SPECTRAL LINE PARAMETERS FOR THE 9 BAND OF ETHANE

Malathy Devi & Chris Benner, W&MRinsland & Smith, NASA Langley

Bob Sams & Tom Blake, PNNLJean-Marie Flaud, CNRS, France

Keeyoon Sung & Linda Brown, JPLArlan Mantz, Connecticut College

Why we did?What we did?How we did?Challenges

WHY ETHANE? WHY THE 9 BAND?1. 12-µm emission features of ethane are seen in the spectra from outer solar system bodies of Jupiter, Saturn, Neptune and Titan.

2. The 9 band, especially the RQ0 sub-band (0 =~ 822 cm-1) is of considerable interest due to its importance in molecular astrophysics and also because it is

often used in remote sensing applications.

3. Laboratory measurements are required to convert the raw observational data of planetary observations into information useful for quantification,.

4. laboratory measurements normally include parameters such as line positions, intensities, pressure-broadened widths and shifts as a function of temperature.

The purpose of the present investigation is to provide new and accurate measurements of individual spectral line parameters for RQ0 AND several other Q, P, R sub-band transitions.

Titan’s atmosphere consists predominantly of N2 and measurable quantities of several organic molecules including ethane. Present investigations involve spectra of ethane and ethane broadened with nitrogen at various temperatures (298 K to 149 K), pressures and absorption path lengths.

Comparison of a Spectrum of Titan to our Laboratory SpectrumTitan’s

AtmosphericSpectrum

showing the C2H6

features.Courtesy of: Henry Roe,

Lowell Observatory

A laboratory spectrum

recorded with the Bruker

125HR FTS at JPL

RQ(J, 2) Sub-band is shown in both figures.

43 high resolution spectra with sample temperatures between 149 K and 298 K are fitted simultaneously.

(17 pure C2H6 and 26 C2H6+N2 spectra).Temperature

(K)Gas Mixture C2H6 Volume

Mixing RatioPath (cm) Pressure

Range (Torr)Number of Spectra

297.2 C2H6 1.0 324.0±2.0 0.3 1

298.2 C2H6 1.0 20.00 ±0.02 4.5-36 5

297.2 C2H6+N2 0.011-0.045 324.0 12-57 5

298.2 C2H6+N2 0.08-0.2 20.00 30-181 6

273.2 C2H6 1.0 20.00 3.8- 16 3273.2 C2H6+N2 ~0.2 20.00 26-53 3248.2 C2H6 1.0 20.00 4-6 3248.2 C2H6+N2 ~0.2 20.00 26-50 3223.2 C2H6 1.0 20.00 4-6 3223.2 C2H6+N2 ~0.2 20.00 25-50 3211.0 C2H6 1.0 20.00 4-6 3211.0 C2H6+N2 ~0.2 20.00 25-50 3148.3 C2H6 1.0 20.387±0.001 6.2 1149.7 C2H6+N2 ~0.17 20.387 34.554 1

Top: A spectrum of

pure (99.5%) C2H6 at 149 K

with 6.2 Torr in a 20.387 cm

cell

Bottom: A low temperature (149 K) N2-broadened

C2H6 spectrum with a volume

mixing ratio of 0.17

1. We applied a multispectrum fitting techniquea to simultaneously fit all 43 spectra of C2H6 in the 9 band. High resolution (0.0016-0.005 cm-1) spectra were recorded with two different FTS (PNNL and JPL).

2. Intensities & separations for torsional split components, widths and their temperature dependences were constrained in the analysis to determine the line parameters for both torsional split components. No pressure shifts needed to fit.

3. Accurate line positions and absolute intensities were retrieved for over 1700 transitions of 9. N2- and self-broadened half width coefficients and their temperature dependences were also obtained for more than 1350 transitions at various sample temperatures between 149 K and 298 K.

4. Variations of the observed widths and their temperature dependences with respect to J, K quanta are discussed. Present results are compared with previously reported measurements and calculations.======a D. Chris Benner, C.P. Rinsland, V. Malathy Devi, M.A.H. Smith, and D.A. Atkins. JQSRT 1995;53:705-721.

A few sub bands near the prominent RQ0

RQ0 sub-band.(a) Low pressure C2H6 spectrum illustrating the high density of lines.

(b) N2-broadened C2H6 spectrum. Features of Individual J transitions are completely obscured.

Multispectrum technique allows fitting such spectra to great advantage.

Only two of the 43 spectra used in the analysis are shown here.

0.1 cm-1 wide interval in RQ0 sub-band.

a) low-pressure spectrum with L= 324 cm; P= 0.3 Torr C2H6

at T= 297.2 K

Each transition has two components with different J, symmetry species

and intensity.

(b) C2H6+N2 spectrum with P(total)=180 Torr, L=20 cm; T=298.2 K.

Examples of Torsional Splittings & Statistical Weights

RQ0 sub-band

A few sample

spectra ofC2H6+N2

recorded at various

temperatures pressuresand paths are shown

Different sub-band

series(PQ, RP, PP)

Effects of temperature and pressure

upon the relative

strengths of the various transitions

SOME ANALYSIS DETAILSFor the PNNL spectra, the calibrations of the wavenumber scales

were done relative to OCS line positions from the NIST wavenumber Ref. data [Maki & Wells]

For the JPL spectra the calibration was achieved with respect to positions of 2 water vapor transitions.

The initial values for line positions, intensities, N2- and self-Widths taken from updates to HITRAN2008 database.

1. Positions, intensities, N2- and Self Widths, T-dependences of N2- and Self Widths measured for over 1330 transitions in 17 Q sub-bands (and several PP, RR, RP and PR sub-bands).

2. Standard Voigt line profile was sufficient (no line mixing or speed dependence) to fit individual manifolds in all sub-bands.

3. Measured N2- and Self-Widths vs. J and K quanta are studied.

4. Temperature dependence exponents for N2- and Self Widths vs. M (M = J′=J″ for Q sub-bands) are discussed.

In the global least squares fits, the pressure-broadened half-width coefficients were determined using the

expression:

2

000

0

1

0002

0 ),)(()1)(,)((),(n

L

n

LL T

TTpselfb

T

TTpNbpTpb

bL (p, T) is the Lorentz half-width (in cm-1) of the spectral line at pressure p and temperature T.

bL0(Gas)(p0, T0) is the Lorentz half-width coefficient of the

line at the reference pressure p0 (1 atm) and temperature T0 (296 K) of the broadening gas (either N2 or C2H6 in this case).

is the ratio of the partial pressure of C2H6 to the total sample pressure in the cell.

43 spectra fitted simultaneously.

Tick marks at the top correspond to

all transitions (>430) included

in the fit. HB=HOT BAND

(9+4-4)

No pressure-induced shifts, line mixing or

speed dependence

required to fit the spectra.

RQ0 sub-band head.The top panel shows

the fitted spectra and the bottom

panel represents the weighted fit

residuals on a magnified vertical

scale.

Overlap & blending of

fundamental and hot-band

transitions further complicates the

analysis.

Well separated J,K transitions in this PP series vary from 7 to17(J″) and 1 to 6 (K″). The torsional split components overlap at high

pressures.

Hot-band transitions (9+-) are

marked with (*)

No pressure-induced shifts

were required to fit even these well-separated

lines.

LINE INTENSITIES IN C2H6 9 BAND1. Intensity measurements in the most recent HITRAN and GEISA databases are based upon analysis by J. Vander Auwera, N. Moazzen-Ahmadi, Jean-Marie Flaud. Astrophys J. 2007;662:750-757. Those values are used as initial input in the present analysis.

2. Line Intensities measured in this work are lower by ~15% from HITRAN 2008 values.

3. Line intensities in the databases were normalized to the band intensity from a medium resolution spectrum recorded at PNNL based on the integrated area under the entire region of the band.

4. Intensities are based upon 43 high-resolution spectra recorded by TWO different Bruker FTS (PNNL and JPL). Measurements are obtained fitting all spectra simultaneously. Absolute uncertainties in intensity measurements are estimated to about ±5%. Differences from the databases are probably due to the difficulty measuring the intensities in medium resolution spectrum?

Triangles and circles are the

measured widths from

multispectrum fit.

The stars correspond to widths that are

calculated from

constrained n by empirical linear fits:

n= a+b (J-c)

Measured N2- and Self-Widths vs. M

(M=J′=J″ for Q sub-bands).

Units of widths are cm-1 atm-1 at 296 K.

LEFT: rQ bandsRED: Self-WidthsBLUE: N2-Widths

RIGHT: pQ bandsRED: Self-WidthsBLUE: N2-Widths

Ratio of Self-Widths to N2-

Widths =1.40±0.05

Measured values of n from multi-

spectrum fit.

Measured T-dependence

exponents (n) of N2- and Self-Widths vs. M (M=J′=J″) in 17

Q sub-bands.

LEFT: rQ Sub-bandsRED: Self-WidthsBLUE: N2-Widths

RIGHT: pQ Sub-bands

RED: Self-WidthsBLUE: N2-Widths

n for Self-Widths < n for

N2-Widths

Temperature dependence exponents

For N2- and Self Widths

obtained fitting an empirical

linear equation

“Fitted” Temperature dependence exponents (n) for N2- and Self-

Widths vs. M (M=J′=J″ for Q sub-bands).

LEFT: rQ Sub-BandsRED: Self-WidthsBLUE: N2-Widths

RIGHT: pQ Sub-Bands

RED: Self-WidthsBLUE: N2-Widths

n for self-Widths < n for N2-

Widths

Measured N2- and Self-Widths vs. M (M=J′=J″ for all Q

sub-bands).

RED: Self-WidthsBLUE: N2-Widths

“Fitted” n using an empirical linear

Equation: n= a + b(J-c)

BLUE: N2-WidthsRED: Self-Widths

Mean N2- to Self-Width =

1.40±0.05

(a)N2-Widths vs. K″

BLUE: pQ Sub-Bands

RED: rQ Sub-Bands

(b) Self-Widths vs. K”

BLUE: pQ Sub-Bands

RED: rQ Sub-Bands

K″+0.05*(J″-K″) is used for

pattern recognition

Half-width coefficients are in

Units of cm-1 atm-1 at 296 K

Values for a and b using n=a+b(J-c), and the half-width coefficients for the highest J (=M=J″=J′ in Q sub-bands) for 9 measured rQ sub-bands.

Sub-band For N2 broadening For self broadening For N2 & self

broadening

a b Width at J

a b Width at J

c and J

pQ(J,K=9) 0.884(5) 0.0076(11) 0.998(17) 0.592(6) 0.0092(12) 0.731(18) 16, 31

pQ(J,K=8) 0.799(5) 0.0096(12) 0.933(18) 0.591(6) 0.0109(13) 0.744(19) 16, 30

pQ(J,K=7) 0.841(3) 0.0131(7) 1.02(12) 0.647(5) 0.0115(8) 0.809(13) 16, 30

pQ(J,K=6) 0.878(3) 0.0158(6) 1.10(10) 0.694(5) 0.0172(7) 0.936(11) 16, 30

pQ(J,K=5) 0.787(3) 0.0131(5) 1.06(1) 0.579(4) 0.0137(6) 0.867(13) 13, 34

pQ(J,K=4) 0.821(2) 0.0087(3) 0.987(7) 0.650(4) 0.0079(5) 0.800(9) 13, 32

pQ(J,K=3) 0.878(2) 0.0162(4) 1.18(7) 0.792(3) 0.0085(5) 0.964(9) 13, 32

pQ(J,K=2) 0.781(2) 0.0054(3) 0.895(7) 0.690(4) 0.0041(4) 0.775(9) 13, 34

pQ(J,K=1) 0.808(2) 0.0123(3) 1.04(6) 0.706(3) 0.0074(4) 0.847(9) 13, 32

The half-width coefficients are in units of cm-1 atm-1 at 296 K.

Comparison of Temperature Dependences (n) of Self-Widths:This Study vs. Nguyen et al. (J. Mol. Spectrosc. 2008;39:429-434)

a This study. The error bars for positions and temperature dependence exponents are twice the standard deviation.b Nguyen et al. Reported temperature dependence exponents were calculated from their measured self-broadened half-width coefficients at three different temperatures (242.2, 226.2 and 150.2 K).

Line (cm-1)a n (This work)Voigt profilea

n Rautian profileb

pQ(17,9) 798.93257(1) 0.602 ± 0.012 0.636 ± 0.123pQ(16,9) 798.97339(1) 0.592 ± 0.012 0.620 ± 0.104pQ(15,9) 799.01179(1) 0.583 ± 0.012 0.621 ± 0.110

pQ(13,5) 809.13646(1) 0.579 ± 0.008 0.659 ± 0.119pQ(12,5) 809.16832(1) 0.566 ± 0.008 0.707 ± 0.113pQ(11,5) 809.19773(1) 0.552 ± 0.010 0.643 ± 0.133pQ(7,5) 809.29055(1) 0.497 ± 0.012 0.669 ± 0.104pQ(6,5) 809.30729(1) 0.483 ± 0.014 0.742 ± 0.098

pQ(13,2) 816.86769(1) 0.690 ± 0.008 0.735 ± 0.099pQ(12,2) 816.90092(1) 0.686 ± 0.008 0.758 ± 0.128pQ(11,2) 816.93136(1) 0.682 ± 0.008 0.664 ± 0.115

SUMMARY & CONCLUSIONS

1. 43 high-resolution (0.0016-0.005 cm-1) spectra of pure and N2-broadened C2H6 are fitted

simultaneously to retrieve:

2. Positions, absolute intensities, N2- and self-Widths and the temperature dependences of N2- and self-Widths measured for over 1300 J, K transitions in 17 Q sub-bands and several pP, rR, rP and pR sub-

bands.

3. Mean Ratio of Self-Widths to N2-Widths is 1.40±0.05

4. No pressure-induced shifts, line mixing or speed dependence were detected in the spectra.

(PTO)

SUMMARY & CONCLUSIONS

T-dependence exponents for N2-Widths are larger than for self-Widths and their values as well as ratios vary with J, K quanta. The “n” values are fitted to empirical linear relationship:

n = a+b (J-c), ✕for each broadening gas in each sub-band structure.

Line intensities from this work are found to be ~15% lower than those listed in the HITRAN 2008 Database. The reason for this discrepancy has recently been investigated from measurements on a (new) spectrum recorded at JPL and the new intensities confirm our present measurements.

C2H6:  Keeyoon (JPL) recorded a (normal sample) ethane spectrum at room temperature in the 12 µm region and Linda Brown compared it with a synthetic spectrum using the HITRAN 2008 linelist (a rough estimation). The cell path of 14.93 cm needed adjustment to 13.25 cm to fit the observed spectrum with the calculated one.

Importance of Present Laboratory Investigation

1. Using the parameters determined in this study the spectrum of ethane can be computed at any temperature below 296 K at infinite resolution. 

2. From there one simply applies the instrument line shape to compare to a spectrum of Titan.

3. The importance of the intensity problem is that it affects the retrieved mixing ratio on a one to one basis. 

4. Lorentz widths are temperature and pressure dependent and we have described that dependence with great accuracy. 

5. We have also got enough information (except for the unidentified lines) to describe the line intensity as a function of temperature-and no new lines will appear at lower temperatures than presently obtained (149 K).

ACKNOWLEDGMENTSThe experimental spectra for the present study were recorded at the W. R. Wiley Environmental Molecular Sciences Laboratory, a national scientific user facility sponsored by the Department of Energy’s Office of Biological and Environmental Research located at Pacific Northwest National Laboratory (PNNL) and the Jet Propulsion Laboratory (JPL) in Pasadena, California. PNNL is operated for the United States Department of Energy by the Battelle Memorial Institute under Contract DE-AC05-76RLO1830.

NASA’s planetary atmospheres program supported the work performed at NASA Langley Research Center and the College of William and Mary. The research at the JPL and Connecticut College was performed under contracts and grants with NASA.