The four-centre reaction I *2+ F2 studied by laser-induced chemiluminescence in molecular beams
Abstract
The four-centre reaction between electronically-excited I*2 and ground-state F2 has been studied under crossed-beam conditions by observing the resulting IF* chemiluminescence. The I2* is prepared in the v′= 43 level of the B3Π(0+u) state using the 514.5 nm line of the argon ion laser as an excitation source. The IF* emission is observed to the blue of the laser line and a spectroscopic analysis indicates that the IF* reaction product that is formed in the B3Π(0+) state has a non-Boltzmann vibrational distribution extending to the predissociation limit. For the I*2(B)+ F2(X) reaction, the energetics allow the formation of two IF product molecules, both of which may be electronically-excited, but do not allow the formation of IF*(B)+ I + F. Although the energetically-favourable ground state reaction I2(X)+ F2(X)→ IF(X)+ IF(X) appears not to take place, the reaction I*2(B)+ F2(X)→ IF*(B)+ IF(X or A) occurs with a reaction cross section estimated to be between 10–50 × 10–16 cm2. From the correlation of orbital symmetries (Woodward-Hoffmann rules), the four-centre reaction I2+ F2 as well as I*2+ F2 both appear to be formally forbidden. A discussion is presented suggesting possible ways in which this symmetry restriction for I*2+ F2 can be relaxed.