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.

68th International Symposium on Molecular Spectroscopy

Ohio State UniversityJune 17-21, 2013

Wei-Li Li, Tian Jian, Gary V. Lopez, and Lai-Sheng Wang

Department of Chemistry, Brown University, Providence, Rhode Island

UO2−: a Highly Correlated System

Electronic Structure of the Uranyl: UO22+

Denning, R. G., Electronic structure and bonding in actinyl ions and their analogs. J Phys Chem A 2007, 111 (20), 4125-4143.

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 (3σu, 3σg, 1πu, 1πg) has electron density over both U and O atoms, suggesting the U-O bond order. LUMOs (1δu and 1φu) are the 5fδ and 5fφ components of the U 5f shell, non-bonding character.

Fill the LUMO will make the UO2, which was expected to have a 1Σg

+

or 3Φu ground state.

Photoelectron Spectrum of UO2

Allen, G. C.; Baerends, E. J.; Vernooijs, P.; Dyke, J. M.; Ellis, A. M.; Feher, M.; Morris, A. J Chem Phys 1988, 89, 5363.

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).

UO2

915 cm-1

UO2 in Neon Matrix

IR Spectrum of UO2

Which is the ground state:

3Φu or 3Hg ?

Hunt, R. D.; Andrews, L. J Chem Phys 1993, 98, 3690Zhou, M. F.; Andrews, L.; Ismail, N.; Marsden, C. J Phys Chem A 2000, 104, 5495.Li, J.; Bursten, B. E.; Andrews, L.; Marsden, C. J. J. Am. Chem. Soc. 2004, 126, 3424.

776 cm-1

UO2 in Argon Matrix

3Φu

3Hg

FIG. 1. Segments of the REMPI spectrum of UO2 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 360-440 nm range.

Lue, C. J.; Jin, J.; Ortiz, M. J.; Rienstra-Kiracofe, J. C.; Heaven, M. C. J. Am. Chem. Soc. 2004, 126, 1812.

Electronic Structure of UO2

The ground state is 3Φu.

What is the ground state of UO2? what is the electronic structure?

M

M*

VD

E

AD

E

M-

M + e-

e- K

E

Configuration coordinate

Ele

ctro

n B

ind

ing

En

erg

y

h

Detachment continuum

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.

Photoelectron Spectroscopy: Principle

Photoelectron Spectroscopy of Size-selected Clusters

Cu–

LS Wang et al., J. Chem. Phys. 102, 9480 (1995)Laser Vaporizationcluster source

Ion extraction

Time of flight MS

Magnetic-bott

le PES

4s-1 3d-1

4s-1 4p+1

UO2−

The target comes without inert gas protection. The residual oxygen on the uranium target surface is enough to produce the UO2

− cluster.

Photoelectron Spectra of UO2−

Frequencies:

250 ± 50 cm−1

860 ± 50 cm−1

Binding Energy (eV)

Theoretical Calculation of UO2− and UO2

Qualitative scalar-relativistic valence orbital energy-level scheme for U and O atoms on the left and right sides, and Dh UO2

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(7s25f1) configuration, all those electronic configuration are not expected. This suggests very strong auto-excitation happens during the electron detachment process.

Assignment of the UO2− Spectra

Cu–

4s-1 3d-1

4s-1 4p+1

Cu – : [Ar] 3d10 4s2

Corresponding to the Cu : [Ar] 3d10 4p

Conclusion

• Joint PES and ab initio studies were used to investigate the electronic structures UO2 and UO2

– . • 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 WangTsinghua University

Prof. Jun Li

Dr. Jing Su