Formation, relaxation, and quenching of Br₂ excited states in CCl₄

The photoexcitation, relaxation, and quenching of bound electronic excited states of Br₂ in CCl₄ are studied by time-resolved photothermal grating and near-IR fluorescence spectroscopies. A long-lived component is observed having a lifetime of 18 ns that depends on Br₂ and oxygen concentrations in the sample. This long-lived component is assigned to the decay of the lowest excited state, A′(³II_2u). From the amplitudes of photothermal diffraction, the quantum yield of the A′ state formation is estimated to be 0.50 ± 0.08. A Stern-Volmer plot of the measured decay rate of the A′ state versus Br₂ concentration, including results obtained from both photothermal grating and near-IR fluorescence measurements, indicates that self-quenching of A′ state proceeds at a rate that is slower than diffusion limited.