Ion imaging studies of the Br(2PJ) atomic products resulting from Br2 photolysis in the wavelength range 260–580 nm

Abstract

The formation of bromine atoms in their ground ²P_3/2^o (Br) and spin–orbit excited ²P_1/2^o (Br*) states as a result of photolysis of Br₂ has been investigated at 260 nm and at 22 different wavelengths in the range 360–580 nm using the technique of photofragment ion imaging. Image analysis yields both the angular distributions and spin–orbit branching ratios of the photofragments at the various wavelengths, which serve to confirm and extend previously reported measurements of these quantities.

Only ground state Br atoms are observed following excitation in the wavelength range 360–430 nm. The distribution of recoil velocity vectors for this Br+Br product channel is found to be well described by an anisotropy parameter β=-1.0±0.1, consistent with previous interpretations of the intense parent absorption at these wavelengths in terms of a perpendicular (ΔΩ=±1) transition from the X¹Σ_g⁺(0_g⁺) ground state to an excited state of ¹Π_u(1_u) symmetry. Images recorded following excitation within the wavelength range 430–500 nm display a second ring attributable to the Br+Br* product channel. The angular anisotropy of this inner ring is described by β≈1.5 at excitation wavelengths ≈450 nm, but this declines to β≈0.6 at the longest excitation wavelength. The sign and the variation in magnitude of these β values suggest that parallel (ΔΩ=0) excitation to the B³Π_u(0_u⁺) state is the principal route to Br+Br* products at the shorter wavelengths, but that an alternative perpendicular route to these products involving initial excitation to the A³Π_u(1_u) state and subsequent transfer to a repulsive 1_u state may play a role at the longer wavelengths. The relative intensities of the two velocity sub-groups in the ion images provide a direct measure of the Br+Br versus Br+Br* channel branching ratio: the latter makes only a minor contribution to the total product flux at λ_phot=430 nm, but this increases to ≈79% at λ_phot=500 nm. None of the images show any evidence of contributions from the possible Br*+Br* product channel. In accord with thermodynamic expectation, only ground state Br+Br products are observed following excitation at wavelengths λ>520 nm. These display a recoil anisotropy consistent with formation via transitions(s) of perpendicular character: the product flux is attributed to dissociation following excitation to the repulsive inner limb of the A³Π_u(1_u) potential curve.