Destruction of AlF: a quantum study of its ground-state photodissociation

Abstract

ABSTRACT Photodissociation by ultraviolet photons is the key destruction pathway for aluminium monofluoride (AlF) in the envelope of the carbon star IRC +10216 from the stellar photosphere up to the outer layers. However, there is no available photodissociation data for AlF, which hampers the prediction of the abundances of Al-bearing molecules in astrochemical models. Here, we present an ab initio study of AlF photodissociation. Potential energy curves of seven singlet states for AlF were computed by the internally contracted multireference single and double configuration-interaction method and aug-cc-pCV5Z-DK basis set, along with the transition dipole moments from excited singlet states to the ground state. State-resolved cross sections for the direct photodissociation from 36 349 ground rovibrational levels ( υ″≤120, J″≤360) to six singlet excited states were calculated by the quantum mechanical method. We found that the 21Π←X 1Σ+, 31Π←X 1Σ+, and 41Π←X 1Σ+ transitions have extremely strong absorption for lower wavelengths, especially between the Lyman and  Lyman α ones. Photodissociation cross sections in local thermal equilibrium were estimated for gas temperatures from 500 to 20 000 K. Finally, the cross sections were utilized to calculate the photodissociation rates in the interstellar and blackbody radiation fields. The obtained photodissociation cross sections and rates can be used to determine the abundance of AlF in astrochemical models.