Volume63A, number3 PHYSICSLETTERS 14 November1977

MEASUREMENTS OF THE ENERGY SHIFTS OF THE K ABSORPTION EDGEAND K X-RAYS OF SEVERAL URANIUM COMPOUNDS

P.L. LEEDepartmentofPhysicsandAstronomy,California State University,Northridge,California 91324, and

California Institute of Technology,Pasadena, California91125,USA

and

F. BOEHM andP. VOGELCalifornia Instituteof Technology,Pasadena, California91125, USA

Received11 February1977Revisedmanuscriptreceived15 August1977

The energyshifts in the K absorptionedgesand the Ka1 and K~313X raysin severaluraniumcompoundswere

measuredwith theCaltech2 m bentcrystal spectrometer.Theresultsarecomparedwith free-ionrelativisticself-consistentfield calculations.

Chemicalshift of the inner electronshells [1] pro- actinides[3] ,mayhelp to understandthe role of thevides a tool for studying the configurationsof those 5f electronsin chemicalbonding.electronsparticipating in thechemicalbond.As The X ray energieswere measuredwith theCaltechvalenceelectronsare added orremovedfrom an atom 2 m bentcrystal spectrometer,using the310 planeofthe energyeigenvaluesof the inner electron states a quartzcrystalin theCauchoisgeometry.Theexperi-change.In medium heavy atoms thisenergy changeis mental procedureis similar to that describedin ref. [2].roughly the samefor the ls, 2p, 3p and 4pelectrons The uraniumcompoundswerepressedinto pills ofassociatedwith the K X raysand amounts to about 1.5 inch diameter andthicknessof 0.235 gm/cm

2.10 eV. Accordingly, the changein X ray energyis The pills were placedin front of an X-ray tube whichvery small, typically of the orderofO.l to 1.0eV. wasoperated at160 keV and l2mA. In the caseofClearly a directmeasurementof the relatively largeis the Ka

1 line, threeseriesof experimentswereper-level shift is desirable.Asidefrom lowerZ (Z ~ 30) formed.In thefirst series, theU02,U30~,and U03compounds[6], no shift of the K absorptionedgehas samplesweremeasuredconsecutivelyin thesequencebeenstudiedbefore. It is the purposeof the present describedin ref. [2]. In the secondseries,U02, UC,letterto reporttheobservationof the chemicalshift UO2SO4 3H2O andUO3HPO4 4H20 weremeas-of the K absorptionedge(ls level) in compoundsof uredconsecutively.In the final series,U02 and U03uranium.In addition,measurementsof theK X ray weremeasuredasa check.A five-parameterGaussianshifts arepresented. with slopeandbackgroundleast-squaresfit is applied

Experimental studies of thechemicalshifts ofK X to the data and thecentroidparameterdeterminingrays fromheavyatomsusingcurvedcrystalspectrome- the chemicalshift is obtained.For Kj31 ~, one singletershavebeen reported in recentyears [1]. The re- seriesconsistingof U02,UC, UO2SO4 3H20 andsultsof these studiesare particularly interesting in the UO3HPO4 4H20was madeunder conditionssimilarcaseof rare earthwhereshiftswere found tobe rela- to thatof Ka1.tively largebearingoutthe dominantrole of the 4f The uraniumoxidessamplesused in theK edgeelectrons[2]. In actinides the 5felectronsplay a role shift measurementhavean optimumthicknessof 1.65analogousto that of the 4felectronsin rare earth.Our gm/cm

2,or roughly seven timesthat of thesamplesexperiment,giving information complementary to that usedearlier.The transmittedbremsstrahlungfrom theusuallyused in studies of the electronicstructureof X ray tubewasdetected in the spectrometer.As the

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Volume63A,number3 PHYSICS LETTERS 14 November1977

Table 1 Table2Observedchemicalshifts.TheenergydifferenceE(UO

2) — Relativisticself-consistentfield calculationfor differentcon-E(cornpound)in eV is given figurationsof the uraniumion. The differencein eigenvalues

andX-ray energiesfor thelisted configurationsis shownKa1 K~1,3 K edge

Level E(5f’)—E(5~) E(6d1)—E(6d°)E(7s2)—E(7s1)

U02 (reference) — — — __________________________ _______________________________

UC 0.05 ±0.03 —0.02 ±0.05 ~ 17.15 12.36 9.71U3O8 0.21 ±0.02 —6.8 ±0.8

2P1/2 17.71 12.63 9.69U0

3 0.23 ±0.02 —5.6 ±0.8 2p312 17.84 12.65 9.68UO2SO4°3H20 0.16±0.03 0.36±0.07 3p312 18.37 12.57 9.64UO3HPO4~4H2O 0.19 ±0.03 0.41±0.08 3d5,2 18.36 12.59 9.644P3/2 18.11 12.39 9.60

4d512 18.10 12.39 9.60

bremsstrahlungspectrumhas a strongslopein the XRayKaj 0.69 0.29 —0.04

energy regionconsidered, theedgeprofile wassuper- 1 22 0 21 —0 08

imposedon a slopingbackground.A similarback- La1 0.52 0:06 —0.04groundappears inall threeoxides,andits presence Kedge —17.15 —12.36 —9.71

didnot affect the shiftof the edge.A least-squaresfitof the following function is appliedto themeasuredprofile: Table3

Averageshifts betweenthe tetravalent andsixvalentcorn-P5—x. +

y(x1) = P1 + P2x1+ P3x~+ p4 tan’ I pounds(E(4 ) — E(6~))P6

Exp “Theory” dThe measuredenergyshiftsE(U02) — E(compound)

of Kc~1,Kj11 ~ and theK-edgearepresented in table1. Kaj 0.19 eV 0.22 eV

The resultsfall roughly into two groups.Theshifts Kp1,3 0.39eV 0.39 eV

betweenthe tetravalent U02 andUC arequitesmall, Kedge —5.6 eV —5.5eVwhile theyare largebetween U02 andthe six-valent La1 0.14 eV [51 0.17 eV

compounds.We notice that theKj31 ~ shift is abouttwice aslargeasthe Ka1,while the K-edgeshiftsare a Self-consistentfield calculation resultof table 2,an order of magnitudelargerand of the oppositesign. [E(5f’) — E(5f°)],multipliedby theempirical factor0.32.

Thesefindingsare in line with previousobservationsin rare earth[1,2]. calculatedvalues.However,the experimentalresults

In orderto interpretour resultsin a semiqualitative havethe characteristicpattern(ratios of shifts of theway, we calculated the shifts of the atomicis112, differentX-ray lines and of theK-edge)predicted for2P1/2’ 2P3/2’ 3P3/2’3d

512,4d5/2, and4P3/2levels removalof the Sf electrons.

usingthe freeion relativistic self-consistentfield pro- If we scale downour calculationfor the shiftduegram(Dirac-Hartree-Slater)[4]. The resultsare pre- to one Sf electron (table 2,column 2) by an empiricalsented intable 2. Columns2, 3, and 4give thechange factor 0.32 andcomparewith our experimentalvaluesin the energyof the levels for changesin the electronic for tetravalent andsix valent compounds,we find re-configuration of theSf, 6d and 7s states,respectively. markableagreement,asillustratedin table 3. We canThe bottomlines of table2 give the energyshifts for therefore concludethat thechargetransfer ofa frac-the K andL X rays. tion of Sf electronsexplainsthe experimentalfacts.

Assumingthat the6-valentcompoundshavetwo Onehasto remember,however, that the predictedless SfelectronsthanU0

2, we cancompareour ex- shifts for the 7s and 6d electrons (particularly inXperimentalresultswith the calculatedvalues.Sucha rays)are muchsmallerthan for the Sf electrons.Thuscomparison,usingour experimentalresultsin table 1 the experimentdoesnotprovide information aboutand the calculatedvaluesin column 2 of table2 shows the role of the 6d and7s electronsin thevalencethat the experimental shiftsaremuchsmallerthan the change.

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Measurementsof severalL X raysof U werere- F. Boehrn,in: Atomic inner-shellprocesses,ed. B. Crase-

portedby Makarov et al. [5]. Their results areconsis- mann(Academic Press, 1975)p. 412.

tent with ourinterpretationof the shifts in theK X [2] P.L. Lee,F. BoehmandP. Vogel, Phys.Rev. A9 (1974)614.

raysand theK edge. [3] AJ. FreemanandD.D. Koelling, in: The actinides:

Electronicstructureandrelatedproperties,vol. 1, ed.A.J.Freemanand J.B.Darby Jr. (Academic,New York,

References 1974)p. 51.[4] R.S.HagerandE.C. Seltzer,Nucl. DataA4 (1968) 1.[5] L.L. Makarovetal., VestnikLeningradskogoUniversiteta

[1] 0.1. Sumbaev,in: Proc.Intern. Conf. on Innershell (J.Univ. Leningrad)No. 16 (1975) 87.ionizationphenomenaandfuture applications(USAEC [6] C. Suguiro,J. Chem.Phys.59 (1973)4907;TechnicalInformationCenter,Oak Ridge, Tennessee, R.G. Shulmanetal., Proc.Nat. Acad.Sci. (USA) 731972)p. 346; (1976) 1384.

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