CO2Pressure Shift and Broadening of Water Lines Near 790 nm

Yan Lu,Xin-fei Li,An-wen Liu,Shui-ming Hu

Hefei National Laboratory for Physical Science at the Microscale,University of Science and Technology of China,Hefei 230026,China

ARTICLE

CO2Pressure Shift and Broadening of Water Lines Near 790 nm

Yan Lu,Xin-fei Li,An-wen Liu?,Shui-ming Hu

Hefei National Laboratory for Physical Science at the Microscale,University of Science and Technology of China,Hefei 230026,China

Carbon dioxide pressure-broadened ro-vibrational transitions belonging to the ν2+3ν3band of H216O have been measured with a sensitive cavity ring-down spectrometer.Water vapor of relatively low pressures(＜0.5 Torr)was used to limit the self-collisions among water molecules.After the calibration using the precise atomic transitions of Rb and a thermostabilized Fabry-P′erot interferometer,10-5cm-1frequency accuracy has been achieved. Line parameters are derived from least-squares fitting of the spectra using the“soft”collision model.The retrieved line parameters can be applied in the study of water absorption in the CO2-rich atmospheres of planets like Venus and Mars.

Water,Carbon Dioxide,Pressure-induced half width,Pressure-induced line shift,Cavity ring-down spectroscopy

I.INTRODUCTION

The water molecule has an extremely rich spectrum throughout the infrared region.Accurate knowledge of the radiative properties of water vapor is needed in chemical,atmospheric and astronomical studies. Tremendous scientif i c efforts have been put into both experimental and theoretical studies of the spectrum of water vapor.A huge archive of the transitions can be found in some widely used spectroscopic databases including HITRAN[1],GEISA[2],and HITEMP[3]. The spectroscopic parameters,including the transition frequencies,line strengths,and collision-induced line widths,are widely used in various applications.

The H216O lines near 0.79μm,belonging to the ν2+3ν3and ν1+ν2+2ν3bands,have been extensively studied by Fourier-transform spectroscopy[4-7], intra-cavity laser absorption spectroscopy(ICLAS) [8-10]and cavity ring-down spectroscopy(CRDS) [11].Using a CRDS calibrated with atomic transitions,we have determined the positions of strong H216O lines(S＞10-25cm/molecule)with an accuracy of 3×10-5cm-1[12].The line parameters and assignments[13]have been updated in the 2009 edition of HITRAN[1].However,the collision-induced effects vary largely between different perturbing gases.The CO2pressure-induced H2O line shift and broadening in this spectral region have not been studied,but the information is particularly useful in the study of CO2-rich gas samples,for example,the atmosphere of Venus.

The atmosphere of Venus consists of 96.5%CO2and 3.5%N2.Though water vapor only contributes less than 0.01%of the atmosphere of Venus,it plays an important role in the climate of Venus[14].In this case,the spectroscopy of the CO2perturbed H2O lines is essential in the study of Venus.Here we present laboratory measurements of the H2O lines near 0.79μm broadened by CO2.We select these water lines because they are close to the ν1+5ν3band of CO2,which spreads in the 782-789 nm region.The ν1+5ν3band has also been the f i rst CO2band observed from the spectrum of Venus[15,16],because it is sufficiently strong(but not saturated)and sits in a transparent window of the earth’s atmosphere.

II.EXPERIMENTS

Natural carbon dioxide gas samples(stated purity of 99.99%)with pressures between 50 and 300 Torr were used.The sample pressure was measured with a capacitance gauge(MKS 627B)with an accuracy of 0.15%. Water vapor presented in the sample cell as an impurity of the sample gas.The pressure of the water vapor,being well below 1 Torr,can be estimated from the intensities of its absorption lines observed in the measurements.The spectrum was recorded at room temperature(296 K)with a cavity ring-down spectrometer based on a continuous-wave Ti:Sapphire laser.

The details of the CRDS spectrometer have been presented in Refs.[17,18]and will be only brief l y described here.A laser beam of about 50 mW is coupled into a 1.4 m long ring-down cavity.The cavity mirrors have a ref l ectivity of 99.995%and one of them is mounted on a piezoelectric actuator.The piezoelectric actuator is driven with a triangle wave from a function generator to repeatedly match the longitudinal mode of the ring-down cavity to the laser wavelength.The Ti:sapphire laser is run in a step-scan mode.At each step,typically about 80 ring-down events are recorded.The decay time τ is retrieved from a fit of the exponential decay curve.The sample absorption coefficient can be determined from the equation

TABLEI Line parameters of theO lines of ν2+3ν3near 0.79μm(in cm-1).Values in parentheses are 1σ standard deviations at the last quoted digit.

TABLEI Line parameters of theO lines of ν2+3ν3near 0.79μm(in cm-1).Values in parentheses are 1σ standard deviations at the last quoted digit.

aRotational assignment,given asbLine position from the HITRAN database[1].cLine position from Ref.[12].Signif i cant digits are omitted.dLine position derived from this work.eLine strength at 296 K,in 10-25/(cm molecule),from the HITRAN database[1].fCO2pressure-induced line shift coefficient,in cm-1atm-1.gCO2pressure-induced line width(HWHM)broadening coefficient,in cm-1atm-1.hRatio of the CO2-broadening coefficient to the air-broadening coefficient,γCO2/γair.

LineaνHITRANbνcνdSeδCO2 f γCO2 g Γh 211←21212578.74010.739573(15)0.739523(62)9.52+0.00575(24)0.1893(16)1.97 202←10112609.07280.071607(20)0.071548(36)31.9+0.01895(16)0.2018(09)1.89 321←22012635.54560.544696(31)0.544797(42)5.00-0.00295(25)0.1546(10)1.78 414←31312637.65500.654340(23)0.654286(54)11.5-0.00159(24)0.1257(08)1.43 524←42312667.75140.750327(12)0.750356(42)17.7-0.01635(17)0.1148(09)1.37 523←42212687.06620.066076(22)0.066206(37)5.19-0.03078(21)0.1663(12)1.85

where c is the speed of light,and τ0is the ring-down time of an empty cell.

Precise frequency calibration is accomplished by using the longitudinal modes of a 10 cm long thermostabilized Fabry-P′erot interferometer(FPI)made of ultra-low-expansion(ULE)glass and installed in a vacuum chamber.The f i nesse of the ULE-FPI is about 2000 near 0.79μm.Using the precise atomic transitions of87Rb at 795 nm[19]and 780 nm[20],the absolute frequencies of the longitudinal modes of the ULE-FPI in the region of 780-795 nm have been determined with an accuracy better than 0.6 MHz(2×10-5cm-1)[18]. The spectrometer has been applied to measure the absolute line positions of H2[21],C2H2[22],and H2O[12] in this region with relative uncertainty of 10-9.

III.RESULTS AND DISCUSSION

Six well isolated H216O lines of the ν2+3ν3band are studied in this work.They are located in the 788-795 nm region and their strengths vary from 5×10-25cm/molecule to 3×10-24cm/molecule.The positions,strengths and rotational assignments from the HITRAN database[1]are given in Table I.For each line,the absorption profile was measured several times under different CO2pressures.The spectral profiles of the H216O line at 12687.066 cm-1measured at different CO2pressures are presented in Fig.1.Pressureinduced effects including frequency shift and broadening are clearly shown in Fig.1.

FIG.1 The normalized profiles of the CO2-broadened H216O line at 12687.066 cm-1.

Using a least-squares fit of the spectral profile,the line parameters,including the position,intensity,and width,can be derived from each spectrum.Figure 2 shows the spectrum and fitting residuals of the observed H216O line at 12635.546 cm-1.As shown in Fig.2(b) and(e),the conventional Voigt profile cannot fit the observed spectrum well.The“soft”collision model[23], which takes into account the collision effects,was applied in the line-by-line fitting of the recorded spectrum. The Gaussian width was f i xed at the calculated value of Doppler broadening width,while other parameters,including the position,intensity,Lorentzian width(half width at half maximum,HWHM)and Dick narrowing coefficient,were derived from the fit.As shown in Fig.2(c)and(f),the observed line profiles are reproduced with the fitting residuals being at the experi-mental noise level.

FIG.2 Fit of the profile of the H216O line at 12635.546 cm-1broadenedbyCO2.(a)Observedspectrumofthe 12635.546 cm-1line broadened by CO2of 50 Torr.(b)and (c)are the residuals of the fit of spectrum(a)using the Voigt and Galatry profiles,respectively.(d)Observed spectrum of the H216O line broadened by CO2of 300 Torr.(e)and(f) are the residuals of the fit of spectrum(d)using the Voigt and Galatry profiles,respectively.

The H216O line positions derived from the fitting have a linear dependence on the CO2pressure.The results are illustrated in Fig.3.Linear fit of the line positions yields the water line center at zero CO2pressure and the collision-induced line shift coefficient.The results are collected in Table I.The obtained line shift coefficients vary considerably from-0.03 cm-1atm-1(523←422line at 12687.066 cm-1)to about+0.02 cm-1atm-1(202←101line at 12609.073 cm-1).The line positions obtained in this work agree well with our previous measurements using low-pressure pure water vapor [12].The differences are within the combined standard deviations of both studies,except for the line at 12687.066 cm-1.The position of this line determined in this work differs from our previous study by 1.3×10-4cm-1,which is about 3.5 times of the statistical deviation.It is probably a result of the considerably large CO2pressure-induced frequency shift coefficient of this line and the uncertainty of the CO2pressure: one Torr difference of the CO2pressure will induce a 4×10-5cm-1difference on the H2O line position.

The CO2pressure-induced half widths,which are also derived from fit of the observed spectra,are shown in Fig.4.The CO2pressure-induced broadening coefficients of respective transitions can be derived from a linear fit of the half widths,and the results are listed in Table I.The uncertainty on the γCO2values obtained in this work is 0.7%in average.As an comparison, Sagawa et al.reported the CO2-broadening coefficients of water lines in the 18-102 cm-1region with an average uncertainty of 2.4%using a terahertz time-domain spectrometer[24],and Poddar et al.reported the γCO2values of 10 for water lines in the 819-833 nm region with an average uncertainty of 2%[25].

FIG.3 CO2pressure-induced line shifts of six H216O lines of the ν2+3ν3band.

FIG.4 CO2pressure-induced half widths of six H216O lines of the ν2+3ν3band.

It is worth noting that owing to the high sensitivity of the cavity ring-down spectrometer applied in this work, we are able to obtain sufficient signal-to-noise ratio with relatively low water vapor pressures,which are typically 0.1-0.5 Torr in the present work.Such a low water vapor pressure reduces the interference of the broadening effects from self-collisions among the water molecules. According to the broadening coefficients given in HITRAN[1],the self-broadening coefficients γH2Oare typically one order of magnitude larger than the airbroadening coefficients γair.We have shown that the self-broadening effect can be detectable when the water vapor pressure is over 2 Torr[12].In Ref.[24],4 Torr of water vapor was used,and 20 Torr of water vapor was used in Ref.[25].The ratios of the CO2-broadening coefficients γCO2from this work to the air-broadening coefficients γairfrom HITRAN vary from 1.37 to 1.97.It coincides with that of the transitions with the same rotational quantum number J of the bands ν1,2ν2,ν3[26], and 2ν1+ν2+ν3[25].Further investigation is needed for a better understanding of the collisions between the water molecule and other perturbing gases.

IV.CONCLUSION

Six CO2-broadened H216O absorption lines in the 788-795 nm region are studied by a cavity ring-down spectrometer calibrated with precise atomic transitions. The line positions and the pressure-induced frequency shifts have been determined with an accuracy in the order of 10-5cm-1.The CO2pressure-induced broadening coefficients are determined with an relative uncertainty less than 1%.The measured CO2-broadening coefficients are found to be 1.37-1.97 times of the airbroadening coefficients.The determined γCO2coefficients of H2O transitions can be useful in the studies of the radiative transfer on Venus and Mars,and also in the modeling of combustion process.

V.ACKNOWLEDGMENTS

This work is supported by the National Natural Science Foundation(No.21225314 and No.20903085), theNationalBasicResearchProgramofChina (No.2013CB834602)and the Fundamental Research Funds for the Central Universities.

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ceived on May 20,2013;Accepted on May 27,2013)

?Author to whom correspondence should be addressed.E-mail：awliu@ustc.edu.cn