Abstract
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.
Highlights
With the development of increasingly sophisticated physical chemistry tools, both experimental and theoretical, that arose in the later part of the 1900s, the study of small fragments of matter with sizes of 2–10 atoms or molecules flourished
Three and a half decades ago, Kaldor, Cox and Zakin, several of the earlier investigators of these species referred to as clusters, asked “How large does the cluster have to be before solid state theoretical description applies? What are the magnetic properties of “naked” and “dressed” clusters? Is there catalytic chemistry possible on such clusters, etc.?”1 This exciting eruption of research was built on the seminal work by Moskovits and Hulse,[2] who co-deposited metal vapor and inert gases to create small fragments of matter
As will be noted below, “hard” clusters can have complex electronic structures and a high density of rovibronic levels of the anion that lie above the detachment limit, which can lead to indirect electron ejection processes
Summary
With the development of increasingly sophisticated physical chemistry tools, both experimental and theoretical, that arose in the later part of the 1900s, the study of small fragments of matter with sizes of 2–10 atoms or molecules flourished. As will be noted below, “hard” clusters can have complex electronic structures and a high density of rovibronic levels of the anion that lie above the detachment limit, which can lead to indirect electron ejection processes One such example is shown in Fig. 1(b): The incident photon is resonant with such an excited level of the anion, which can lead to internal conversion to a very high vibrational level of the anionic ground state and electron loss through thermionic emission, which favors low e−KE statistically. Multiple valence electrons occupy the Mo-local orbitals Because they are energetically close-lying, the PE spectrum will show multiple overlapping transitions associated with, for example, the detachment of the “red” and “green” electrons, with differences in the e−KE values associated with the different final states equaling the energy difference between the close-lying states. Vibrational and electronic action spectra leverage the mass selectability of chromophore and mass analysis of daughter ions.[77,78]
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