Dipole effects in the photoelectron angular distributions of the sulfur monoxide anion

Photoelectron angular distributions (PADs) in SO⁻ photodetachment using linearly polarized 355 nm (3.49 eV), 532 nm (2.33 eV), and 611 nm (2.03 eV) light were investigated via photoelectron imaging spectroscopy. The measurements at 532 and 611 nm access the X³Σ⁻ and a¹Δ electronic states of SO, whereas the measurements at 355 nm also access the b¹Σ⁺ state. In aggregate, the photoelectron anisotropy parameter values follow the general trend with respect to electron kinetic energy (eKE) expected for π*-orbital photodetachment. The trend is similar to O₂⁻, but the minimum of the SO⁻ curve is shifted to smaller eKE. This shift is mainly attributed to the exit-channel interactions of the departing electron with the dipole moment of the neutral SO core, rather than the differing shapes of the SO⁻ and O₂⁻ molecular orbitals. Of the several ab initio models considered, two approaches yield good agreement with the experiment: one representing the departing electron as a superposition of eigenfunctions of a point dipole-field Hamiltonian, and another describing the outgoing electron in terms of Coulomb waves originating from two separated charge centers, with a partial positive charge on the sulfur and an equal negative charge on the oxygen. These fundamentally related approaches support the conclusion that electron-dipole interactions in the exit channel of SO⁻ photodetachment play an important role in shaping the PADs. While a similar conclusion was previously reached for photodetachment from σ orbitals of CN⁻ (Hart, Lyle, Spellberg, Krylov, Mabbs, J. Phys. Chem. Lett., 2021, 12, 10086-10092), the present work includes the first extension of the dipole-field model to detachment from π* orbitals.