Creation date: March 2024

Last update: June 2024    

Key words: collisional line-broadening parameters, exoplanetary atmospheres, exotic molecular pairs, high temperatures, semi-classical calculations

 

COLlisional-LINE-broadening database provides semi-classical rotationally independent estimates [1,2] of pressure-broadening coefficients (in cm^-1 atm^-1) for a range of “exotic” spectroscopically active molecules relevant to exoplanetary atmospheres. The list of perturbers is currently limited to N₂, O₂, H₂, He, Ar, CO, NO, CO₂, H₂O, CH₄ and NH₃; the self-perturbation case is also included.

The pressure-broadening coefficients γ are given for the reference temperature 296 K and can be scaled to a desired temperature T via the formula

γ(T)= γ(296) (296/T)^0.5

The estimates are based on the input data (referenced below) including the interaction index m (= 4, 6, 8, 10 for dipole-dipole, dipole-quadrupole, quadrupole-quadrupole and dipole-"induced dipole" leading interactions), molecular masses m_a and m_p for the active and perturbing molecules as well as kinetic molecular diameters d_a and d_p (or Lennard-Jones σ parameters) found in the literature. When different values of d_a are available, multiple pressure-broadening coefficients are given. One of them, marked by an asterisk, is selected for including in the ExoMol database. (Typically, the selected value corresponds to the measured/calculated kinetic diameter closest to the data-set averaged value or to the most recent value.)

For each molecular pair, an estimate T_min (in K) is provided for the low-temperature limit: for temperatures approaching this limit and below the semi-classical treatment looses its validity. Each "minimal-temperature" value is taken as the depth of the isotropic potential energy of the colliding molecular pair: if the temperature (kinetic energy of the relative molecular motion) is above T_min, the molecules separate after the collision (open trajectory); in the opposite case, orbiting collisions take place and the semi-classical formulae do not work. For the majority of the considered collision partners, T_min values lie far below the reference temperature 296 K, so that the line-broadening estimates provided for 296 K are physically meaningful and can be scaled to any desired temperature T>T_min. However, for some collision partners (KCl with KCl, CO2, H2O, NH3 and self-perturbed LiF and MgO), the information found in the literature on the isotropic interaction potentials argues in favor of T_min values superior to 296 K (see files for the cited active species), and the provided line-broadening coefficients are valid only for T > T_min > 296 K. It should be also noted that the use of the broadening coefficients at T smaller than T_min will lead to an underestimation of the real broadening (contributions from molecular quasi-complexes not accounted for).