The photodissociation dynamics of CFBr excited into the Ã(1A″) state

Abstract

The dynamics of the photolysis reaction, CFBr+hν→CF+Br, have been investigated for photolysis energies in the range, ν̄=23 500–26 000 cm−1 (λ=385–435 nm). These energies correspond to excitation into the Ã(1A″) state of CFBr with 2500–5000 cm−1 of excess vibrational energy. Following dissociation of jet-cooled CFBr, the internal energy (Ω, Λ, J) of the nascent CF fragments (X 2Π, υ=0) was probed by laser induced fluorescence spectroscopy. Two distinct types of product state distributions were observed. At energies above T00+3360 cm−1 the populations of the Π1/22 and Π3/22 spin–orbit states of CF were equal, while A″ lambda doublet states were preferred over A′. These populations are consistent with a direct dissociation mechanism on the à state, over a barrier with a height of 3360 cm−1. The strong state mixing in the vicinity of the barrier ensures a statistical mixture of final spin–orbit states. The preference for the A″ lambda doublet states is consistent with the two lone electrons in in-plane orbitals pairing up in the final CF product, leaving one unpaired electron in an out-of-plane orbital, lying parallel to the J vector of the recoiling fragment. For excitation at energies below T00+3360 cm−1 the ground spin–orbit state of CF (2Π1/2) is preferred, while the lambda doublet populations are equal. The interpretation of these populations is that at these energies à state CFBr is stable with respect to dissociation over the barrier. The molecule crosses to either the X̃ or ã state where it encounters a deep attractive potential well. The subsequent slower dissociation rate allows the molecule to follow a more adiabatic pathway producing the lowest spin–orbit state of CF, and for any preference for lambda doublet states to be lost.