Small clusters have captured the imaginations of experimentalists and theorists alike for decades. In addition to providing insight into the evolution of properties between the atomic or molecular limits and the bulk, small clusters have revealed a myriad of fascinating properties that make them interesting in their own right. This perspective reviews how the application of anion photoelectron (PE) spectroscopy, typically coupled with supporting calculations, is particularly well-suited to probing the molecular and electronic structure of small clusters. Clusters provide a powerful platform for the study of the properties of local phenomena (e.g., dopants or defect sites in heterogeneous catalysts), the evolution of the band structure and the transition from semiconductor to metallic behavior in metal clusters, control of electronic structures of clusters through electron donating or withdrawing ligands, and the control of magnetic properties by interactions between the photoelectron and remnant neutral states, among other important topics of fundamental interest. This perspective revisits historical, groundbreaking anion PE spectroscopic finding and details more recent advances and insight gleaned from the PE spectra of small covalently or ionically bound clusters. The properties of the broad range of systems studied are uniquely small-cluster like in that incremental size differences are associated with striking changes in stability, electronic structures, and symmetry, but they can also be readily related to larger or bulk species in a broader range of materials and applications.

Mixed-metal oxides have proven to be effective catalysts for the hydrogen evolution reaction, often outperforming either of the binary metal oxides. The reactivity of MnxMoOy− (x = 1, 2; y = 3, 4) clusters toward H2O was investigated via time-of-flight mass spectrometry with clear evidence of cluster oxidation and corresponding H2 production, specifically for MnxMoO3− (x = 1, 2) clusters. Unlike previously studied MoxOy− clusters, which assumed a broad distribution of stoichiometries (typically x ≤ y ≤ 3x), both MnMoOy− and Mn2MoOy− preferentially formed y = 3 and 4 compositions in significant quantities under our source conditions. The electronic and molecular structures of the MnxMoOy (x = 1, 2; y = 3, 4) anion and neutral clusters were probed with anion photoelectron spectroscopy and analyzed with supporting density functional theory calculations. Our studies suggest that both metal centers are involved in initial cluster–water complex formation, while Mo is the center that undergoes oxidation; hence, reactivity terminates when Mo is saturated in its highest oxidation state of +6. Across these four clusters, Mn remains relatively reduced and is stable in a high-spin electronic configuration. The preferential reactivity of water molecules toward the Mo center rather than Mn is rationalized by the much lower relative oxophilicity of Mn.

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Cluster anions are commonly made in Smalley-type laser ablation sources, which generate a wide range of species in different charge states, potentially causing ion loss due to charge recombination in thermalization channels. We present an anion photoelectron spectroscopy study of anions selectively funneled into a thermalization channel using a planar ion funnel (PIF). This funnel can increase ion current and allow selection of different portions of the ion plume generated in the cluster source. The vibrational temperatures of funneled anions are evaluated by analysis of hot band transitions, and we present the advantages and disadvantages of implementing this mass spectrometric tool in studies that rely on ablation for ion production.

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