Spectral Line Shape Analysis Using Hartmann‐Tran Profile for Tunable Diode Laser Absorption Spectroscopy of Water Vapor at 1.39 μm

Abstract

In this study, spectral line shape analysis was conducted for water vapor absorption at 1.39 μm with a Hartmann‐Tran profile (HTP), which is capable of being efficiently and accurately reduced to (speed dependent) Voigt and (speed dependent) Rautian profiles. The near‐IR absorption lines of water vapor were measured using tunable diode laser absorption spectroscopy (TDLAS). The spectrum fitting code was home‐written based on the Trust region reflective algorithm and the HTP. Voigt profile (VP), Rautian profile (RP), speed‐dependent VP (SDVP), and speed‐dependent Rautian profile (SDRP), which were reduced from the HTP, were used as model functions to test the validity of the near‐IR absorption lines of water vapor in terms of line shape profile. With the SDVP, the standard deviation of the fitting residuals was at a 10−3 level relative to the most intense peak height in a range of 1.39 μm at 2.5 Torr and room temperature, while the global standard deviation was less than 0.5 × 10−3, thus showing the best result among them. This suggests the need to consider the speed‐dependent collision width in the self‐broadening of water vapor.