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Tunable diode laser spectroscopy and line parameters of water vapor in the near infrared

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In this study, we present the technique of tunable diode laser spectroscopy and the spectroscopic parameters of 13 lines of H2O in the 11980 -12260 cm−1 spectral region. Spectra were recorded at room temperature for a wide range of pressures (2 - 15 Torr for pure H2O and 50 - 760 Torr for H2O in air).
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Tunable diode laser spectroscopy and line parameters of water vapor in the near infrared JOURNAL OF SCIENCE OF HNUE Mathematical and Physical Sci., 2014, Vol. 59, No. 7, pp. 157-164 This paper is available online at http://stdb.hnue.edu.vn TUNABLE DIODE LASER SPECTROSCOPY AND LINE PARAMETERS OF WATER VAPOR IN THE NEAR-INFRARED Ngo Ngoc Hoa and Nguyen Manh Nghia Faculty of Physics, Hanoi National University of Education Abstract. In this study, we present the technique of tunable diode laser spectroscopy and the spectroscopic parameters of 13 lines of H2 O in the 11980 -12260 cm−1 spectral region. Spectra were recorded at room temperature for a wide range of pressures (2 - 15 Torr for pure H2 O and 50 - 760 Torr for H2 O in air). Line parameters were adjusted from experiments using the least square method, Fortran and three line-shape models: the Voigt profile (VP), the (hard collision) Rautian profile (RP) and the speed dependent Voigt profile (SDVP). A comparison between our results with the HITRAN database 2012 shows good agreement. Keywords: Water vapor, line-shape, tunable diode laser spectroscopy.1. Introduction Water vapor is a key molecule in the Earth’s atmosphere, its distribution the objectof several remote sensing experiments. The latter provide recordings of atmosphericabsorption spectra whose inversion yields the atmospheric humidity vertical profile. Forsuch applications, precise knowledge of the spectroscopic parameters of the H2 O linesis needed. In this work, line parameters (position, integrated intensity and broadeningcoefficient) are deduced from laboratory spectra assuming a specific line-shape. For this,the Voigt profile [1] is used in most of the available studies. Within this model, twocollisional parameters are used, i.e. the Lorentz broadening and shifting coefficients, theDoppler width being fixed to its theoretical value. As is well known, [2-4], the Voigtmodel can lead to discrepancies with measured spectra since it does not take into accounttwo velocity effects: (a) collision-induced velocity changes, leading to the so-called Dickenarrowing and (b) the speed dependence of the pressure-induced width and shift. Usingmore refined models that take into account such effects, several studies showed that theline broadening determined by the Voigt profile can be underestimated up to 10% [2, 4]while the error for integrated intensity is from 0.3 to 2% [5, 6].Received August 12, 2014. Accepted October 13, 2014.Contact Ngo Ngoc Hoa, e-mail address: hoa.nn@hnue.edu.vn 157 Ngo Ngoc Hoa and Nguyen Manh Nghia Three spectral shape models have been used: the Voigt, the Rautian (to take intoaccount the Dicke narrowing effect) and the speed dependent Voigt models (to takeinto account the speed dependence of the collisional parameters). The correspondingabsorption coefficients for a single line are given by [7]: √ S.PH2 O ln2 α (σ) = √ V Re {W (σ − σ0 , ΓD , ∆, Γ)} , (1.1) π ΓD √ { } S.PH2 O ln2 W (σ − σ0 , ΓD , ∆, Γ + B) α RP (σ) = √ Re √ , (1.2) π ΓD 1 − πBW (σ − σ0 , ΓD , ∆, Γ + B)   √     S.PH2 O ln2  +∞ e−t [1 − Z ′ (t)] 2  α SDV P (σ) = Im ∫ √ dt , (1.3) π 3/2 ΓD  −∞ ln2    (σ − σ0 ) − t − Z(t)  ΓDwhere PH2 O is the partial pressure of H2 O and S is the integrated intensity of the line. σ0and ΓD are the unperturbed spectral position of the transition, and the Doppler width Γ,△ and B are the collisional half-width (HWHM), the pressure induced line-shift, and thenarrowing parameter, respectively. The complex ...

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