing parameters are compared to self-broadening results available in the literature and to air-broadened parameters determined for these transitions from the same set of spectra.

Multispectrum analysis of 12CH 4 in the ν4 band: I. : Air-broadened half widths, pressure-induced shifts, temperature dependences and line mixing

Lorentz air-broadened half widths, pressure-induced shifts and their temperature dependences have been measured for over 430 transitions (allowed and forbidden) in the ν 4 band of 12CH 4 over the temperature range 210–314 K. A multispectrum non linear least squares fitting technique was used to simultaneously fit a large number of high-resolution (0.006–0.01 cm −1) absorption spectra of pure methane and mixtures of methane diluted with dry air. Line mixing was detected for pairs of A-, E-, and F-species transitions in the P- and R-branch manifolds and quantified using the off-diagonal relaxation matrix elements formalism. The measured parameters are compared to air- and N 2-broadened values reported in the literature for the ν 4 and other bands. The dependence of the various spectral line parameters upon the tetrahedral symmetry species and rotational quantum numbers of the transitions is discussed. All data used in the present work were recorded using the McMath–Pierce Fourier transform spectrometer located at the National Solar Observatory on Kitt Peak.

Low-temperature measurements of HCN broadened by N 2 in the 14-μm spectral region

Half-width and pressure-induced shift coefficients; the temperature-dependence exponents of the half-widths and the temperature-dependence coefficients of pressure-induced shifts have been measured for N 2-broadened transitions in the ν 2 band of HCN. Line positions and intensities were also determined. A total of 34 laboratory absorption spectra, recorded at 0.002–0.005 cm −1 resolution with two different Fourier transform spectrometers, were used in the determination of the spectral line parameters. The total pressures of the HCN–N 2 samples ranged from less than 1 torr up to nearly 1 atm, and temperatures were between 211 and 300 K. A multispectrum nonlinear least-squares fitting technique employing a modified Voigt line profile, including speed dependence and line mixing via the off-diagonal relaxation matrix formulation, was used in the analysis. Speed-dependence parameters were determined in the P and R branches of the ν 2 band of H 12C 14N, and in the ν 2 Q branches of H 12C 14N and H 13C 14N the off-diagonal relaxation matrix elements that characterize line mixing were included in the analysis to fit the data. Present results are compared with previous measurements reported in the literature.

Line mixing effects in the ν2 + ν3 band of methane

This study provides the first direct experimental measurements of the off-diagonal relaxation matrix element coefficients for line mixing in air-broadened methane spectra for any vibrational band and the first off diagonal relaxation matrix elements associated with line mixing for pure methane in the ν 2 + ν 3 band of 12CH 4. The speed-dependent Voigt profile with line mixing is used with a multispectrum nonlinear least squares curve fitting technique to retrieve the various line parameters from 11 self-broadened and 10 air-broadened spectra simultaneously. The room temperature spectra analyzed in this work are recorded at 0.011 cm −1 resolution with the McMath-Pierce Fourier transform spectrometer located at the National Solar Observatory, Kitt Peak, Arizona. The off-diagonal relaxation matrix element coefficients of ν 2 + ν 3 transitions between 4410 and 4629 cm −1 are reported for eighteen pairs with upper state J values between 2 and 11. The observed line mixing coefficients for self broadening vary from 0.0019 to 0.0390 cm −1 atm −1 at 296 K. The measured line mixing coefficients for air broadening vary from 0.0005 to 0.0205 cm −1 atm −1 at 296 K.

Line mixing and speed dependence in CO 2 at 6227.9 cm −1: Constrained multispectrum analysis of intensities and line shapes in the 30013 ← 00001 band

Line position, intensity and line shape parameters (Lorentz widths, pressure shifts, line mixing, speed dependence) are reported for transitions of the 30013 ← 00001 band of 16O 12C 16O ( ν 0 = 6227.9 cm −1). The results are determined from 26 high-resolution, high signal-to-noise ratio spectra recorded at room temperature with the McMath-Pierce Fourier transform spectrometer. To minimize the systematic errors of the retrieved parameters, we constrained the multispectrum nonlinear least squares retrieval technique to use quantum mechanical expressions for the rovibrational energies and intensities rather than retrieving the individual positions and intensities line by line. Self- and air-broadened Lorentz width and pressure-induced shift, speed dependence and line mixing (off-diagonal relaxation matrix elements) coefficients were adjusted individually. Errors were further reduced by simultaneously fitting the interfering absorptions from the weak 30012 ← 00001 band of 16O 13C 16O as well as the weak hot bands 31113 ← 01101, 32213 ← 02201, 40014 ← 10002 and 40013 ← 10001 of 16O 12C 16O in this spectral window. This study complements our previous work on line mixing and speed dependence in the 30012 ← 00001 band ( ν 0 = 6347.8 cm −1) [V.M. Devi, D.C. Benner, L.R. Brown, C.E. Miller, R.A. Toth, J. Mol. Spectrosc. 242 (2007) 90-117] and provides key data needed to improve atmospheric remote sensing of CO 2.

Line mixing and speed dependence in CO 2 at 6348 cm −1: Positions, intensities, and air- and self-broadening derived with constrained multispectrum analysis

Intensity and line shape parameters which predict spectral lines with absolute accuracies better than 0.3% have been determined for transitions of the 30012 ← 00001 band of 16O 12C 16O centered near 6348 cm −1 from 26 high resolution, high signal-to-noise ratio spectra recorded at room temperature with the McMath–Pierce Fourier transform spectrometer. To maximize the accuracies of the retrieved parameters, the multispectrum non-linear least squares retrieval technique was modified to adjust the rovibrational constants ( G, B, D, etc.) and intensity parameters, including Herman–Wallis terms, rather than retrieving the individual positions and intensities. Speed-dependent Voigt line shapes with line mixing were required to remove systematic errors in the fit residuals. Self- and air-broadening (widths and pressure-induced shifts, speed dependence parameters) and line mixing (off-diagonal relaxation matrix elements) coefficients were thus obtained in the multispectrum fit. Remaining errors were minimized by fitting the weak 30011 ← 00001 band of 16O 13C 16O as well as the weak hot bands 31112 ← 01101, 32212 ← 02201, 40012 ← 10001, and 40013 ← 10002 of 16O 12C 16O that contribute interfering absorptions in this spectral window. This study presents the most extensive set of measurements to date for self- and air-broadening and self- and air-shift coefficients of a near infrared band of CO 2. This is also the first study where line mixing parameters have been experimentally determined for any parallel CO 2 band.

Line mixing in self- and foreign-broadened water vapor at 6 μm

Line mixing has been measured for two pairs of transitions in the ν 2 band of H 2 16O. The rotational assignments of the four lines (in terms of the rotational quantum numbers J, K a and K c) are (a) two P branch lines: (1 0 1)←(2 1 2) mixing with (2 1 2)←(3 0 3) at 1539.061 and 1540.300 cm −1, respectively; and (b) two R branch lines: (3 0 3)←(2 1 2) mixing with (2 1 2) ←(1 0 1) at 1652.400 and 1653.267 cm −1, respectively. Pressure broadening coefficients of H 2O (halfwidths, pressure shifts and off-diagonal relaxation matrix elements) are reported for seven broadeners (self, H 2, He, CO 2, N 2, O 2 and air) from retrievals performed by multispectrum fitting implemented with line mixing. The effect is largest for self- and H 2-broadening.