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.