Photodissociation of aryl halides in the gas phase studied with femtosecond pump-probe spectroscopy

The photodissociation dynamics of iodo-, bromo- and chlorobenzene upon excitation at 266 nm have been studied by femtosecond pump-probe spectroscopy combined with time-of-flight (TOF) mass spectrometric detection. The kinetics are characterized by a single time constant for chlorobenzene (1 ns) and bromobenzene (28 ps), and by two time constants for iodobenzene (700 and 350 fs). The time constant of 350 fs is due to direct dissociation. The increase of the other time constants with decreasing halogen mass supports the assignment of these constants to the decay of the initially excited (π,π∗) states to the repulsive triplet (n,σ∗) state due to spin–orbit coupling.