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(2P1/2–2P3/2) 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*(2P1/2) 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.