Photoelectron imaging spectroscopy of Ag₃⁻ in the S₀ and S₁ states

We explore laboratory-frame photoelectron angular distributions (LF-PADs) originating from the outermost two orbitals, σ_u [the highest occupied molecular orbital (HOMO)] and σ_g (HOMO−1), of the silver trimer anion (Ag₃⁻) in the S₀(¹Σ_g⁺) state. The experiment was performed by our novel photoelectron imaging technique using a high-repetition-rate tunable laser [T. Horio et al., J. Chem. Phys. 162, 026101 (2025)]. The LF-PAD for σ_u is found to be highly energy-dependent in a photoelectron kinetic energy (PKE) range from 0 to 1.57 eV; an isotropic LF-PAD with an anisotropy parameter β ∼ 0 is observed at the detachment threshold, exemplifying the Wigner threshold law, whereas the β value decreases down to β = −0.4 at 0.46 eV, followed by an increase up to β = 0.6 at 1.57 eV, as PKE increases. A small dip discernible in the PKE range of 1.2-1.3 eV suggests an influence of the autodetachment process on the β value, which is via bound electronic state(s) embedded in the D₀(²Σ_u⁺) + e⁻ continuum. On the other hand, the LF-PAD for σ_g exhibits a strong anisotropy parallel to the laser polarization with β of ∼1 in 0-0.39 eV. These contrasted trends are qualitatively reproduced by theoretical modeling of LF-PAD that accounts for photoelectron partial waves allowed for each photodetachment process. Furthermore, two-photon detachment spectra via the excited S₁(¹Σ_u⁺) state are presented, where the relative band intensities for the two detachment channels, D₀(²Σ_u⁺) + e⁻ ← S₁(¹Σ_u⁺) and D₁(²Σ_g⁺) + e⁻ ← S₁(¹Σ_u⁺), are discussed in terms of their leading electronic configurations.