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, σu [the highest occupied molecular orbital (HOMO)] and σg (HOMO-1), of the silver trimer anion (Ag3-) in the S0(1Σg+) 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 σu 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(2Σu+) + e- continuum. On the other hand, the LF-PAD for σg 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(1Σu+) state are presented, where the relative band intensities for the two detachment channels, D0(2Σu+) + e- ← S1(1Σu+) and D1(2Σg+) + e- ← S1(1Σu+), are discussed in terms of their leading electronic configurations.