Photodissociation of Cl2 on the shorter wavelength side of the first absorption band has been known to yield a small but significant amount of Cl(2P1/2) from the C 1∏u state, despite its adiabatic correlation to the two Cl(2P3/2) atoms. We calculated some potential energy curves of the ground and excited states of Cl2 by the spin-orbit configuration interaction method and examined the possibilities of several nonadiabatic transition mechanisms. It was found that the radial Rosen-Zener-Demkov (RZD)-type nonadiabatic transition from the C 1∏u to the third Ω = 1u (3Σ+ 1u (σg → σ*u)) state is responsible for the production of Cl(2P1/2), and the rotational nonadiabatic transition probability from the C 1∏u to the B 3∏0+u state is negligibly small. The wavelength dependence of the product branching ratio Cl(2P1/2)/Cl(2P3/2) and the anisotropy parameter β(Cl(2P1/2)), which was calculated from the electronic transition moments to the A 3∏1u, B 3∏0+u, and C 1∏u states with the RZD transition mechanism, was in good agreement with their experimental behavior. This RZD model and Young's double slit model could also reproduce the quantum-mechanical interference pattern in the orientation of the total angular momentum J = 1/2 of the products Cl(2P1/2).
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