The photodissociation dynamics of CFBr exited into the A(~1A') state

摘要

The dynamics of the photolysis reaction, CFBr + h#upsilon# -> CF + Br, have been investigated for photolysis energies in the range, #upsilon# = 23 500 - 26 000 cm~(-1) (#lambda# = 385 - 435 nm). These energies correspond to excitation into the A(~1A") state of CFBr with 2500-5000 cm~(-1) of excess vibrational energy. Following dissociation of jet-cooled CFBr, the internal energy (#OMEGA#, #LAMBDA#, J) on the nascent CF fragments (X ~2#PI#, #upsilon# = 0) was probed by laser induced fluorescence spectroscopy. Two distinct types of product state distributions were observed. At energies above T_(00) + 3360 cm~(-1) the populations of the ~2#PI#_(1/2) and ~2#PI#_(3/2) 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 A state, over a barrer 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 T_(00) + 3360 cm~(-1) the ground spin-orbit state of CF (~2#PI#_(1/2)) is preferred, while the lambda doublet populations are equal. The interpretation of these populations is that at these energies A state CFBr is stable with respect to dissociation over the barrier. The molecule crosses to either the X or a 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.