The photodissociation dynamics of CFBr excited into the (A)over-tilde((1)A'') state

The dynamics of the photolysis reaction, CFBr+h nu-->CF+Br, have been inves tigated for photolysis energies in the range, (nu)over-bar=23 500-26 000 cm (-1) (lambda=385-435 nm). These energies correspond to excitation into the (A) over tilde tilde((1)A ") state of CFBr with 2500-5000 cm(-1) of excess vibrational energy. Following dissociation of jet-cooled CFBr, the internal energy (Omega, Lambda, J) of 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 c m(-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('). T hese populations are consistent with a direct dissociation mechanism on the (A) over tilde state, over a barrier with a height of 3360 cm(-1). The str ong state mixing in the vicinity of the barrier ensures a statistical mixtu re of final spin-orbit states. The preference for the A " lambda doublet st ates 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-pla ne orbital, lying parallel to the J vector of the recoiling fragment. For e xcitation 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 equ al. The interpretation of these populations is that at these energies (A) o ver tilde state CFBr is stable with respect to dissociation over the barrie r. The molecule crosses to either the (X) over tilde or (a) over tilde stat e where it encounters a deep attractive potential well. The subsequent slow er dissociation rate allows the molecule to follow a more adiabatic pathway producing the lowest spin-orbit state of CF, and for any preference for la mbda doublet states to be lost. (C) 1999 American Institute of Physics. [S0 021-9606(99)01124-1].