Water-vapor absorption line measurements in the940-nm band by using a Raman-shifted dye laser

Zhiping Chu, Thomas D. Wilkerson, and Upendra N. Singh

We report water-vapor absorption line measurements that are made by using the first Stokes radiation(930-982 nm) with HWHM 0.015 cm-' generated by a narrow-linewidth, tunable dye laser. Forty-fiveabsorption line strengths are measured with an uncertainty of 6% and among them are fourteen stronglines that are compared with previous measurements for the assessment of spectral purity of the lightsource. Thirty air-broadened linewidths are measured with 8% uncertainty at ambient atmosphericpressure with an average of 0.101 cm-1 . The lines are selected for the purpose of temperature-sensitiveor temperature-insensitive lidar measurements. Results for these line strengths and linewidths arecorrected for broadband radiation and finite laser linewidth (0.015 cm-' HWHM) broadening effects andcompared with the high-resolution transmission molecular absorption.

Key words: Water vapor, absorption lines, line strength, line width, Raman-shifted laser, water-vaporspectroscopy.

1. Introduction

The remote sensing of water vapor requires accurateknowledge of H20 line parameters in the near infra-red because H2 0 concentration measurements arecritically dependent on the assumed line strengthsand linewidths. The differential absorption lidar(DIAL) technique can be used to determine theconcentration of water vapor remotely by observingthe absorption of pulsed laser radiation as it propa-gates through the atmosphere when the laser is tunedon and off of a water-vapor absorption line.' Thepurpose of the current work is to obtain accuratewater-vapor line parameters in the pcr band that liesin the 930-982-nm range. Because this band con-tains strong lines and lies in an otherwise clearportion of the atmospheric spectrum, it is particularlywell suited for remote sensing in the stratosphere andpolar regions, where H20 is much less abundant thanin the tropical or midlatitude troposphere.2 3

The currently available line parameters for this

When this work was performed the authors were with theInstitute for Physical Science and Technology, University ofMaryland, College Park, Maryland 20742-2431. Z. Chu is nowwith the University of Connecticut, Storrs, Connecticut 06269.U. N. Singh is now with Hughes STX Corporation, working at theGoodard Space Flight Center, Code 915, National and Aeronauticsand Space Administration, Greenbelt, Maryland 20771.

Received 2 July 1992.0003-6935/93/060992-07$05.00/0.© 1993 Optical Society of America.

water-vapor absorption band come from a few previ-ous studies. The most comprehensive water-vaporcompilations in the 940-nm range include the solartable edited by Swensson et al.,4 the high-resolutiontransmission molecular absorption (HITRAN) data-base,5 the measurements by Giver et al.,6 and therecent results by Chevillard et al.7 and Mandin et al. 8

Giver et al.6 measured line strengths and N2 -broadened linewidths for 97 water-vapor lines in the932-961-nm spectral region. These spectra wereobtained by using a multipass absorption cell and aspectrometer with a spectral resolution of 0.046 cm-'FWHM. Chevillard et al.7 and Mandin et al. ana-lyzed the water-vapor spectra recorded with a Fourier-transform spectrometer between 870 and 1052 nmwith 0.02 cm-' resolution. 718 well-isolated linestrengths were measured by using the equivalentwidth method with 7% uncertainty, and 1695 linestrengths were measured by using the faster centraldepth method with an average uncertainty of 15%.200 N2-broadened linewidths were measured with anuncertainty of 7%. The 1991 edition of HITRANincluded line position and line strength measure-ments from Chevillard et al., and linewidths from theimproved calculations9"10 in the 940-nm band.

In preparation for the current work, we carried outexperiments on an efficient Raman-shifted dye lasersystem" because the YAG-pumped dye laser effi-ciency falls off rapidly for wavelengths that are > 700nm. Tunable radiation was generated with a line-width of 0.015 cm-' (HWHM) and a conversion

992 APPLIED OPTICS / Vol. 32, No. 6 / 20 February 1993

efficiency of 37% in the wavelength range 930-982nm. This range was determined from the curve ofdye laser efficiency to ensure an output energy of > 20mJ to obtain a stable Raman-shifted signal. Opticaldepths were first measured at the centers of 14absorption lines between 930 and 953 nm, which hadbeen covered by Giver et al.6 By comparing theseabsorption line strengths with previous measure-ments,6 7 we could determine the spectral purity ofthe Raman-shifted radiation. Furthermore thestrengths and widths of 31 H20 absorption lines weremeasured for the purpose of temperature-sensitiveand temperature-insensitive lidar applications in thewavelength range 961-982 nm, which had not beencovered in the experiment of Giver et al.6 Themeasured line strengths and linewidths were com-pared with observed data and HITRAN-91. The com-parison of the current laser-based measurementswith other empirical measurements6-8 that were takenby using different techniques is important for consol-idating the knowledge of molecular absorption param-eters in this spectral region.

2. Experimental Details

In this experiment, some lines were selected for thepurpose of water-vapor concentration measurementsthat are temperature insensitive from the criterion'2aoro/aT = 0, where uo is the absorption cross sectionat the line center and T is the temperature. Thiscriterion is more meaningful for tuning a narrow-band laser to a line center in lidar measurementsthan the alternative criterion'3 aS/aT = 0, where S isthe line strength and is equal to the integrated crosssection. The other nearby measured lines, beingtemperature sensitive, were selected for measure-ment if they were strong enough and well separatedrelative to the laser linewidth of 0.015 cm-'. Theselines may be used for measurements of atmospherictemperature as well as humidity.14 6

The experimental setup is shown in Fig. 1. Theapparatus has been described in detail in Ref. 11.The dye laser (0.01 cm-' HWHM) was operated in the

Fig. 1. Experimental setup for water-vapor absorption line mea-surements in the 940-nm range. OMA, optical multichannelanalyzer; Osc, oscillator.

range 670-697 nm with a solution of Exciton LDS698 dye in methanol and optically pumped by afrequency-doubled Nd:YAG laser to produce an out-put energy that was Ž 20 mJ. The Raman cell filledwith H2 gas was operated at pressures below 14 atmto ensure that the output linewidth was narrowerthan 0.015 cm-'." The White cell, with a baselength of 1.63 m, was adjusted to have total opticalpath lengths of up to 100 m. All spectra were takenat atmospheric pressure and in the temperaturerange 25-290 C.

The water-vapor absorption linewidths (at low J) inthe pcr' band at sea level atmospheric pressure aretypicallyofthe order of 0.1 cm-1 (HWHM). Thereforethe laser linewidth of 0.015 cm-' (HWHM) is approxi-mately 5 to 10 times narrower than the H20 line-widths. This narrow laser linewidth permits themeasurements to be made at 1 atm without introduc-ing too much line shape distortion and facilitates thedata interpretation, as is discussed below.

To adjust for the laser energy variation pulse bypulse, we took the ratio of the White cell signal B tothe reference signal A. The B/A off-line ratio in thespectra was determined from the regions of no absorp-tion. The relatively clear spectral regions were se-lected to ensure that the variation of the absoluteoff-line ratio was less than 0.2%.

Each reading of the B/A ratio was averaged from10 laser pulses. Generally 10 readings (100 pulsestotal) were averaged to produce a B/A datum to beused in the optical depth calculation. The opticaldepth at the line center ro is

[ (B/A )min vo=-n(B/A )' (1)

where (B/A )min is the line center ratio and (B/A )ma isthe off-line ratio. When 0 was determined, the B/Aratio at the half-optical-depth To/2 could be calculated.The dye laser was then tuned off the line center tomatch the B/A reading with the calculated value(B/A)~O/2. The B/A ratio was recorded at the To/2condition, and the corresponding wavelength wasread from a Laser Technics-100 Fizeau wavemeter.Similar observations were made with the laser tunedto the other side of the absorption line. The mea-sured linewidth Ymeas was calculated from the differ-ence between the two frequency settings (vl, v2) forwhich the optical depth was 7o/2; Ymeas = (V2 - vl)/2.

The White cell temperature was measured by athermometer and a Hanna instrument, and the Whitecell relative humidity was also measured by theHanna instrument, which has both thermal andhumidity sensors. The two temperature readingsfrom the thermometer and Hanna sensor agreed well.In order to get accurate absolute line strengths, theHanna instrument was calibrated at the NationalInstitute of Standards and Technology (NIST). TheHanna temperature readings were compared with aNIST digital thermometer (HT series 5810) andshowed good agreement. The Hanna humidity sen-

20 February 1993 / Vol. 32, No. 6 / APPLIED OPTICS 993

sor was calibrated by Peter Huang at NIST, who usedthe National Bureau of Standards Two-PressureHumidity Calibration Facility, Mark 2."7 This facil-ity used the two-pressure principle for generating airof known humidity. The relative humidity range ofthe calibration was 35-65% for the temperaturerange 24-290C and a pressure of 1 atm. Finally, theestimated maximum uncertainty in relative humidityduring our spectroscopic measurements was 0.2%.We note that the direct humidity reading from theHanna gauge turned out to be accurate to within0.1% in our range, which was much better than withother gauges that we tried. Therefore the absolutehumidity accuracy was within 0.3%.

3. Data Correction and Analysis

The measured optical depth r(v)meas at frequency vwas converted to absorption cross section (V)meas(square centimeters per molecule) by

(V)meas - L(V)me (2)nH2 oL

where nH2O is the water-vapor number density (mole-cule per cubic centimeters), and L is the absorptionpath length (centimeters). To convert these mea-sured quantities {Vv)meas and Ymeas to the commonlyused Lorentz parameters for each line, several stepsof calculations or calibrations are needed.

A. Water-Vapor Number Density nH2O

Relative humidity RH is defined by

RH = e/e5, (3)

where e is the partial pressure of the vapor in thehumid air and e, is the saturation vapor pressure overa plane surface of the pure phase of liquid or solidwater corresponding to the temperature and mixture.From the measured RH and the e tabulated byWexler,'8 the partial vapor pressure e is calculated fordifferent T and RH. Because the atmospheric pres-sure is 1 atm, the interactions between the moleculescan be neglected, and the ideal gas law is used todetermine the water-vapor molecular density, i.e.,

e RHe4nH0 T T (

where k is Boltzmann's number.

B. Finite Laser Linewidth Correction

Because Raman-shifted radiation has a finite line-width of 0.015 cm-', the measured cross section is anaverage of the H20 line profile (Voigt) weighted by thelaser frequency distribution":

crmeas n L n G(v-vo)

x exp[-nLu(v - vo)]d(v - v0), (5)

where v is the wave number of the line center; aGaussian frequency distribution G(v - v) is as-sumed for the laser, and a Voigt profile V(v - v) isassumed for the absorption cross section or(v - VO)12,19:

a(v - go) = SV(v - VO), (6a)

V(v - VO) = 1 (ln 2) / y ,[ exp(-t2) dt

(ln 2)1/2 (ln 2)1/2

YD YL, D (V - Vo),(6b)

where "ID is the Doppler half-width and "IL is theLorentz half-width. The convolution of the laserprofile with the Voigt absorption line in Eq. (5) can becalculated by parametrically varying the Voigt centralcross section (v = vo) and linewidth yv. Yv wascalculated from "YD = 0.015 cm-' and YL, which wasvaried over the range 0.07 to 0.13 cm-' to cover thepossible linewidth (low J) values at the pressure of 1atm 2 0:

YV = /2[1.0692YL + (0.86639YL2 + 4YD2 )1/2]. (7)

The Voigt cross section u(v = vo) was varied over awide range from 0 to 2 x 10-21 cm2, which corre-sponds to the wide ranges of line strengths in thiswork. Approximately 1100 such convolutions werecalculated to give the measured central absorptioncross section umeas and linewidth Ymeas with highresolution. Figure 2 shows a-v and "yL as functions ofUmeas and Ymeas, so that from the measured quantitiesUmeas and ymeas we can correct for the laser linewidthbroadening effect and determine o-V and yL. oV wasthen converted to line strength S from Eqs. (6).

0.14 ^- YL=~~~~~~~~7~ 0. 13 cm

. 0.13

0 500 ~ ~~~ 120 50 20

~a 0.12

0.09

i 0.09 0.1 -~~~~~000.09 -~~~~~00

0.07.

0.70 500 1000 1500 ... 2000

Measured Central Absorption Cross section CFmeas (XIO'24cm 2

Fig. 2. Calibrations to laser linewidth (0.015 cm- 1 HWHM)broadening effect for determining Voigt central cross section avand Lorentz linewidth 7L from the measured central cross sectioncrmeas and HWHM Ymeas.

994 APPLIED OPTICS / Vol. 32, No. 6 / 20 February 1993

C. Standard Temperature and Pressure ConditionIn order to compare with other available data (mea-sured or calculated), we normalized the above quanti-ties to STP conditions (p = 1 atm, T = 273 K) byusing' 2-14

so = S( )' exptk ( -)

"YL = YL(pO/p)(T/TO)n,

a-L = so'ITYL

a:'Zs

'a

.,

QU11

en

2.

(8a)

(8b)

(8c)

where n is the temperature-dependence coefficientand varies from line to line. HITRAN-91 collects themeasured and calculated data on n and gives a defaultvalue of n = 0.68 to most of the water-vapor lines thatare not independently measured or calculated.9"l0

D. Spectral Purity Correction

The term spectral purity means the relative absenceof broadband radiation in laser emission. Any radia-tion that is outside the bandpass of an absorption linehaving a high optical depth would cause an erroneoustransmission measurement when the laser is tunedonto the line center frequency. This broadbandradiation could pose a problem for quantitative laserabsorption spectroscopy, just as for molecular concen-tration measurements that are taken by lidar."' 2 'Amplified spontaneous emission and higher-orderStokes radiation were identified as the main possiblegeneration sources for this out-of-band light.

To determine the spectral purity of our Raman-shifted radiation, we remeasured 14 absorption linestrenghts in the wavelength range 930-953 nm,which has been covered by Giver et al. 6 and Chevillardet al.7 To ensure the equivalence of these two stan-dards we intercompared them, as indicated in Fig.3(a), for 53 lines that turn out to have a measured linestrength ratio of R = 0.96.6,7 Table 1 lists thevibrational-rotational assignments of the 14 lines weremeasured. The chosen lines are all quite strong soas to make the extra transmittance caused by anyweak broadband radiation distinguishable if it werepresent. The correction steps of Subsections 3.A-3.C were used first to deduce the line strengths fromthe measured optical depths, and the Lorentz line-widths measured by Giver et al. 6 were applied inSubsection 3.B. Figure 3(b) shows the values of SOmeasured in this work and from Chevillard et al.7versus those given by Giver et al. 6 We found goodagreement in the case of weak absorption but a smalldiscrepancy in the case of strong absorption, whichindicates that the Raman-shifted light contained asmall amount of broadband radiation. The relativediscrepancy between the measured strength and Giv-er's data (AS/S) at the maximum absorption ( 1.6)

1 0

8

6

4

2

00 2 4 6 8

Line strength measured by Giver et a.

600

- ; 500

- 4 400Is a

_ _ 300_a V

c . 200A C;I0 =_ t 100

rfAne'

(a)

1 0

(b)

0 100 200 300 400 500 600

So (10-24 cm-1/molecule/cm2 , Giver et al.)

Fig. 3. (a). Comparison of 53 line strengths of Chevillard et al.7

(S2) with Giver et al.6 (S1 ) at T = 300 K. The average ratio S 2 /S1

is 0.96. (b). S measured on 14 lines in this study and fromChevillard et al. as a function of SO from Giver et al. for thedetermination of spectral purity. The unit used in (a) is inversesquare centimeters times inverse atmosphere and in (b) is inversecentimeter per molecule per square centimeter with a conversionrelation of

(cm-2 atm-) = LTO/T (cm-1/molecule/cm 2 ),

where To = 273.15 K, T = 300 K in (a), and L = 2.686754 x 1019cm-3 at STP.

was 5%. From relation AS/S = AT/T, the absoluteerror in transmittance is

AT= -Tln T- 0.016.

This means that 1.6% of the Raman-shiftedradiation was emitted outside the narrow spectralband. Therefore the optical depth in Eq. (1) shouldbe written as"

[(B/A)min (B/A)broad[(B/A)m.i (B/A)m. _

= -ln(Tnarrow + Tbroad). (10)

By means of Eq. (10), we applied small correctionsto all the measured optical depths.

20 February 1993 / Vol. 32, No. 6 / APPLIED OPTICS 995

(9)

Table 1. Positions, Energy Levels, Assignments and Strengths SO of the 14 H20 Absorption Lines Selected Here for Spectral Purity Confirmationa

So so so

X (nm, air) v (cm-', vac) E" (cm-') J" Ka" Kc" J' Ka' Kc' V1' V2 V3 (measured) (Giver et al.6

) (Chevillard et al.7 )

935.775 10683.38 142.28 3 1 3 4 1 4 2 0 1 254.0 237.0 220.09936.941 10670.12 134.90 2 2 1 3 2 2 2 0 1 307.4 314.1 313.18943.062 10600.85 136.16 2 2 0 2 2 1 2 0 1 216.6 207.7 204.40943.768 10592.87 212.16 3 2 1 3 2 2 2 0 1 272.9 287.0 264.01948.164 10543.78 136.76 3 0 3 2 0 2 2 0 1 545.6 574.0 573.74949.345 10530.71 206.30 3 2 2 2 2 1 2 0 1 81.3 76.3 69.18949.435 10529.72 173.36 3 1 2 2 1 1 2 0 1 405.3 420.2 520.49949.745 10526.28 212.16 3 2 1 2 2 0 2 0 1 213.3 216.6 211.63950.093 10522.42 224.84 4 1 4 3 1 3 2 0 1 377.4 424.0 406.35950.169 10521.57 222.05 4 0 4 3 0 3 2 0 1 169.8 166.0 156.82951.933 10502.06 275.50 4 1 3 3 1 2 2 0 1 100.9 100.4 99.18952.231 10498.78 325.35 5 0 5 4 0 4 2 0 1 325.7 360.0 334.97952.510 10495.70 315.78 4 2 2 3 2 1 2 0 1 75.7 74.4 72.97953.594 10483.77 399.46 5 1 4 4 2 3 3 0 0 95.9 95.3 85.72

aThese were measured by Giver et al.6 and Chevillard et al.7 S must be multiplied by 10-24. All lines except = 953.594 nm aretemperature insensitive.

4. Results and Discussion measurement precision) for 31 lines are also pre-Our measurements included all isolated water-vapor sented in Tables 2 and 3. The accuracies in linelines that were stronger than 10-23 cm-'/molecule/ strength SO and Lorentz linewidth YLO are better thancm2 in the wavelength range 961-980 nm, which is of 6% and 8%, respectively.interest for lidar applications but was not covered by The line strength comparison between this workGiver et al.6 For our absorption length of 100 m, the and Chevillard et al.7 is shown in Fig. 4. The ratio S°measured optical depths lie in the range 0.15-1.0, (this work)/S 0 (Chevillard et al.) averages to 1.03.depending on the line strength and the RH. Seven- The measured air-broadened Lorentz linewidths YL0

teen lines were selected for temperature-insensitive (air) for 30 lines were compared with HITRAN-91, aslidar measurements. Table 2 lists the vibrational- seen in Fig. 5. Among these 30 lines, 20 lines wererotational assignments of these 17 lines, which are all measured by Mandin et al.8 with an uncertainty ofin the 1 2 1 vibrational band. The vibrational- 7%, and this comparison is also shown in Fig. 5. Therotational assignments of 14 other lines (which are average ratio of the 30 measured values of yL0 relativetemperature sensitive) are given in Table 3, which to HITRAN is 1.059 and the 20 YLE from Mandin et al. tocovers a wide range of E"; twelve of these lines are in HITRAN is 0.958, giving a 10% difference between ourthe 1 2 1 band and two are in the 2 0 1 band but data and Mandin's, which, while within the sum oforiginate in a high J state. The measured SO, YL0, two measurement uncertainties, indicates a need forand crL0 with standard deviations (which indicate the further measurement.

Table 2. Positions, Energy Levels, Assignments, Measured Strengths S0, Air-broadened Lorentz Linewidths A, and Line Center Cross Sections crL for17 H20 Absorption Lines (Temperature Insensitive) at STP Conditionsa

S ° 'YLO UL 0

X (nm, air) v (cm1, vac) E" (cm'1) J" Ka" Kc" J' Ka' Kc' V1' V2' V3' (cm/molecule) (cm-') (Cm2)

964.520 10365.00 285.22 3 3 1 3 3 0 12 1 19.9+ 0.9 - -

964.556 10364.61 285.42 3 3 0 3 3 1 1 2 1 54.5 + 1.1 0.095 ± 0.002 183.1 ± 7.6965.973 10349.42 206.30 3 2 2 3 2 1 1 2 1 10.2 ± 0.9 0.109 + 0.009 29.7 ± 5.1966.232 10346.65 134.90 2 2 1 2 2 0 1 2 1 67.4 + 1.8 0.109 ± 0.003 197.6 ± 10.1966.465 10344.16 136.16 2 2 0 2 2 1 1 2 1 24.3 ± 1.7 0.105 + 0.008 73.7 ± 10.4967.060 10337.79 212.15 3 2 1 3 2 2 1 2 1 27.9 ± 1.0 0.103 ± 0.004 86.2 ± 6.1970.892 10296.98 173.36 3 1 2 3 1 3 1 2 1 8.4 ± 0.7 0.095 + 0.008 28.1 + 4.9973.064 10274.02 206.30 3 2 2 2 2 1 1 2 1 16.0 ± 1.1 0.118 ± 0.008 43.2 ± 5.9973.500 10269.40 212.15 3 2 1 2 2 0 1 2 1 25.2 ± 1.6 0.109 ± 0.007 73.5 ± 9.1974.349 10260.45 136.76 3 0 3 2 0 2 1 2 1 54.2 ± 1.1 0.098 ± 0.002 176.0 + 7.5974.931 10254.32 173.36 3 1 2 2 1 1 1 2 1 47.1 ± 1.1 0.112 ± 0.003 134.6 ± 6.2975.382 10249.58 300.36 4 2 3 3 2 2 1 2 1 29.5 + 1.7 0.105 ± 0.006 89.9 ± 10.6975.773 10245.51 224.84 4 1 4 3 1 3 1 2 1 50.4 + 2.9 0.110 ± 0.006 146.3 ± 16.9976.276 10240.17 222.05 4 0 4 3 0 3 1 2 1 17.9 + 1.3 0.109 ± 0.008 52.1 + 7.8977.386 10228.55 275.50 4 1 3 3 1 2 12 1 12.2 ± 1.1 0.101 ± 0.009 38.5 + 6.6977.941 10222.77 326.62 5 1 5 4 1 4 12 1 12.8 ± 1.0 0.085 ± 0.007 48.2 ± 7.6978.241 10219.64 325.35 5 0 5 4 0 4 1 2 1 34.0 + 1.5 0.101 + 0.005 107.2 ± 9.5

aSO and crLO must be multiplied by 10-24.

996 APPLIED OPTICS / Vol. 32, No. 6 / 20 February 1993

Table 3. Positions, Energy Levels, Assignments, Measured Strengths SO, Air-Broadened Lorentz Linewidths y'Y, and Line Center Cross Sections Lfor 14 H20 Absorption Lines (Temperature Sensitive) at STP Conditionsa

s o 'LO OILL

X (nm, air) v (cm'1, vac) E" (cm'1) J" Ka" Kc" J' Ka' Kc' V 1 ' V2 ' V3 ' (cm/molecule) (cm-') (cm2 )

961.005 10402.93 782.41 7 2 5 6 2 4 2 0 1 32.3 ± 1.0 0.096 ± 0.004 106.6 ± 7.8961.545 10397.09 920.17 9 0 9 8 0 8 2 0 1 30.3 ± 0.9 0.088 ± 0.003 110.2 ± 7.3962.120 10390.85 79.50 2 1 2 3 1 3 1 2 1 62.1 ± 2.0 0.104 0.003 190.5 ± 12.1962.270 10389.21 488.11 4 4 1 4 4 0 1 2 1 34.7 ± 1.4 0.090 ± 0.004 122.5 ± 10.1962.638 10385.31 42.37 1 1 0 2 1 1 1 2 1 42.9 ± 1.1 0.106 ± 0.003 128.9 ± 6.4963.734 10373.42 23.79 1 0 1 2 0 2 1 2 1 65.5 ± 1.8 0.110 ± 0.003 189.4 ± 10.1974.647 10363.63 382.52 4 3 2 4 3 1 1 2 1 23.7 ± 0.7 0.097 ± 0.003 77.6 ± 4.8966.087 10348.19 79.50 2 1 2 2 1 1 1 2 1 19.4 ± 0.8 0.104 ± 0.004 59.3 ± 4.8966.998 10338.46 37.13 1 1 1 1 1 0 1 2 1 19.2 ± 0.9 0.108 ± 0.005 56.5 ± 5.3968.038 10327.34 42.37 1 1 0 1 1 1 1 2 1 39.0 ± 1.1 0.106 0.003 117.4 ± 6.3970.145 10304.94 23.79 1 0 1 0 0 0 1 2 1 40.2 ± 1.2 0.112 ± 0.003 113.8 ± 6.5971.531 10290.22 79.50 2 1 2 1 1 1 1 2 1 38.8 ± 1.0 0.114 ± 0.003 108.5 ± 5.3972.458 10280.42 95.18 2 1 1 1 1 0 1 2 1 15.5 ± 0.8 0.116 ± 0.006 42.3 ± 4.3980.170 10199.53 447.25 5 1 5 6 1 6 1 2 1 21.8 ± 0.8 0.098 ± 0.004 70.9 ± 5.4

aSO and arL0 must be multiplied by 10-24.

In Fig. 6, the air-broadened Lorentz linewidthsfrom this work are plotted according to the quantumnumber J of the lower rotational state. The solidboxes give the averages for different J. A trend for

S

C

S

a

.?,U

°u:

rt

CA

80 -

70 -

60 -

s -

40 -

30

20

10-

0 ., ., ., ., I.,II ..I0 10 20 30 40 50 60 70 80

SI (10-24 cm-'/molecule/cm2, Chevillard eta!.)

Fig. 4. Comparison of the current line strength measurements of31 spectral lines, which are taken by using the stimulated Ramanscattering technique with the results from Chevillard et al.7

0.12

Ga

a

ait

c-a

*_

0S

0.11 -

0.1 -

0.09 -

0.08 -

0.07:

0.06 -

0.05 -_0.05

. . . . . 0 90.06 0.07 0.08 0.09

I . 1 . 10.1 0.11 0.12

't (cm'l/atm, HITRAN-91)

Fig. 5. Air-broadened Lorentz linewidths in this measurementare plotted as functions of the calculation data from HITRAN-91.

UI 0.11 0 a

0.1 °

ai o.i 0F~ ~~E X

0.0 - , 0

0

0.08 . . .0 2 4 6 8 10

Rotational number J

Fig.6. Rotational quantum number J" dependence of the Lorentzlinewidths -yLO (partially filled boxes). The filled boxes indicate theaverages for different J".

the J variation of the air-broadened linewidths can beseen in the graph; the linewidths decrease with thequantum number J. When J increased from 3 to 5,an average YLO decrease by 13% was found. Similareffects were reported by Giver et al.6 for H20 linesbraodened by N2 in the 940-nm region; a 15% de-crease was found,6 which corresponds to the increaseof J from 3 to 5.

5. Conclusion

A high-resolution spectroscopic study of water vaporin the 940-nm wavelength region has been conductedby using narrow-band Raman-shifted dye laser radia-tion. The air-braodened Lorentz linewidths and linestrengths have been measured. This work makes acontribution to the basic spectroscopy of water vaporand will be useful for atmospheric remote sensingapplications for which accurate line parameters arerequired. For DIAL measurements in the 940-nmrange, this work provides useful information on thestimulated Raman scattering techniques, which re-mains a competitive technique for use as a lidar

20 February 1993 / Vol. 32, No. 6 / APPLIED OPTICS 997

i U

.*:It

gOfas

It

- Mandin et a. a * -measured U 0 0

* 1 ~~~10 IT

o " U 9a 0

03

03

transmitter, namely, to shift tunable radiation intothe infrared, where molecular lines are relativelystrong, while maintaining sufficient optical power forpractical lidar applications. Further measurementsin this H2 0 band are needed, such as the temperature-dependent linewidth coefficients and line center fre-quency shifts that are due to pressure.

The authors appreciate the contributions to thiswork by Leo Cotnoir, Bryan Bloomer, George Treacy,William Grant, Geary Schwemmer, the IPST SpaceScience Shop, and Peter Huang (NIST). The re-search is supported, in part, by NASA CooperativeAgreement NCC 1-25, which is monitored by EdwardV. Browell of the Langley Research Center. We alsoappreciate the support by John Theon and RameshKakar (NASA Headquarters) for our research pro-gram on Raman-shifting techniques and near-infra-red spectroscopy of atmospheric constituents.

References1. E. V. Browell, T. D. Wilkerson, and T. J. McIlrath, "Water

vapor differential absorption lidar development and eval-uation," Appl. Opt. 18, 2472-3483 (1979).

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