1. Vasiliy Goncharov, Jiande Han, Leonid Kaledin,
Probing actinide electronic structure using fluorescence and multi-photon ionization spectroscopy
Vasiliy Goncharov, Jiande Han, Leonid Kaledin,
and Michael Heaven
Department of Chemistry
DoE

2. Motivation
In principle, computational quantum chemistry methods may be used to predict the properties of hazardous radioactive materials. Computational prediction is desirable as this could greatly reduce the need for difficult and expensive laboratory studies.
Computational methods for treating heavy element compounds are being developed but they need to be tested against reliable data for gas phase molecules. There is a critical need for such experimental data, and this information can be obtained from studies of less hazardous Th and U compounds.

3. Key issue for actinide chemistry
Role of the f-electrons in bonding and electronic
structure.
Challenges for experiment
Open f- and d- shells result in high densities of
electronic states.
Refractory species  high temperatures.
Challenges for theory
Strong relativistic effects.
Large numbers of electrons.

4. Examples of the difficulties in comparing
experiment and theory for actinide compounds
Calculations for UO2 have yielded predictions for the
ground state of (5ffu)2 3S-g, (5ffu) (5fdu) 3Hg or 3S-g and (5ffu)(7ssg) 3Fu
Data for UO2 obtained in rare gas matrices show
anomalously large guest-host interactions
DG1/2 = 914 cm-1 (Ne), 776 cm-1 (Ar)
(Zhou, Andrews, Ismail & Marsden JPCA, 104, 5495 (2000))
Most high level theoretical calculations yield an ionization energy for UO2 above 6 eV. Electron impact measurements yield 5.4 eV

5. Ionization energies are often used in the
determination of bond energies
Uncertainties in the IE’s
propagate through the
thermodynamic data base

6. Simple electronic structure model for ionic actinide compounds
M.O. theory does not provide an easily interpretable
zeroth-order picture
Ligand field theory works well for lanthanides - is
it suitable for actinides?
Basic concept - Mn+ perturbed by Ln-
Successful if the f-orbitals are very compact
R. W. Field, Ber. Bunsenges. Phys. Chem. 86, 771 (1982)

7. Example of LFT applied to CeO1
(Field, Linton et al.) 2 2 3
Jf=7/2 3 3 4 4
Ce2+(4f6s)
Ce2+(4f6s)O2- 1 0- 2 1 3
Jf=5/2 3 2 2 Ja
W=Ja
W=Ja-1
W=Ja-2

8. Spatial extent of lanthanide (Nd) and actinide (U) atomic orbitals
N. Edelstein, J. Alloy. Comp. 223, 197 (1995)

9. Electronic spectroscopy of UO
(Kaledin, McCord & Heaven JMS 164, 27 (1994))
Ground state
U2+(5f37s,5I4)O2- X(1)W=4
First excited state (294 cm-1)
U2+(5f27s2,3H4)O2- X(1)W=4
LFT prediction with no adjustable parameters, X(1)W=1
Low-lying states can be interpreted using an adjustable
parameter LFT model. Is this meaningful?

10. Spectroscopic studies of actinide oxides using
multi-photon ionization techniques
Resonance enhanced multi-photon ionization (REMPI)
Photo-ionization efficiency curves (PIE)
Mass-analyzed threshold ionization (MATI)
Zero kinetic energy (ZEKE) photoelectron
spectroscopy

11. Multi-Photon Ionization Processes
M+ + e-
hv1
hv2
REMPI
ZEKE
PIE
MATI
Pulsed
electric
field

12. ZEKE & Mass-selected REMPI spectrometer
Microchannel plate
(cation detection)
MO+
Metal target
Skimmer
hv2
Einzel
lens
Pulsed
valve
Grids
MO+He
Microchannel plate
(electron detection)
Vaporization
laser
hn1

13. Spectroscopy of ThO+ - a simple test case
Theoretical expectations - single unpaired electron
outside of a closed shell metal ion core
Ground state:
Th3+(7s)O2-, X2S+
First excited state manifold:
Th3+(6d)O2-, 2D, 2P, 2S+

14. REMPI Spectrum for ThO reveals new
vibronic transitions
Low-resolution scan
Wavelength, nm

15. Rotationally resolved 1-0 band of the F’(0+)-X(0+) transition

16. Photoionization of ThO
Intermediate resonance at cm-1
IP=
53260(5) cm-1
6.6035(6) eV
e=21.4 V/cm
Threshold at
cm-1
ThO+ ion signal
Literature
value
6.1(1) eV
Second Photon Energy /cm-1

17. PFI-ZEKE Spectra of ThO, X 2+ (v+= 0) state
X 2+, v+ = 0, N+
scanned
W = 0+
ThO
O, v’ = 0, J’
fixed
W = 0+
ThO
X, v” = 0, J” 
X 2+ state, v+ = 0
Bo+ = (6) cm-1
IE(ThO)= (2) cm-1

18. PFI-ZEKE Spectra for ThO+
Rotational structure of ThO+ A, =5/2, v=0
ThO+
A, v+=0
Broken lines show zero-field energies 2
A’, v=0 1
ThO
X, v=0

19. Spectroscopic data for ThO+
State
To /cm-1 (Theory1)
T /cm-1 (This Work)
B /cm-1
e /cm-1
exe /cm-1
X 2+
 = 0
 = 1
 = 2
 = 6
 = 7
0, {52020}
0, {IE = (2)}
950.0(1)
1895.3(1)
5627.0(1)
6547.2(5)
0.3450(6)
0.3439(5)
0.3434(5)
0.3409(10) –—
954.97(6)
2.45(3)
1 23/2
 = 3
 = 4
2477
2933.7(1)
3846.2(1)
5656.8(1)
6554(1)
0.3373(8)
0.337(1)
0.3379(13)
917.1
2.35
1 25/2
5886
5814.4(1)
6729.9(1)
0.3410(2)
0.340(1)
[915.5(2)]
1 21/2
 = 5
9167
7404.1(1)
8303.6(1)
9198.5(1)
(2)
(2)
0.3365(11)
0.3354(10)
0.3334(6)
0.3330(7)
0.333(2)
904.22(2)
2.339(3)
1. Rajni Tyagi, PhD Thesis, OSU Advisor, R. M. Pitzer

20. Ionization makes the Th-O bond weaker but stiffer
IE(Th)= eV
Th+ + O
IE(ThO)= eV
Hence, the ThO+
bond is weaker
D0+
IE(Th)
D0-D0+=0.296 eV
Th + O
but its vibrational
frequency is higher
e /cm-1
ThO
ThO D0
IE(ThO)
and
B(ThO+)>B(ThO)

21. Avoided curve crossings are responsible for the
anomalous relationship between the bond energy
and molecular constants
Th+ + O
must correlate with
this limit on
dissociation
Energy /cm-1
X2S+
R/Å
Th3+(7s)O2-
approximate configuration at equilibrium

22. Photoionization spectroscopy of UO
U2+(5f37s, 5I4)O2-  UO*  U3+(5f3, 4I4.5)O2-
IE (electron impact) = 5.6(1) eV
Goncharov & Heaven, RH01

23. He I Photoelectron Spectrum of UO/UO2 Vapor

24. Low-lying states of UO+: What to expect?
11392 cm-1 (Ref. 1)
4I6.5
8024 cm-1 (Ref. 1)
U3+ [5f3]: the lowest energy term–4I:
4I5.5
4265 cm-1 (Ref. 1)
4I4.5
0 cm-1 (Ref. 1)
(Ref. 2)
4I7.5
9796
4I6.5
7251
4600 – 4800 cm-1
U3+ [5f3] + O2-[2p6]: 4I:
3991
4I5.5
600 – 700 cm-1
4I4.5
0 cm-1 W:
1 Jean BLAISE and Jean-François WYART, Selected Constants Energy Levels and Atomic Spectra of Actinides.
2 L. A. Kaledin et. al. Journal of Molecular Spectroscopy 164, (1994)

25. PFI-ZEKE Spectra of UO, X(1)4.5(v+= 0) state
X(1)4.5, v+=0, J+
scanned UO
[19.453]3(v’ = 0)
fixed UO
X(1)4(v” = 0) 
X(1)4.5(v+=0)
Bo+ = (7) cm-1
IE(UO)= (25) eV
e- Impact1: 5.6(1) eV
Theory / Best Value (CASSCF)2: eV
1  E. G. Rauh and R. J. Ackermann, J. Chem. Phys. 60, 1396 (1974).
2  J. Paulovic, L. Gagliardi, J. Dyke, K. Hirao, J. Chem. Phys. 122, (2005).

26. PFI-ZEKE Spectra of UO, [1.132] (1)2.5 state (v+=0)
X(1)4.5, v+=1
(1)3.5, v+=0
UO+
X(1)4.5, v+=0
scanned UO
[19.453]3(v’ = 0)
fixed UO
X(1)4(v” = 0) 
[1.132] (1)2.5 state
Bo+ = (3) cm-1
To=  cm-1

27. UO+ Energy Levels Diagram: 0 – 5200 cm-1 range
(only v+ = 0 & 1 levels are shown, all states originate from U3+[5f3]O2- configuration )

28. Comparison of the experimentally obtained data
to theoretical calculations for UO+
State
This
Work
PFI-ZEKE
(2006)
LFT
Calculations
L. Kaledin
(1994/2006)
MCSCF/CI
Rajni&Pitzer
(2005)
MCSCF/VCI
Krauss&
Stevens
(1983)
U3+ energy
levels
X(1)4.5
0/0
0 {66%4I9/2+8%4H9/2}
X 4I4.5[5f3]
(1)3.5
764.93(20)
633/757
582 {47%4H7/2+15%4I7/2…}
1319 
(1)2.5
(20)
696/1129
856 {20%4 5/2+17%45/2…)
1895
(1)1.5
1284.4(5)
580/1281
1076 {17%4 3/2+8%43/2…)
2094
(1)0.5
1325.5(8)
695/1328
3296
(1)5.5
(20)
3991/4163
3744 {69%4 I11/2+11%4H11/2…)
2563
4265 4I5.5 [5f3]
(2)4.5
(20)
4601/4773
4180 {36%4H9/2+15%4I9/2…}
3599
(2)3.5
(20)
4770/5121
4045
(2)2.5
(20)
4744/5293
(3)3.5
(20)
4287 {58%4H7/2[5f27s]}

29. Trends in bond length and vibrational frequency change resulted
Comparison of the experimentally obtained data to theoretical calculations for UO+
Spectroscopic
data, UO+
This Work
Krauss&Stevens
AREP-MCSCF-SO
Paulovič et. al. /CASSCF
ANO-RCC basis set
Rajni&Pitzer
MCSCF-SO
X, e /cm-1
911.9(2)
92530
912 
X, re /Å
1.798(5)
1.842
1.802
1.812
Trends in bond length and vibrational frequency change resulted
from photo-ionization of ThO and UO
ThO
(Ref. 1)
ThO+
(Ref. 2)
Difference UO
(Ref. 3)
UO+
This work
X, re /Ǻ
1.840
1.804(3)
0.036(3)
1.8383
1.798(5)
0.040(5)
X, e /cm-1
895.77
954.97(6)
59.2(1)
846.5(6)
911.9(2)
65.4(8)
1 Edvinsson, G.; Selin, L.-E.; Aslund, N., Arkiv. Fysik. 30, (1965).
2 V. Goncharov and M. C. Heaven, J. Chem. Phys. (2006).
3 L. A. Kaledin et. al. Journal of Molecular Spectroscopy 164, (1994)

30. m(UO)=3.363 D m(NdO)=3.31 D Tongmei Ma et al. WF09
Indication that LFT may be viable for actinides
provided by recent measurements of the dipole
moment and magnetic g-factor for UO
m(UO)=3.363 D
m(NdO)=3.31 D
Tongmei Ma et al.
WF09

31. Electronic spectroscopy of UO2
Points of interest
Ground state configuration: (5ff) 2, (5ff)(5fd),
or (5ff)(7ss) ?
Ionization energy: theory >0.5 eV above experiment
Gagliardi. et al (JPCA 105, 10602, 2001) conclude that
the experimental values are wrong
Origin of the anomalous vibrational frequency matrix shifts.
914 cm-1 (Ne) vs. 776 cm-1 (Ar)

32. Is the ground state configuration
U(5f2)O2 or U(5f7s)?
2F7/2 3u
5ff5fd 3H 4g 5g 6g 4u
5ff7ss or 3u 3F
2F5/2 2u
Lowest energy for W=Ja f fs

33. Calculations for UO2 by Chang & Pitzer (2002)
Spin-orbit SCF-CI using relativistic core potentials
3H4
5f2 3D 3F
5f7s

34. First observation of the electronic spectrum of gas phase UO2
UO2+ Ion signal
UO2++e-
v2 (fixed) 2g v1 3u 2u
X 3F

35. Visible range spectra for UO2 show
progressions of bending vibrational levels
Note that odd-Dv
transitions were not
observed. This is
consistent with a
linear structure in both the ground and excited states.

36. Rotational resolution could not be achieved using a laser linewidth of 0.06 cm-1
The rotational structure was not resolved. Surprising as the rotational constant should be around 0.16 cm-1. Checks for power broadening and fragmentation yielded negative results - The congestion is a property of UO2
Ground state configuration
determined from vibronic structure

37. Delayed ionization of UO2 at energies just above threshold
Bond dissociation energy
of UO2 (7.85 eV)exceeds
the IP
Mixing of UO2++e- with
highly excited levels of
UO2 lengthens the lifetime.
t=220 ns
Decay of UO2+ interferes
with MATI detection

38. Photo-ionization of UO2
X 3F(2u)
v1=31838 cm-1 v2
UO2++e-
3F(2g)
Photo-ionization of UO2
I.P.=49424(20) cm-1 (6.128(3) eV)
UO2+ Ion signal

39. Ionization potentials (eV) for UO2: Comparison
with previous determinations and theory
Reference
Method
UO2
This work
MATI, PIE
6.128(3)
Rauh & Ackerman ‘74
Electron Impact
5.4(1)
Capone et al. ‘99
Gagliardi et al. ‘01
CASPT2
6.17
Zhou et al. ‘00
B3LYP
6.3
Majumdar et al. ‘02
MP2
6.05
Rajni & Pitzer ‘05
SOCI
5.7
Calculations for the energies of low-lying excited states are
still not converged - see Fleig et al. JCP 124, (2006)

40. Conclusions Spectroscopic studies of the low-lying states of actinide
compounds yield interpretable data. ZEKE is particularly
well suited for mapping the low energy electronic structure.
Relativistic quantum chemistry is making good progress,
but there is a long way to go. Calculations for the simplest
molecules are still very challenging.
Preliminary indications are that LFT yields meaningful
insights for ionic compounds.
Ionization energies for refractory actinide compounds
require systematic re-evaluation.

41. Thanks to - DoE Michael Duncan (UGA) Fredric Merkt (ETH Zurich)
Robert Field (MIT)
Russ Pitzer (OSU)
Laura Gagliardi (U. of Geneva)
Björn Roos (Lund)
DoE

42. Anomalous Behavior of
Matrix Isolated UO2
Jin Jin and Chris Lue

43. Calculated vibrational frequencies
for gas phase UO2
Zhou, Andrews, Ismail & Marsden, JPC A, 104, 5495 (2000)
(spin-free, relativistic DFT)
3Fu v(su)=931 cm-1
3Hg v(su)=814 cm-1
Large vibrational matrix effect is attributed to a
re-ordering of the electronic states caused by a strong interaction between UO2 and Ar

44. Bruce
Bursten

45. Matrix Isolation Apparatus used to Study UO2
in Solid Ar
Cold Mirror (12K)
ABLATION LASER
Configuration for sample preparation

46. Dispersed fluorescence spectrum obtained using
266 nm excitation (Nd/YAG 4th harmonic)

47. Band positions for matrix isolated UO2 and comparison of observed low-lying energy levels with theoretical predictions

48. The emission spectrum is consistent with transitions that terminate on the low-lying 5f7s states, but is this still the lowest energy configuration? u
Non-radiative relaxation g
5f7s u g
5f2

49. Laser excitation spectrum
Detection
band
Fluorescence intensity
Dispersed fluorescence
Gas phase transition nm

50. Conclusions for UO2 in Ar
Emission and excitation spectra indicate that 5f7s is the lowest energy configuration for UO2 in solid Ar
The anomalous effect of Ar on the vibrational
frequency may be due to host-induced state mixing
Studies of the UO2-Ar van der Waals complex will be used to probe the issue of incipient chemical bond formation.

51. Electronic Spectrum of UO
Low resolution
Note the dramatic effect of isotopic substitution on this spectrum. Bright states are mixed with many dark states. Isotopic substitution reorders the perturbations

52. IR Studies of UO2 Isolated in Rare Gas Matrices
Green, Gabelnick et al. ANL, 1973 and 1980
UO2 trapped in solid Ar:
v(sg)=776.10
v(su)=765.45
v(pu)=225.2
Andrews et al. UVA, 1993 and 2000
UO2 trapped in solid Ar:
v(su)=776.0
UO2 trapped in solid Ne:
v(su)=914.8
D=138.8 cm-1

53. Photoionization of atomic uranium
I.P.=49959(1) cm-1
f 3s2 4I9/2
(literature value: (5) cm-1)
U++e-
e=343 V/cm
e=21 V/cm v2
f 3s2p 5I5
v1= cm-1
f 3ds2 5L6

54. Angular momentum selection rules are relaxed
by field-induced mixing of the Rydberg series
Spectrum
for excitation
via J’=7

55. PFI-ZEKE Spectra for ThO+
Rotational structure of ThO+ X2+, v+=1
ThO+
X, v+=1
Broken lines show zero-field energies 2
O, v=0 1
ThO
X, v=0

56. Observed Energy Level Structure for UO2
Low-lying excited state at 360 cm-1 cannot be explained by the 5f2 3H ground state assumption.
The results are in good agreement with the structure expected for 5f7s (first predicted by
Zhou et al. JCPA, 104, )