Spectral Line-Shape Model to Replace the Voigt Profile in Spectroscopic Databases

<p>The standard description of molecular line shapes in spectral databases and radiative transfer codes is based on the Voigt profile. It is well known that its simplified assumptions of absorber free motion and independence of collisional parameters from absorber velocity lead to systematic errors in analysis of experimental spectra, and retrieval of gas concentration.</p> <p>We demonstrate^1<em>,</em>2 that the partially correlated quadratic speed-dependent hardcollision profile³. (pCqSDHCP) is a good candidate to replace the Voigt profile in the next generations of spectroscopic databases. This profile takes into account the following physical effects: the Doppler broadening, the pressure broadening and shifting of the line, the velocity-changing collisions, the speed-dependence of pressure broadening and shifting, and correlations between velocity- and phase/state-changing collisions. The speed-dependence of pressure broadening and shifting is incorporated into the pCqSDNGP in the so-called quadratic approximation. The velocity-changing collisions lead to the Dicke narrowing effect; however in many cases correlations between velocityand phase/state-changing collisions may lead to effective reduction of observed Dicke narrowing. The hard-collision model of velocity-changing collisions is also known as the Nelkin-Ghatak model or Rautian model.</p> <p>Applicability of the pCqSDHCP for different molecular systems was tested on calculated and experimental spectra of such molecules as H₂, O₂, CO₂, H₂O in a wide span of pressures. For all considered systems, pCqSDHCP is able to describe molecular spectra at least an order of magnitude better than the Voigt profile with all fitted parameters being linear with pressure. In the most cases pCqSDHCP can reproduce the reference spectra down to 0.2% or better, which fulfills the requirements of the most demanding remote-sensing applications.</p> <p>An important advantage of pCqSDHCP is that a fast algorithm for its computation was developed<em>^ab</em>^4<em>,</em>5 and allows for its calculation only a few times slower than the standard Voigt profile. Moreover, the pCqSDHCP reduces to many simpler models commonly used in experimental spectra analysis simply by setting some parameters to zero, and it can be easily extended to incorporate the line-mixing effect in the first-order approximation.</p> <p> </p> <p>The idea of using pCqSDHCP as a standard profile to go beyond the Voigt profile for description of H₂O line shapes was recently supported by the IUPAC task group⁶ which also recommended to call this profile with fast computation algorithm the HTP profile (for Hartmann-Tran).