1. INTERNATIONAL SYMPOSIUM ON MOLECULAR SPECTROSCOPY
71st Meeting, University of Illinois, Urbana-Champaign, June 20-24, 2016
SPECTRAL LINE SHAPE PARAMETERS FOR THE n1, n2 AND n3 BANDS OF HDO: SELF AND CO2 BROADENED V. MALATHY DEVI, D. CHRIS BENNER, Department of Physics, College of William and Mary, Williamsburg, VA K. SUNG, T.J. CRAWFORD, Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA A.W. MANTZ, Department of Physics, Astronomy and Geophysics, Connecticut College, New London, CT M.A.H. SMITH, Science Directorate, NASA Langley Research Center, Hampton, VA R.R. GAMACHE, C.L. RENAUD, Department of Environmental, Earth, and Atmospheric Sciences, University of Massachusetts, Lowell, MA G.L. VILLANUEVA, Astrochemistry, NASA Goddard Space Flight Center, Greenbelt, MD.
ACKNOWLEDGMENTS
The research performed at the College of William and Mary is supported by a Grant from NASA’s Mars Fundamental Research Program (NNX13AG66G). The research at Jet Propulsion Laboratory, California Institute of Technology, Connecticut College and NASA Langley Research Center was conducted under contracts and cooperative agreements with the National Aeronautics and Space Administration. Research performed at the University of Massachusetts Lowell is supported by the National Science Foundation through Grant No. AGS

2. OVERVIEW Objectives HDO sample preparation Experimental spectra
Spectral regions used in planetary studies
Examples of multispectrum fits
Data retrievals and analyses
7. Examples of line mixing (relaxation matrix)
8. Sample results: n1, n2 and n3 bands
9. Theoretical calculations using the MCRB formalism
10. Summary and conclusions

3. Principal Objectives
Accurate knowledge of CO2-broadened HDO (HDO-CO2) half width coefficients and their temperature dependences is important for proper interpretation of Mars data and to determine reliable D/H.
Combined measurements of HDO and H2O abundances are ideal and the best means to obtain Mars atmospheric D/H.
Room temperature measurements of H2O-CO2 in the n2 and rotational bands are available, but none so far for HDO in any band, even at room temperature.
At present, only unconfirmed theoretical predictions of broadening coefficients are available, impacting the predictions of the H2O and HDO retrievals and thereby D/H.
Significant systematic inaccuracies in the knowledge of the widths and their temperature dependences can adversely affect the retrievals of D/H.
Temperature dependences of CO2-broadened HDO line widths are critical since the uncertainty in the temperature dependence of half-widths leads to large errors in the retrieved mixing ratios and hence in column abundances.

4. HDO-CO2 half width coefficients and their temperature dependence exponents are important
Transitions belonging to the n2 (Th. Encrenaz, 2005) and n1 bands of HDO (Villanueva et al., 2011, 2012) have been observed in Mars atmospheric data.
HDO n3 band transitions were selected for the Mars Science Laboratory (MSL) mission at JPL [Chris Webster and Paul Mahaffy (2011)] using a Tunable Diode Laser spectrometer system.
Recent maps of atmospheric H2O and HDO across the martian globe [Villanueva et al. 2016] from ground-based observations showed many HDO lines especially in the n1 band near 2720 cm-1.
Inaccurate half widths and their Temp. dependence exponents can adversely affect the Mars D/H as illustrated in the next slide.

5. Curve of growth (integrated line area vs
Curve of growth (integrated line area vs. column density) for a strong (optically thick) HDO transition (2 2 0 ← of the n1 band) illustrating the potential effects of incorrect line widths on the retrieved HDO column densities. Using the N2-width (red curve) vs. the N2-width×1.8 (blue curve) as a proxy for HDO-CO2 widths would cause an error of ~46% in the retrieved column abundance. Using the wrong temperature dependence exponents for the half widths would produce similar effects in the retrieved column abundances.

6. Pure HDO does not exist in nature due to isotopologue
exchange reaction.
HDO samples are prepared by mixing high purity distilled H2O
and 99% pure D2O.
50%-50% mixtures of H2O and D2O were used resulting in:
H2O (50%) + D2O (50%) → H2O (25%) + HDO (50%) + D2O (25%)
HDO Band
Band strength
(cm-2 atm-1 at 296 K
Spectral region
Expected application n1
A type:
B type:
cm-1
(3.67 mm)
NIRSPEC/Keck-II, TEXES n2
A type:
B type:
cm-1
(7.13 mm)
Express, OMEGA, PFS on board the Mars n3
A type:
B type:
cm-1
(2.70 mm)
Express satellite SOIR. MAVEN mission, MSL etc.
A Type (parallel), B Type (perpendicular) or a mixture of both (hybrid).
n1, n2 and n3 band centers of H2O are: 3657 cm-1 (2.73 mm), 1595 cm-1 (6.26 mm), 3756 cm-1(2.66 mm), respectively.

7. DATA ANALYSIS Spectra were recorded using the Bruker 125HR FTS at JPL.
High Resolution FTS data and Multispectrum fittings
Spectra were recorded using the Bruker 125HR FTS at JPL.
A cm (straight path) and m (multipass Herriott) coolable cells designed and built by Prof. Arlan Mantz contained the gas samples.
Sample temperatures range from ~230 to 296 K.
The spectral resolutions were between and cm-1.
HDO (H2O+D2O+HDO) pressures were between 0.55 and 14.3 Torr.
Total HDO and CO2 pressures varied from 34 to 317 Torr. The volume mixing ratios (vmr) of HDO were in the ~ range (0.5 for HDO-HDO).
n2 lines of H2O and the n3 lines of CO2, as appropriate, were used for wavenumber calibration.
DATA ANALYSIS
Spectra obtained for each band were analyzed separately, but for each band, all spectra were fit simultaneously using the multispectrum fitting technique by Benner et al. (W&M).
Isotopologue abundances in each spectrum were determined from least squares fittings. These values were also needed in addition to the knowledge of vmr.

8. CO2-BROADENED HDO SPECTRA
n1 band
n2 band
n3 band

9. Configuration of the Bruker 125 HR FTS at JPL
Experimental conditions of HDO and HDO-CO2 spectra
Configuration of the Bruker 125 HR FTS at JPL
Light Source
Globar and Tungsten
Beam Splitter
KBr, CaF2
Detector
HgCdTe, InSb
Focal length: collimating lens
418 mm
Source aperture diameter mm
Filter band pass (cm-1)
, &
Resolution (cm-1) Unapodized
Maximum Optical Path Difference cm
Sample Pressure HDO (Torr)
Total pressure: HDO and CO2
~ (Torr)
Volume mixing ratios of HDO (HDO and HDO-CO2 mixtures)
~ for HDO-CO2 and 0.5 for HDO
Temperature (K) ~
Cell path length (m)
and
Scanning time (h)
~3-12 for data & 7-17 for empty cell scans
Signal-to-noise
(depends on spectral range and absorption path)
Gas Samples
High-purity distilled H2O and 99% D-enriched D2O
Calibration standardsa
H2O and CO2
Calibration of wavenumber scales obtained relative to n2 water vapor lines or the n3 CO2 lines in the evacuated FTS chamber
For the low temperatures (250 K or less) the coolable multipass Herriott cell
with m path was used.
CO2 sample with natural isotopologue abundance was used in HDO-CO2 data

10. Sample temperature (K)c Volume mixing ratio of HDOd
HDO and HDO-CO2 spectra analyzed in the n1 band
Serial #
File name
Cell length
(m)a
Total Pressure
(Torr)b
Sample temperature (K)c
Volume mixing ratio of HDOd
Only HDO 1
4377x2550.ars
0.2038
0.55
296.80
0.5 2
4383x2550.ars
9.56 3
4387x2550.ars
14.30 4
4394x2550.ars
2.95
CO2 broadened HDOe 5
4400x2550.ars
317.1
0.0412 6
4406x2550.ars
153.9
0.040 7
4515x2550.ars
96.0
271.30
0.030 8
4520x2550.ars
97.1
255.00
0.0023 9
4624x2550.aus
20.941
37.21
250.00
0.0058 10
4614x2550.ars
33.55
230.00
1 atm =101.3kPa = 760 Torr.
a Cell lengths are known with uncertainties of %
b Gas sample pressures are measured with uncertainties of ± 0.05% of full-scale pressure readings.
c Cell (gas sample) temperatures are measured with uncertainties of ± K.
d For an HDO sample the maximum amount of HDO is 0.5 of the total pressure. In addition to the volume mixing ratios, isotopologue abundances of major isotopologues (e.g., HD16O, HD18O, H2O, D2O) are to be determined in each spectrum in fitting the spectra simultaneously. See text for further details.
e A natural CO2 sample was used in the mixtures of HDO-CO2.

11. Equations used in retrieving the line shape parameters
(1)
(2)
(3)
𝑏 𝐿 (p, T) is the Lorentz halfwidth (in cm-1) of the spectral line at pressure p and temperature T.
𝑏 𝐿 0 and  0 represent Lorentz broadening and pressure-shift coefficients in (cm-1 atm-1 at 296 K), respectively.
c is the ratio of the partial pressure of HDO to the total sample pressure in the cell.

12. Multispectrum fit of HDO n3 lines near 3776 cm-1
All major transitions seen in this fitted interval are HDO.
1) ← at
(4) cm-1
2) ← at
(4) cm-1
(Transitions marked 1 and 2 are involved in weak collisional mixing)
The strongest line at
(3) cm-1
is ←

13. Example fits for line mixing in the n1 and n2 bands

14. MEASURED PARAMETERS

15. Measured line parameters including the Off-Diagonal Relaxation
Matrix Element Coefficients: A few examples

16. Line position differences (Measured – Predicted) and Ratios of Line Intensities (Measured/Predicted) in the v2 and v3 bands of HDO.
Top: Position differences for the majority of lines are within ± cm-1. Weaker lines show larger diff.
Bottom: On average, the measured intensity ratios are higher than predicted values, with a mean of 1.05 ±0.07.
Top: Position differences for the majority of lines are within ± cm-1.
Bottom: The mean intensity ratios (PS/predicted) is ~1.0; but vary within ± 9.0% to predicted values.

17. HDO-CO2 Lorentz half-width coefficients and temperature-dependence exponents in n2 and n3: Measurements and MCRB calculations.
Top: Measured and calculated (MCRB) Lorentz half width coefficients for HDO-CO2 (n2 band).
Bottom: Measured and calculated T-dependences of HDO-CO2 half width coefficients.
Top: Measured and calculated (MCRB) Lorentz half width coefficients for HDO-CO2 (n3 band).
Bottom: Measured and calculated T-dependences of HDO-CO2 half width coefficients.

18. Pressure shift and temperature dependence of pressure shift coefficients: HDO-CO2
Top: HDO-CO2 pressure shift coefficients in the n2 band.
Bottom: Temperature dependence of HDO-CO2
pressure-shift coefficients (varies between and cm-1 atm-1 K-1).
Top: HDO-CO2 pressure shift coefficients in the n3 band.
Bottom: Temperature dependence of HDO-CO2 pressure-shift coefficients varies between and cm-1 atm-1 K-1.

19. Comparisons between Measured and calculated CO2-broadened HDO Widths, Shifts and T-dependence of Widths.
Green (x) symbols represent measurements and blue (x) symbols MCRB calculations
Top panel: HDO-CO2 half widths vs. line positions
Middle panel: Temperature dependence exponents for HDO-CO2 widths vs. line positions
Bottom panel: HDO-CO2 pressure shift coefficients vs. line positions

20. CONCLUSIONS AND FUTURE PLANS
Line positions, intensities, CO2-broadened halfwidth and pressure-shift coefficients have been measured for a large number of transitions in all three fundamental bands (n1, n2 and n3) of HDO.
Room temperature self-broadened (HDO-HDO) halfwidth and pressure-shift coefficients have also been determined for most of the same transitions in all three bands for HDO.
Line mixing via off-diagonal relaxation matrix formalism was quantified for self- and HDO-CO2 collisions in several transition pairs in n1, n2 and n3.
Temperature dependences of HDO-CO2 halfwidth coefficients are also determined for a number of transitions in all the three bands.
Theoretical modeling of line parameters are performed and a new list has been generated for the cm-1 region for HDO.
FUTURE STUDY
Limited time and challenges involved in the experiments precluded analyses of low temperature HDO-HDO and determination of H/D from the same dataset. These studies are important and should be pursued in the future.

21. Thank you