Transition strengths and potential curves for the valence transitions in Br2 from a reanalysis of the ultraviolet-visible absorption at low resolution

Abstract

The absorption profile of gaseous Br2 in the 320–675 nm region is reanalyzed using a quantum mechanical nonlinear least-squares approach. For the first time the A(1u 3Π)←X(1Σg+) transition is included along with the two stronger transitions, B(0u+ 3Π)←X and C(1u 1Π)←X. The analyzed data include absorption spectra at temperatures between 23 °C and 440 °C along with specific estimates of B←X and (A+C)←X absorption. A new computational device facilitates inclusion of the discrete region of the spectrum (λ>510 nm) in the analysis: The A←X and B←X transitions are treated as entirely bound-free, which is accomplished computationally by just removing the attractive branches of the A and B potentials. The new analysis determines the molar absorptivities of all three transitions within ∼1 L mol−1 cm−1. Although the high precision is partly due to the inclusion of the specific B←X and (A+C)←X data, test computations show that the resolution can be achieved reliably by fitting just the T-dependent spectra. At 23 °C the component peaks are: 10.6 L mol−1 cm−1 at 525 nm (A←X), 82.5 L mol−1 cm−1 at 480 nm (B←X), and 158 L mol−1 cm−1 at 412 nm (C←X).