Effects of isomer coexistence and solvent-induced core switching in the photodissociation of bare and solvated (CS₂)₂⁻ anions

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The photodissociation of the (CS₂)₂⁻ dimer anion, known to exist in the form of several electronic and structural isomers, has been investigated at 532, 355, and 266 nm. The observed anionic fragments are CS₂⁻ and C₂S₂⁻ at 532 nm, and C₂S₂⁻, CS₂⁻, CS₃⁻, S₂⁻, and S⁻ at 355 and 266 nm. In addition to the photon energy, the fractional yields of the photofragments depend on the ion source conditions and solvation of the dimer anion. Specifically, the (C₂S₂⁻+S₂⁻)/CS₂⁻ product ratio is significantly higher when (CS₂)₂⁻ is formed in the presence of water in the precursor gas mixture, even though the parent anion itself does not include H₂O. On the other hand, an abrupt decrease in the above product ratio is observed upon the addition of solvent molecules (CS₂ or H₂O) to the (CS₂)₂⁻ anion. Since the variation of this product ratio exhibits positive correlation with the relative intensity of the photoelectron band assigned to the C_2v (²B₁) covalent structure of C₂S₄⁻ by Habteyes et al. [J. Phys. Chem. A 112, 10134 (2008)], this structure is suggested as the primary origin of the C₂S₂⁻ and S₂⁻ photoproducts. The switching of the fragmentation yield from C₂S₂⁻ and S₂⁻ to other products upon solvation is ascribed to the diminished presence of the C_2v (²B₁) dimer-anion structure relative to the CS₂⁻ based clusters. This population shift is attributed to the more effective solvation of the latter. The CS₂⁻ based clusters are suggested as the origin of the S⁻ photoproduct, while CS₃⁻ is formed through the secondary S⁻+CS₂ intracluster association reaction. ©2009 American Institute of Physics