Theoretical Study of the Low-Lying Electronically Excited States of OBrO

Abstract

Motivated by the possible importance of OBrO in atmospheric photochemistry, multireference configuration interaction calculations of the low-lying excited states were carried out to obtain information about the electronic vertical spectrum up to excitation energies of about 6 eV from the ground state, including the transition dipole moments, and about possible photodissociation pathways, based on one-dimensional cuts through the potential energy surfaces for dissociation into BrO + O and Br + O2, respectively. In addition, for probing the angle dependence the bending potentials were also calculated. From all computed eight doublet states (two/four of each symmetry in C2v Cs) only the 12A2 state at 2.7 eV possesses a large transition dipole moment with the 12B1 ground state, whereas for all other states this quantity is very small or zero. Therefore the 12A2 state should play a decisive role in OBrO photochemistry. Close to the 12A2 state two other states were found at 2.4 eV (12B2) and 2.5 eV (12A1) so that interactions of these three states should certainly influence possible dissociation processes. For this reason, besides direct adiabatic photodissociation of the 12A2 state into BrO + O also predissociation via these close-lying states can be expected, leading to a very complex photodissociation mechanism for excitation energies around 2.5 eV. Moreover, in this energy range photodissociation into Br + O2 is only possible through the 12B2 state (after initial excitation of the 12A2 state) because only for this state a small barrier of 0.7 eV relative to its minimum is estimated from the calculation of a simplified C2v minimum energy path. For the 12A1 and 12A2 states, rather large barriers are predicted. The next higher-lying states, with excitation energies of 3.9 eV (22A1) and 4.5 eV (22B2) are well separated from lower- and higher-lying states and from each other, but due to their small transition dipole moments, they should be probably of minor importance for the OBrO photochemistry. The last two states considered in our study are predicted to lie close together at 6.0 eV (22A2) and 6.1 eV (22B1) and are strongly repulsive upon dissociation into BrO + O. Finally, it should be noted that our calculations demonstrate the expected qualitative similarity to the results previously obtained for the corresponding OClO system.