1. LINE SHAPE PARAMETERS OF WATER VAPOR TRANSITIONS IN THE 3645−3975 cm−1 REGION
V. MALATHY DEVI and D. CHRIS BENNER
The College of William and Mary, Williamsburg, VA, U.S.A.
ROBERT R. GAMACHE, BASTIEN VISPOEL and CANDICE L. RENAUD
University of Massachusetts Lowell, Lowell, MA, U.S.A.
MARY ANN H. SMITH
NASA Langley Research Center, Hampton, VA, U.S.A.
ROBERT L. SAMS and THOMAS A. BLAKE
Pacific Northwest National Laboratory, Richland, WA, U.S.A.
MJ05 – 72nd International Symposium on Molecular Spectroscopy, June 19-23, 2017

2. Air-broadened H2O line parameters and their temperature dependences are needed for quantitative analysis of spectroscopic observations of Earth’s atmosphere, both to retrieve atmospheric water vapor abundances and to model the H2O absorption overlapping features of other species.
Spectroscopic databases such as HITRAN and HITEMP contain line positions, intensities, and assignments for hundreds of thousands of transitions of H2O and its isotopologues.
However, measured line shape parameters (widths, pressure-induced shifts, temperature dependences, etc.) are available for only a small fraction of the transitions listed in the database.
Calculations of H2O line shape parameters (e.g., Gamache and Laraia, J. Mol. Spectrosc. 257 (2009) ), necessary for completing the databases, require laboratory measurements for validation.
The majority of published measurements of air-, N2-, or O2-broadened line shape parameters for H2O have been for the ν2 band system near 1600 cm-1.
Present Work: Analyze air- and self-broadened H2O spectra recorded at K to determine temperature dependences of air-broadened line shape parameters for ν1 and ν3 transitions in the 3645 − 3975 cm-1 spectral region.

3. Experimental Conditions: Bruker IFS 120 HR (Pacific Northwest National Laboratory)
Source: Tungsten
Beam splitter: CaF2
Detector: InSb
Spectral coverage: –4750 cm-1
Maximum path difference(2L): ~125 cm
Unapodized resolution (FWHM): cm-1
(Two pure H2O spectra at cm-1)
FTS input aperture size: < 2 mm
Number of co-added scans: 66−512
Signal-to-RMS noise: >1500
Cell path lengths: and cm
Broadening gases: H2O, Air
Pressures: to 613 Torr
Temperatures: to 353 K

4. Summary of Lab Spectra Analyzed
Serial #
Cell Length (cm)
Total gas pressure (Torr)
Sample gas temperature (K)
Volume mixing ratio of H2O
Only H2O 1
19.95
0.130
296.20
1.0 2
0.101 3
0.0235 4
0.0690 5
0.0415
296.21 6
9.906
2.032
298.00 7
4.16
H2O + air 8
150.1
0.0051 9
305.5
0.0072 10
450.4 11
100.5
268.20 12
252.7
268.19 13
400.6 14
600.0 15
100.8
353.23 16
250.3 17 18
612.9
353.24
0.0200
Notes: 1 atm =101.3kPa = 760 Torr. Spectral resolution was cm-1 for #6 and #7, and cm-1 for all others.

5. Examples of recorded spectra
All spectra shown were recorded with the cm sample cell.

6. Analysis
Nonlinear least squares multispectrum fitting [D. Chris Benner et al., JQSRT 53 (1995) ] is used to retrieve spectroscopic parameters consistent with the entire set of laboratory spectra.
All spectra are calibrated to the same wavenumber scale with reference to CO 2-0 line positions.
Initial line list taken from HITRAN2012 [L. S. Rothman et al., JQSRT (2013) 4-50].
Voigt line shape is initially assumed, and quadratic speed dependence is included if residuals indicate it is necessary; line mixing is allowed for pairs of lines expected to mix.
Room-temperature self-broadened spectra are fit first; then the air- broadened spectra at room, higher, and lower temperatures are added sequentially.

7. Example Fit of Air- and Self-Broadened H2O Spectra
A total of 18 spectra were fit.
170 transitions (tick marks in top panel) in this 17 cm-1 interval
Voigt line shape profile with quadratic speed dependence

8. Example Fit of Air- and Self-Broadened H2O Spectra
A total of 18 spectra were fit.
Expanded view of a small region of the 20 cm-1 interval fit ( cm-1).
Positions of H2O transitions indicated by tick marks in top panel.
Voigt line shape profile with quadratic speed dependence.
Line mixing occurs between transitions marked 1&4, 3&5, 3&4 and 4&5 (line 2 does not mix with others).

9. Measured Air-Broadened Line Widths and Shifts and their Temperature Dependences
Temperature dependence of widths: b(T) = b0 × (T0/T)n, where b0 is the width at T0 = 296 K.
Temperature dependence of shifts: δ(T) = δ0 + δ' × (T − T0), where δ0 is the shift at T0 = 296 K.

10. Measured Self-Broadened Line Widths and Shifts
Temperature dependences were not measured because self-broadened spectra were recorded only at room temperature (296 − 298 K).

11. Comparison with Other Measurements and HITRAN
Positions and Intensities, ν3 band only
Self-broadened Widths and Shifts, ν1 and ν3 bands
References: L. S. Rothman et al., JQSRT 130 (2013) 4-50; Q. Zou and P. Varanasi, JQSRT 82 (2003) 45-98; I. V. Ptashnik et al., JQSRT 177 (2016) ; J. Loos et al., JQSRT (2017) in press [two articles].
Self-shifts are not reported in HITRAN 2012 or in Ptashnik et al.

12. Comparison with Other Measurements and HITRAN
Air-broadened Widths and T-dependences, ν1 and ν3 bands
Air-broadened Shifts and T-dependences, ν3 band only
References: L. S. Rothman et al., JQSRT 130 (2013) 4-50; Q. Zou and P. Varanasi, JQSRT 82 (2003) 45-98; I. V. Ptashnik et al., JQSRT 177 (2016) ; J. Loos et al., JQSRT (2017) in press [two articles].
Air-broadened parameters are not reported in Ptashnik et al., and air-shift T-dependences are only reported in the present study and in Loos et al.

13. Measured ν3 Parameters with Line Mixing#
Quantum
Assignment n
(cm-1) S
(cm/molecule at 296K) 
H2O-air
Widtha nb
Shifta δ̍
(H2O-air)b
Wij
H2O-aira
H2O-H2Oa 1
3 3 0  4 3 1
3 2 1  4 2 2
1.306e-20
3.296e-20
(7)
(4)
0.692(5)
0.731(3)
(6)
(3)
+0.35(4)e-05
+0.35(4)e-05c
0.0044(1)
0.0(F) 2
4.647e-20
4.852e-20
(5)
(5)
0.669(2)
0.762(2)
(3)
(3)
+2.59(5)e-05
+0.82(6)e-05
0.0029(1) 3
2 2 0  3 2 1
2 1 1  3 1 2
8.725e-20
1.695e-19
(4)
(3)
0.726(2)
0.745(2)
(2)
(2)
+0.78(48)e-06
+0.27(4)e-05
0.0078(1)
0.031(5) 4
5 5 1  5 5 0
5 5 0  5 5 1
3.072e-20
8.140e-21
(4)
(4)c
0.636(3)
0.636(3)c
(2)
(2)c
+2.00(4)e-05
+2.00(4)e-05c
0.026(4) 5
5 4 2  5 4 1
5 4 1  5 4 2
3.534e-20
1.174e-20
(4)
(4)c
0.615(2)
0.615(2)c
(2)
(2)c
+1.79(4)e-05
+1.79(4)e-05c
0.0013(1)
0.121(7) 6
3 2 2  3 2 1
3 3 1  3 3 0
1.031e-19
1.647e-19
(6)
(4)
0.735(2)
0.695(2)
(3)
(3)
+0.54(5)e-05
+0.123(4)e-04
0.0049(0)
0.040(1) 7
3 0 3  2 0 2
3 2 2  2 2 1
8.125e-20
9.662e-20
(4)
(3)
0.821(2)
0.777(2)
(2)
(2)
−0.119(4)e-04
+0.178(4)e-04
0.0025(1) 8
5 0 5  4 0 4
4 1 3  3 1 2
6.179e-20
1.770e-19
(4)
(3)
0.688(3)
0.760(2)
(4)
(2)
−0.21(6)e-05
+1.80(3)e-05
0.0035(1)
0.048(3) 9
4 2 2  3 2 1
1.180e-19
(5)
(3)
+1.13(4)e-05
0.033(1) 10
0.0094(1) 11
5 1 5  4 1 4
1.800e-19
(2)
0.664(1)
(1)
+0.48(3)e-05
0.0118(1) 12
7 3 4  6 3 3
7 2 5  6 2 4
6.795e-21
1.094e-20
(41)
(30)
0.818(11)
0.875(7)
(30)
(27)
+0.67(3)e-04
+0.30(2)e-04
0.0170(3)
0.071(2)
aWidths, Shifts, and Off-diagonal relaxation matrix element coefficients, Wij , all have the same units (cm-1 atm-1 at 296 K).
bT-dependence of the width, n, is unitless, and the T-dependence of the shift, δ̍, has units (cm-1 atm-1 K-1).
cValues are constrained in the fits to those listed in the line just above it.
F indicates a value fixed in the fits. Values in gray-shaded cells are those retrieved from the fit shown on Slide 8.

14. Summary
We have measured line positions, intensities, air-broadened half-width and pressure shift coefficients and their temperature dependences for strong transitions in the ν3 and ν1 bands of H216O, by multispectrum fitting FTS spectra recorded from 268 to 353 K, using the speed-dependent Voigt line shape.
Air-width and air-shift coefficients were measured for over 200 transitions, but their temperature dependences were obtained for only about 100 transitions.
Room-temperature self-width and self-shift coefficients were determined for nearly all of the same 200 transitions.
Line mixing parameters were experimentally determined for 12 pairs of ν3 transitions.
Results are in good agreement with other recent measurements of these parameters, but our results extend to higher J values.
Future Work
Compare with results of Modified Complex Robert-Bonamy (MCRB) calculations.
Provide preliminary results for use in atmospheric retrievals.
Final results will be contributed to future spectroscopic database updates (e.g., HITRAN).

15. Acknowledgments
Cooperative agreements and contracts with National Aeronautics and Space Administration have supported the research performed at the College of William and Mary and at NASA Langley Research Center through the Upper Atmosphere Research Program and the AQUA Validation Program.
Research performed at the University of Massachusetts Lowell is supported by the National Science Foundation through Grant No. AGS and AGS
The Department of Energy’s Office of Biological and Environmental Research located at the Pacific Northwest National Laboratory (PNNL) supported part of this research. PNNL is operated for the United States Department of Energy by Battelle under contact No. DE-AC06-76RLO 1830.

16. Backup Slide: Other experimental details
Both specially built absorption cells were made of stainless steel, but electroplated and gold-coated to minimize decomposition and sticking of the gas samples and to minimize reactivity when using corrosive gases such as HCN. Both cells have an inner diameter of 5 cm. The cm cell had wedged KCl windows sealed with Viton O-rings, and the cm cell had wedged BaF2 windows also sealed to the ends of the cell with Viton O-rings. Cell lengths are known with an uncertainty of 0.01%.
In each experimental set up, the absorption cell was mounted inside the sample compartment of the spectrometer to avoid errors associated with beam steering due to cell misalignment. A temperature regulated outer jacket surrounds the cell’s inner tube and is plumbed through the spectrometer’s vacuum bulkhead to a circulating bath (Julabo F25-MD). The cell is actively temperature stabilized by either heating or cooling the circulating fluid (a weak solution of propylene glycol in water).
Gas sample pressures are measured with uncertainties of ± 0.05% of full-scale readings of the appropriate MKS Baratron pressure gauges (1, 10 or 1000 Torr range, as appropriate).
Cell (gas sample) temperatures are measured with uncertainties of ±0.02 K (RTD sensor).
Research grade synthetic air sample was used for the air-broadened H2O spectra.
To minimize residual atmospheric H2O signal, the entire spectrometer bench and the sample compartment were kept under vacuum (below 0.03 Torr) using a mechanical vacuum pump. A ½” diameter copper tube, positioned horizontally, ran the length of the moving mirror compartment, and both ends of the tube came out of the spectrometer through vacuum feedthrough fittings. The ends of the tube were bent so they were vertical. This tube was kept filled with liquid nitrogen for the duration of each measurement. Two molecular sieve sorption traps were connected to the sample and detector vacuum chambers, and were cooled to liquid nitrogen temperature during the duration of each measurement.