68th International Symposium on Molecular Spectroscopy Ohio State University June 17-21, 2013 Wei-Li Li, Tian Jian, Gary V. Lopez, and Lai-Sheng Wang Department of Chemistry, Brown University, Providence, Rhode Island UO 2 − : a Highly Correlated System

Electronic Structure of the Uranyl: UO 2 2+ Denning, R. G., Electronic structure and bonding in actinyl ions and their analogs. J Phys Chem A 2007, 111 (20), Uranyl is one of the most stable uranium compounds in the nature. U has an formal oxidation state of VI. Linear structure with two U≡O bonds. HOMO ] 2+ [ HOMOs (3σ u, 3σ g, 1π u, 1π g ) has electron density over both U and O atoms, suggesting the U-O bond order. LUMOs (1δ u and 1φ u ) are the 5f δ and 5f φ components of the U 5f shell, non-bonding character. Fill the LUMO will make the UO 2, which was expected to have a 1 Σ g + or 3 Φ u ground state.

Photoelectron Spectrum of UO 2 Allen, G. C.; Baerends, E. J.; Vernooijs, P.; Dyke, J. M.; Ellis, A. M.; Feher, M.; Morris, A. J Chem Phys 1988, 89, FIG. He I photoelectron spectrum of the vapor above an equimolar VO, (s) IV (s) mixture heated to 2250 ± 30 K. Ordinate: Counts. Abscissa: Ionization energy (eV). UO 2

915 cm -1 UO 2 in Neon Matrix IR Spectrum of UO 2 Which is the ground state: 3 Φ u or 3 H g ? Hunt, R. D.; Andrews, L. J Chem Phys 1993, 98, 3690 Zhou, M. F.; Andrews, L.; Ismail, N.; Marsden, C. J Phys Chem A 2000, 104, Li, J.; Bursten, B. E.; Andrews, L.; Marsden, C. J. J. Am. Chem. Soc. 2004, 126, cm -1 UO 2 in Argon Matrix 3Φu3Φu 3Hg3Hg

FIG. 1. Segments of the REMPI spectrum of UO 2 taken in the near UV spectral range. In the gas phase. Figure 1. Emission spectra for UO2 trapped in solid Ar at 12 K. These traces were recorded using 266 nm excitation. (a) Fluorescence in the nm range. Lue, C. J.; Jin, J.; Ortiz, M. J.; Rienstra-Kiracofe, J. C.; Heaven, M. C. J. Am. Chem. Soc. 2004, 126, Electronic Structure of UO 2 The ground state is 3 Φ u.

What is the ground state of UO 2 ? what is the electronic structure?

Detaching electron from the anionic ground state to access the neutral ground and excited states. This technique depicts the electronic structures of the neutral species.

Cu – LS Wang et al., J. Chem. Phys. 102, 9480 (1995) Laser Vaporization cluster source Ion extraction Time of flight MS Magnetic-bottle PES 4s -1 3d -1 4s -1 4p +1 UO 2 − The target comes without inert gas protection. The residual oxygen on the uranium target surface is enough to produce the UO 2 − cluster.

Photoelectron Spectra of UO 2 − Frequencies: 250 ± 50 cm −1 860 ± 50 cm −1 Binding Energy (eV)

Theoretical Calculation of UO 2 − and UO 2 Qualitative scalar-relativistic valence orbital energy-level scheme for U and O atoms on the left and right sides, and D  h UO 2  in the middle Non-bonding character. Bonding length does change much upon electron detachment. Bonding character. Active space for calculation U(5f)U(7s)U(5f) + O (2p)

Photoelectron is one electron process, from the U(7s 2 5f 1 ) configuration, all those electronic configuration are not expected. This suggests very strong auto-excitation happens during the electron detachment process. Assignment of the UO 2 − Spectra Cu – 4s -1 3d -1 4s -1 4p +1 Cu – : [Ar] 3d 10 4s 2Ar Corresponding to the Cu : [Ar] 3d 10 4pAr

Conclusion Joint PES and ab initio studies were used to investigate the electronic structures UO 2 and UO 2 –. Strong electron correlation effect was first observed between the 7s electron pair. The PES features due to this correlation effect were semi-quantitative assigned.

Acknowledgement Brown University Gary V. Lopz Tian Jian Lai-Sheng Wang Tsinghua University Prof. Jun Li Dr. Jing Su