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)