Probing actinide electronic structure using fluorescence and
multi-photon ionization spectroscopy
Vasiliy Goncharov, Jiande Han, Leonid Kaledin,
and Michael Heaven
Department of Chemistry
DoE
•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.
Motivation
Key issue for actinide chemistryRole of the f-electrons in bonding and electronicstructure.
Challenges for experiment
Open f- and d- shells result in high densities ofelectronic states.
Refractory species high temperatures.
Challenges for theory
Strong relativistic effects.
Large numbers of electrons.
Examples of the difficulties in comparingexperiment and theory for actinide compounds
Calculations for UO2 have yielded predictions for theground state of (5fu)2 3-
g, (5fu) (5fu) 3Hg or 3-g
and (5fu)(7sg) 3uData for UO2 obtained in rare gas matrices showanomalously large guest-host interactions
G1/2 = 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
Ionization energies are often used in thedetermination of bond energies
Uncertainties in the IE’spropagate through thethermodynamic data base
Simple electronic structure model forionic actinide compounds
M.O. theory does not provide an easily interpretablezeroth-order picture
Ligand field theory works well for lanthanides - isit suitable for actinides?
Basic concept - Mn+ perturbed by Ln-
FI SO LFˆ ˆ ˆ ˆH H H H
2LF L i
i
H Z e / r
R. W. Field, Ber. Bunsenges. Phys. Chem. 86, 771 (1982)
Successful if the f-orbitals are very compact
Ce2+(4f6s)
4
2
3
3Jf=7/2
Jf=5/2
=JaJa
23
43
=Ja-1
12
3
2
=Ja-2
0-
1
2
1
Ce2+(4f6s)O2-
Example of LFT applied to CeO
(Field, Linton et al.)
Spatial extent of lanthanide (Nd) and actinide (U) atomic orbitals
N. Edelstein, J. Alloy. Comp. 223, 197 (1995)
Electronic spectroscopy of UO
(Kaledin, McCord & Heaven JMS 164, 27 (1994))
18606.70 18609.35 18612.00
Q(6
)
Q(1
2)
Excitation Energy (cm-1)
Q(1
8)
Q(2
4)
R(5
)
R(6
)
[20.491]6-(1)5(v=1)
Fluo
resc
ence
Int
ensi
ty
Ground state
U2+(5f37s,5I4)O2- X(1)=4
First excited state (294 cm-1)
U2+(5f27s2,3H4)O2- X(1)=4
LFT prediction with no adjustable parameters, X(1)=1
Low-lying states can be interpreted using an adjustableparameter LFT model. Is this meaningful?
Spectroscopic studies of actinide oxides usingmulti-photon ionization techniques
Resonance enhanced multi-photon ionization (REMPI)
Photo-ionization efficiency curves (PIE)
Mass-analyzed threshold ionization (MATI)
Zero kinetic energy (ZEKE) photoelectronspectroscopy
Multi-Photon Ionization Processes
M
M*
M+ + e-
hv1
hv2
hv1
hv2
REMPI ZEKEPIE MATI
Pulsedelectricfield
MO+He
Microchannel plate(cation detection)
Einzellens
h
Skimmer
Pulsedvalve
ZEKE & Mass-selected REMPI spectrometer
Vaporizationlaser
Metal target
hv2
MO+
Microchannel plate(electron detection)
Grids
Spectroscopy of ThO+ - a simple test case
Theoretical expectations - single unpaired electronoutside of a closed shell metal ion core
Ground state:
First excited state manifold:
Th3+(6d)O2-, 2, 2, 2+
Th3+(7s)O2-, X2+
Wavelength, nm
REMPI Spectrum for ThO reveals new vibronic transitions
Low-resolution scan
Rotationally resolved 1-0 band of the F’(0+)-X(0+) transition
24500 24600 24700
MATI PIE
Second Photon Energy /cm-1
Th
O+ io
n s
ign
al
Photoionization of ThO
Threshold at24653.5 cm-1
Intermediate resonance at 28578.8 cm-1
=21.4 V/cm IP=
53260(5) cm-1
6.6035(6) eV
Literaturevalue
6.1(1) eV
53250 53260 53270 53280 53290 53300 53310 53320 53330
via J'(O) =0
via J'(O) =1
via J'(O) =2
via J'(O) =3
via J'(O) =5
via J'(O) =7
via J'(O) =9
N+= 0 N+= 6N+= 4 N+= 8 N+= 10 N+= 12
Total Energy, cm-1
via J'(O) =11
PFI = 0.285 V/cm
FWHM = 1 cm-1
PFI-ZEKE Spectra of ThO, X 2+ (v+= 0) state
:)1(0 NNB
X 2+ state, v+ = 0
Bo+ = 0.3450(6) cm-1
IE(ThO)=53253.8(2) cm-1
X 2+, v+ = 0, N+
ThO+
=0+
O, v’ = 0, J’ ThO
ThOX, v” = 0, J” =0+
fixed
scanned
59060 59070 59080 59090 59100 59110 59120 59130 59140
J+= 25/2J+= 21/2J+= 17/2J+= 13/2J+= 9/2
Total Energy, cm-1
J+= 5/2
via J'(A')=0
via J'(A')=3
via J'(A')=7
via J'(A')=11
PFI-ZEKE Spectra for ThO+
X, v=0
A’, v=0
A, v+=0
ThO
ThO+
1
2
Rotational structure of ThO+ A, =5/2, v=0
Broken lines show zero-field energies
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 = 53253.8(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 23/2
= 0
= 1
= 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 25/2
= 0
= 1
5886 5814.4(1)
6729.9(1)
0.3410(2)
0.340(1)
[915.5(2)]
–—
1 21/2
= 0
= 1
= 2
= 3
= 5
9167 7404.1(1)
8303.6(1)
9198.5(1)
10088.7(2)
11855.0(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 2005. Advisor, R. M. Pitzer
Th+ + O
Th + O
D0+
D0
IE(Th)
IE(ThO)
Ionization makes the Th-O bond weaker but stiffer
IE(Th)=6.3067 eV
IE(ThO)=6.6027 eV
Hence, the ThO+
bond is weaker
D0-D0+=0.296 eV
but its vibrationalfrequency is higher
e /cm-1
ThO 895.77ThO+ 954.97
and
B(ThO+)>B(ThO)
Th+ + O
X2+
0
20000
40000
60000
80000
1.0 1.5 2.0 2.5 3.0 3.5 4.0
Ene
rgy
/cm
-1
R/Å
Th3+(7s)O2-
Avoided curve crossings are responsible for theanomalous relationship between the bond energyand molecular constants
approximate configuration at equilibrium
must correlate withthis limit ondissociation
Photoionization spectroscopy of UO
U2+(5f37s, 5I4)O2- UO* U3+(5f3, 4I4.5)O2-
IE (electron impact) = 5.6(1) eV
Goncharov & Heaven, RH01
He I Photoelectron Spectrum of UO/UO2 Vapor
Low-lying states of UO+: What to expect?
U3+ [5f3]: the lowest energy term–4I:
1 Jean BLAISE and Jean-François WYART, Selected Constants Energy Levels and Atomic Spectra of Actinides.
4I4.5
4I5.5
4I6.5
0 cm-1 (Ref. 1)
4265 cm-1 (Ref. 1)
8024 cm-1 (Ref. 1)
U3+ [5f3] + O2-[2p6]: 4I:
4I4.5
4I5.5
4I6.5
0 cm-1
3991
7251
(Ref. 2)
2 L. A. Kaledin et. al. Journal of Molecular Spectroscopy 164, 27-65 (1994)
600 – 700 cm-1
4600 – 4800 cm-1
4I7.511392 cm-1 (Ref. 1)
4I7.5 9796
48640 48650 48660 48670 48680 48690 48700 48710
J+: 4.5 5.5 6.5 7.5 8.5 9.5 10.5 11.5 PFI = 0.285 V/cm
via J' =11
via J' =8
via J' =6
via J' =4
Total Energy, cm-1
PFI-ZEKE Spectra of UO, X(1)4.5(v+= 0) state
:)1(0 JJB
Bo+ = 0.3467(7) cm-1
IE(UO)=6.031065(25) eV
UO+
UO
UO
e- Impact1: 5.6(1) 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, 144317 (2005).
X(1)4.5, v+=0, J+
X(1)4.5(v+=0)
X(1)4(v” = 0)
[19.453]3(v’ = 0)
Theory / Best Value (CASSCF)2: 6.040 eV
fixed
scanned
49780 49800 49820 49840 49860 49880
33/229/225/221/217/213/29/2
via J'=15
via J'=12
via J'=8
via J'=6
via J'=4
Total Energy/ cm-1
J+: 5/2
PFI-ZEKE Spectra of UO, [1.132] (1)2.5 state (v+=0)
:)1(0 JJB
Bo+ = 0.3364(3) cm-1
To=1132.42 cm-1
X(1)4.5, v+=0UO+
[19.453]3(v’ = 0) UO
UOX(1)4(v” = 0)
(1)3.5, v+=0
(1)2.5, v+=0
[1.132] (1)2.5 state
X(1)4.5, v+=1
fixed
scanned
0
800
1600
2400
3200
4000
4800
5600
0
4I5.5
[5f 3]
0 1
U3+ion
1
0
1
0
0
0
5.5
1 1
B0=0.3364(3)
B0=0.3421(5)
B0=0.3467(7)
0 0
1
0.51.52.53.5
En
ergy
/cm
-1
4.5
0
907.15
905.07
903.93901.9 900.4
907.52
4I4.5
[5f 3]
UO+ Energy Levels Diagram: 0 – 5200 cm-1 range (only v+ = 0 & 1 levels are shown, all states originate from U3+[5f3]O2- configuration )
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 0 {66%4I9/2+8%4H9/2} 0 X 4I4.5[5f3]
(1)3.5 764.93(20) 633/757 582 {47%4H7/2+15%4I7/2…} 1319 (1)2.5 1132.42(20) 696/1129 856 {20%4 5/2+17%45/2…) 1895 (1)1.5 1284.4(5) 580/1281 1076 {17%4 3/2+8%43/2…) 2094 (1)0.5 1325.5(8) 695/1328 3296 (1)5.5 4177.83(20) 3991/4163 3744 {69%4 I11/2+11%4H11/2…) 2563 4265 4I5.5 [5f3]
(2)4.5 4758.45(20) 4601/4773 4180 {36%4H9/2+15%4I9/2…} 3599 (2)3.5 5161.96(20) 4770/5121 4045 (2)2.5 5219.37(20) 4744/5293 (3)3.5 4982.44(20) 4287 {58%4H7/2[5f27s]}
Comparison of the experimentally obtained data to theoretical calculations for UO+
Spectroscopic
data, UO+
This Work
Krauss&StevensAREP-MCSCF-SO
Paulovič et. al. /CASSCF
ANO-RCC basis set
Rajni&Pitzer
MCSCF-SO
X, e /cm-1 911.9(2) 92530 912
X, re /Å 1.798(5) 1.842 1.802 1.812
Comparison of the experimentally obtained data to theoretical calculations for UO+
ThO
(Ref. 1)
ThO+
(Ref. 2)
Difference UO
(Ref. 3)
UO+
This work
Difference
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)
Trends in bond length and vibrational frequency change resulted from photo-ionization of ThO and UO
1 Edvinsson, G.; Selin, L.-E.; Aslund, N., Arkiv. Fysik. 30, 283-319 (1965).2 V. Goncharov and M. C. Heaven, J. Chem. Phys. (2006).3 L. A. Kaledin et. al. Journal of Molecular Spectroscopy 164, 27-65 (1994)
R(4)
R(4)
-4 +40
Zero Field
E=706V/cm, //
Stark Shift (MHz)
Zero Field
+40-4
E=706V/cm, //
-500 -300 -100 +100 +300 +500
MJ=0
MJ=0
Optical Stark spectra f or UOR(4)
R(4)
-4 +40
Zero Field
E=706V/cm, //
Stark Shift (MHz)
Zero Field
+40-4
E=706V/cm, //
-500 -300 -100 +100 +300 +500
MJ=0
MJ=0
R(4)
R(4)
-4 +40
Zero Field
E=706V/cm, //
Stark Shift (MHz)
Zero Field
+40-4
E=706V/cm, //
-500 -300 -100 +100 +300 +500-500 -300 -100 +100 +300 +500
MJ=0
MJ=0
Optical Stark spectra f or UO
Indication that LFT may be viable for actinidesprovided by recent measurements of the dipolemoment and magnetic g-factor for UO
(UO)=3.363 D
(NdO)=3.31 D
Tongmei Ma et al.WF09
Electronic spectroscopy of UO2
Points of interest
Ground state configuration: (5f) 2, (5f)(5f), or (5f)(7s) ?
Ionization energy: theory >0.5 eV above experimentGagliardi. et al (JPCA 105, 10602, 2001) conclude thatthe experimental values are wrong
Origin of the anomalous vibrational frequency matrix shifts.
914 cm-1 (Ne) vs. 776 cm-1 (Ar)
Is the ground state configuration U(5f2)O2 or U(5f7s)?
5f5f3H
4g
5g
6g
or 5f7s
2F7/2
2F5/2
f fs
2u
3u
4u
3u
3
Lowest energy for =Ja
Calculations for UO2 by Chang & Pitzer (2002)
Spin-orbit SCF-CI using relativistic core potentials
3
3H4
3
5f7s
5f2
First observation of the electronicspectrum of gas phase UO2
X 3
v1
v2 (fixed)
UO2++e-
2g
2g - X32u
2g - X33u
31350 31500 31650 31800 31950
First photon enenrgy /cm-1
vb=0
vb=0
vb=1
vb=1U
O2+
Ion
sig
nal
2u
3u
Visible range spectra for UO2 showprogressions of bending vibrational levels
Note that odd-vtransitions were notobserved. This isconsistent with alinear structure in both the ground and excited states.
17388 17472 17556 17640 17724
UO
2+ I
on S
igna
l
First Photon Energy (cm-1)
0-0
1-1
2-22-0
3-1
4-2
2g-3(3u)
Rotational resolution could not be achieved using a laser linewidth of 0.06 cm-1
17488 17492 17496 17500
UO
2+ I
on
Sig
nal
4g-3(3u)
0-0
a
17435 17440 17445 17450 17455
UO
2+ I
on
Sig
nal
Second Photon Energy (cm-1)
4g-3(3u)
1-1
b
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 configurationdetermined from vibronic structure
7.0 7.5 8.0 8.5
Ion
Sig
nal
Flight Time /s
U+
UO+
UO2
+
Delayed ionization of UO2 at energies just above threshold
Bond dissociation energyof UO2 (7.85 eV)exceedsthe IP
Mixing of UO2++e- with
highly excited levels ofUO2 lengthens the lifetime.
=220 ns
Decay of UO2+ interferes
with MATI detection
X 3(2u)
v1=31838 cm-1
v2
UO2++e-
2g
Photo-ionization of UO2
I.P.=49424(20) cm-1 (6.128(3) eV)
17400 17430 17460 17490 17520 17550
Second photon energy /cm-1
UO
2+ I
on
sign
al
Ionization potentials (eV) for UO2: Comparisonwith 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 Electron Impact 5.4(1)
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 arestill not converged - see Fleig et al. JCP 124, 104106 (2006)
Conclusions
Spectroscopic studies of the low-lying states of actinidecompounds yield interpretable data. ZEKE is particularlywell 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 simplestmolecules are still very challenging.
Preliminary indications are that LFT yields meaningfulinsights for ionic compounds.
Ionization energies for refractory actinide compoundsrequire systematic re-evaluation.
Thanks to -
Michael Duncan (UGA)
Fredric Merkt (ETH Zurich)
Robert Field (MIT)
Russ Pitzer (OSU)
Laura Gagliardi (U. of Geneva)
Björn Roos (Lund)
DoE
Anomalous Behavior of Matrix Isolated UO2
Jin Jin and Chris Lue
Calculated vibrational frequenciesfor gas phase UO2
Zhou, Andrews, Ismail & Marsden, JPC A, 104, 5495 (2000)
(spin-free, relativistic DFT)
3u v(u)=931 cm-1
3Hg v(u)=814 cm-1
Large vibrational matrix effect is attributed to are-ordering of the electronic states caused by a strong interaction between UO2 and Ar
ADF/PW91 Linear Transit Energy Curves for UO2 + 5Ar UO2(Ar)5 (D5h)
Dotted curve: 3Hg curve stabilized by 0.23 eV, the differential SO stabilization of the 3Hg state per Gagliardi, Roos, et al. Also cf. Maron, Schimmelpfennig, Vallet, Teichteil, Wahlgren, et al., Chem. Phys. 1999, SOCI on PuO2
2+: 22.4 kcal/mol SO splitting of 3Hg state vs. 0.06 kcal/mol for 3u state.
BruceBursten
ABLATION LASER
Cold Mirror (12K)
Matrix Isolation Apparatus used to Study UO2
in Solid Ar
Configuration for sample preparation
360 370 380 390 400 410 420 430 440
Flu
ores
cenc
e In
tens
ity
Wavelength /nm
X32u
X32u, v=1
31u
33u
32u
Dispersed fluorescence spectrum obtained using266 nm excitation (Nd/YAG 4th harmonic)
Band Position Energy Theory b Theory c Assignment
27036 0 0 0 X3
2u 26628 408 431 403 3
3u 26265 771 X3
2u, v=1 25942 1094 1088 1935 3
2u 25635 1401 1566 2340 3
1u
Band positions for matrix isolated UO2 and
comparison of observed low-lying energy levels with theoretical predictions
The emission spectrum is consistent with transitions that terminate on the low-lying 5f7s states, but is this still the lowest energy configuration?
g
u
g
u
5f2
5f7s
Non-radiative relaxation
360 370 380 390 400 410 420 430 440
Wavelength /nm
X32u
X32u, v=1
31u
33u
32uDetection
band
358 360 362 364 366 368 370
Excitation wavelength /nm
Dispersed fluorescence
Laser excitation spectrum
Flu
ores
cenc
e in
tens
ity
Gas phase transition - 366.9 nm
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 vibrationalfrequency 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.
Electronic Spectrum of UO
Note the dramatic effect of isotopic substitution on this spectrum. Bright states are mixed with many dark states. Isotopic substitution reorders the perturbations
Low resolution
IR Studies of UO2 Isolated in Rare Gas Matrices
Green, Gabelnick et al. ANL, 1973 and 1980
UO2 trapped in solid Ar:
v(g)=776.10 v(u)=765.45 v(u)=225.2
Andrews et al. UVA, 1993 and 2000
UO2 trapped in solid Ar: v(u)=776.0
UO2 trapped in solid Ne: v(u)=914.8
=138.8 cm-1
Photoionization of atomic uranium
f 3ds2 5L6
v1=17908.17 cm-1
v2
U++e-
I.P.=49959(1) cm-1
f 3s2p 5I5
f 3s2 4I9/2(literature value: 49958.4(5) cm-1)
31900 31950 32000 32050
U+
Ion
Sign
al
Second Photon Energy (cm-1)
PIE MATI
=343 V/cm =21 V/cm
0 1 2 3 4 5 6 7 8 9 10
53250
53255
53260
53265
53270
53275
53280
53285
53290
53295
N+
Angular momentum selection rules are relaxedby field-induced mixing of the Rydberg series
Spectrumfor excitation
via J’=7
54200 54210 54220 54230 54240 54250 54260 54270 54280
via J'(O)=0
via J'(O)=3
via J'(O)=5
via J'(O)=7
via J'(O)=9
via J'(O)=11
Total Energy, cm-1
N+= 0 N+= 6N+= 4 N+= 8 N+= 10 N+= 12
PFI-ZEKE Spectra for ThO+
X, v=0
O, v=0
X, v+=1
ThO
ThO+
Broken lines show zero-field energies
1
2
Rotational structure of ThO+ X2+, v+=1
31908
31838
0
120
360489
17859
618
179361802418111
2g
2u
3u
4g
3183
8
3178
8
3147
8
3141
9
1749
9
1744
7
1740
6
1766
4
176 2
1
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 byZhou et al. JCPA, 104, 5495 2000)