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.

Electronic spectroscopy of bromomethylenes in a supersonic free jet expansion

“…Based upon the known 2 v 0 and 1 1 0 2 v 0 progressions, these five bands can be assigned to the CBr 2 A ˜À X ~215 0 (17 864 cm À1 ), 1 1 0 2 13 0 (17 964 cm À1 ), 2 16 0 (18 048 cm À1 ), 1 1 0 2 14 0 (18 147 cm À1 ) and 2 17 0 (18 231 cm À1 ) bands. Kable and co-workers 36,37 reported a nonradiative pathway in the A state of CFCl and in that of CFBr based on the observation of the shortened radiative lifetimes in the A ˜state levels. For CFBr, this dissociation pathway corresponds to the C-Br bond cleavage and the dissociation barrier is at approximately 3250 AE 150 cm À1 above the zero-point level of the A ˜state.…”

Geometries, vibrational frequencies, and excitation energies of a series of fluorine-substituted carbenes, FCX (X = H, F, Cl, Br, and I): A high-level multireference configuration interaction study

“…To date, most experimental studies concerned the small fluorine‐substituted carbenes, FCH, and CF 2 . Because the early emission and absorption spectroscopic studies of CF 2 were carried out several decades ago, many experimental efforts have been made to determine the structure and spectroscopic constants of the ground and first excited singlet states of FCH (and/or FCD(isotope of FCH)), CF 2, FCCl, as well as FCBr, , using a variety of laser‐based spectroscopic techniques. For some carbenes, experimental studies for the spectrum and dynamics of the states beyond the A state were carried out recently, for example, the B state of FCH and dissociation dynamics of FCCl and FCBr at 193 nm .…”

Photodissociation dynamics of dichlorocarbene at 248 nm

“…6 At sufficiently high excitation energies, no fluorescence is observed from the decay of Franck-Condon accessible CFBr(A ˜1A 0 0 ) vibronic levels. Knepp and Kable 7 have directly detected the CF photofragment from the predissociation and photodissociation of CFBr(A ˜1A 0 0 ) and determined its internal state distribution for various CFBr excitation energies.…”