Photoelectron Spectra of Gd₂O₂^– and Nonmonotonic Photon-Energy-Dependent Variations in Populations of Close-Lying Neutral States

Abstract

Photoelectron spectra of Gd2O2 – obtained with photon energies ranging from 2.033 to 3.495 eV exhibit numerous close-lying neutral states with photon-energy-dependent relative intensities. Transitions to these states, which fall within the electron binding energy window of 0.9 and 1.6 eV, are attributed to one- or two-electron transitions to the ground and low-lying excited neutral states. An additional, similar manifold of electronic states is observed in an electron binding energy window of 2.1–2.8 eV, which cannot be assigned to any simple one-electron transitions. This study expands on previous work on the Sm2O– triatomic, which has a more complex electronic structure because of the 4f6 subshell occupancy of each Sm center. Because of the simpler electronic structure from the half-filled 4f7 subshell occupancy in Gd2O2 and Gd2O2 –, the numerous close-lying transitions observed in the spectra are better resolved, allowing a more detailed view of the changes in relative intensities of individual transitions with photon energy. With supporting calculations on numerous possible close-lying electronic states, we suggest a potential description of the strong photoelectron–valence electron interactions that may result in the photon-energy-dependent changes in the observed spectra.

New Photoelectron–Valence Electron Interactions Evident in the Photoelectron Spectrum of Gd2O–

“…31 From the standpoint of orbital angular momentum, there is no explanation for the features with perpendicular PAD, unless the orbital angular momentum of the ejected electron is affected by PEVE interactions in a way that was not observed in previous studies on similar strongly correlated systems. 13 Considering the strong coupling between unpaired outervalence electrons and the 4f 7 cores evident from the energy associated with α → β spin flips in the δ orbitals (ca. 0.8 eV, Table 1), it is conceivable that there is similarly strong coupling between the orbital angular momentum of the ejected electron and the projection of the spin angular momentum of the 4f 7 cores.…”

“…9−11 We recently reported the effects of strong electron−neutral interactions in the photoelectron spectra of Sm 2 O − and Gd 2 O 2 − . 12,13 Specifically, the probability of populating excited neutral states via anion detachment increased relative to the ground-state neutral with decreasing photon energy and, therefore, decreasing photoelectron kinetic energy (e − KE); this observation is opposite of what would be expected to arise from threshold effects. 14 Pronounced effects were observed in a number of Sm-rich suboxide clusters, 15,16 but it was not observed in homometallic Ce-homologues, which implicated the greater density of electronic states from Sm's 4f 5 or 4f 6 subshell occupancy, compared to Ce's lone 4f electron.…”

“…12 In order to probe the nuances of the PEVE interactions in more detail, the photoelectron spectra of Gd 2 O 2 − were similarly examined over a range of photon energies. 13 Relative to Sm 2 O − , Gd 2 O 2 − has a simpler electronic structure because the Gd-centers have half-filled 4f 7 subshells and therefore zero orbital angular momentum (strictly speaking, they are not spherically symmetric), 18 and the incremental difference in oxidation state results in two fewer electrons in metal-local orbitals. Gd 2 O 2 − , therefore, provided a canvas for exploring the fundamental physics of this effect.…”

“…The exceptionally high density of electronic states in Sm x O y – anion and neutral suboxide clusters render them difficult to interrogate experimentally and computationally, − and the exact nature of the excited states of Sm 2 O being populated because of PEVE interactions could not readily be determined from the broad manifold of unresolved transitions observed in its anion PE spectrum . In order to probe the nuances of the PEVE interactions in more detail, the photoelectron spectra of Gd 2 O 2 – were similarly examined over a range of photon energies …”

Electronic structure of NdO via slow photoelectron velocity-map imaging spectroscopy of NdO¯

“…23 These features show exceptional agreement with the term energies of NdO determined from the LIF spectra of Linton and coworkers, 27 who identified the corresponding neutral states as belonging to the Ω = 4, 5 manifolds; we thus consider the final neutral states reached in transitions A-G to belong to the same Ω = 4, 5 states. Previous experiments employing photoelectron spectroscopy of lanthanide monoxides 20,[64][65][66][67] have shown that photodetachment from a metalcentered 6s orbital, as is the case for the NdO¯ anion, imposes a ∆Ω = ± ½ selection rule. Our results then strongly suggest detachment from an NdO¯ ground electronic state with Ω = 4.5, thus allowing photodetachment transitions to states with Ω = 4, 5.…”