Accurate quantum yields by laser gain vs absorption spectroscopy: Investigation of Br/Br* channels in photofragmentation of Br₂ and IBr

A two-laser pulse-and-probe technique is used to study photofragmentation of Br₂ and IBr over the wavelength range 450-530 nm. The metastable Br(²P_1/2-²P_3/2) transition is probed by time-resolved laser gain vs absorption spectroscopy using a tunable color center laser. This new approach to the measurement of quantum yields provides highly accurate absolute values for Br*(²P_1/2) production. The peak quantum yield for Br₂ photodissociation is φ = 87% at λ = 500 nm. The difference between the spectral variation of φ and the total absorption spectrum characterizes the A state of bromine, which contributes ≲ 14% to the absorption spectrum at λ = 510 nm. The peak in the Br* yield from photofragmentation of IBr is φ ∼ 73% at λ = 500 nm. The present absolute IBr data together with the previous molecular beam studies suggest a reassessment of the contributions of the continuum states in IBr. The laser gain vs absorption method for obtaining quantum yields is readily generalized to other atoms and molecules.