Quasiclassical and quantum mechanical modeling of the breakdown of the axial recoil approximation observed in the near threshold photolysis of IBr and Br_2

摘要

The photodissociations of jet-cooled IBr and Br_2 molecules have been investigated using high resolution ion imaging methods, at excitation energies just above the thresholds for forming, respectively, I(~2P_3/2~o) + Br(~2P_3/2~o) and Br(~2P_3/2~o) + Br~*(~2P_1/2~o) products from parent molecules in their v" = 0 levels. For such molecules, we observe in both cases, that fragments with larger recoil velocities have markedly reduced angular anisotropy, whereas those from photolysis of IBr molecules with v" = 1 show an essentially constant, limiting anisotropy. Given the monochromaticity of the photolysis radiation, increased recoil velocity of fragments resulting from photolysis of v" = 0 molecules can only be derived from increased parent internal (rotational) energy. The measurements thus provide a particularly clear and direct observation of the breakdown of the axial recoil approximation as applied to the photodissociation of a diatomic molecule, and have been modeled, quantitatively, using both quantum and semiclassical methods together with the best available potential energy curves for the relevant excited states of IBr and Br_2.