Laser determinations of ‘‘hot band’’ quantum yields: Br*(2P1/2) formation in the continuum absorption of Br2 at 510–550 nm

Abstract

Absolute quantum yields of Br* in the photodissociation of thermally excited vibrational levels of Br2 are measured by transient gain vs absorption probing of the Br*(2P1/2)–Br(2P3/2) transition at 2.7μm with an F-center laser. An etalon-narrowed pulsed dye laser with a linewidth of 0.04 cm−1 is used to excite continuum regions between the bound Br2(B–X) transitions of isotopic Br2(∼81% 81Br2) at selected wavelengths between 510 and 550 nm. The threshold wavelength for production of Br*+Br from v″=0, J″=0 is 510.7 nm. At 297 K the Br* yields vary from 85% at 510 nm to 12% at 550 nm. Absorption coefficient data measured with the laser together with the Br* quantum yields at both 297 and 373 K enable a direct determination of the B-state continuum absorption. The data are used to determine the individual contributions of the B 3Π(0+u), 1Π(1u) and A 3Π(1u) states to the continuum absorption in this spectral region. Good agreement is obtained between the measured B-state absorption coefficients and those calculated from the potentials of Le Roy et al., which were obtained by a fit to the total absorption data of Passchier et al. Deviations from Le Roy’s fit at very long wavelengths suggest an adjustment of the inner turning point of the B-state potential near the Br*+Br dissociation limit. The A-state continuum band has a peak absorption coefficient of ∼12ℓ mol−1 cm−1 at around 530 nm.