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CR&T Code0 Technical Report No. 45N

Understanding Core Level Decay ProcessesIn the High-Temperature Superconductors

By

D. E. Ramaker, N. H. Turner and F. L. Hutson

Prepared for Publication

in the

Physical Review B

George Washington UniversityDepartment of Chemistry

Washington, D.C. 20052

December, 1988

Reproduction in whole or in part is permitted forany purpose of the United States Government

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11. TITLE (Inrclude Socunty 0CUSSiaJiofl)Understanding Core Level Decay Procespes in the High-Temperature Super-conductors (Unci.) /I

12. PERSONAL AUTH4OR(S)D. E. Ramaker, N. H. Tur er, and F. L . Hutson

13a. TYPE OF REPORT li b_ TIME COVERED 1 14. DATE OF REPORT (Yar Moonth, Day) IS. PAGE COUNTinterim Technical( FROM ____ O December 1988 '0

6. SUPPLEMENTARY NOTATION

repared for publication in Physical 2view B

17 COSATI CODES IW UJC E (Cont"'..e on ,W~iR* of necerrjry and identify by block '15o.,)

E L GROUP 1SUB-CROuP Superconductivity, Asiger

Spectroscopy, X 1 ray

Emission Spectroscopy, HubbardMoe

A XACT (Convnu# on mieerr. if I.enary and .doniify by block number)

A highly correlated CuOq cluster model is utilized to intprpret the corelevel Auger and xray emission decay spectra for YBa?"O" and CLIO. Theevidence indicates that the initial-state shakeup stitei ?efax to states ofthe same symmetry before the core level decay, provided they have a shakeupexcitation energy much greater than the core level width.

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All 11'rA ,Mo~I. Unclassified

2

Previously reported (1,21 core level Auger (AES) and x-ray emission

(XES) data are interpreted within a highly correlated CuOs cluster model for

the high-temperature superconductors (HTSC's), YSaCu,07.j and Lat.

&BaCuO4 (herein referred as 123 and La), The Lu s4V Auger lineshape is

reported here for the first time and is interpreted consistently with the LVV

lineshape and XES data. This work clearly indicates, contrary to previous

reports [1,31, that the initial-core shake-up (ICSU) states do not directly

decay, but rather relax to the primary core-state before decay. The XES data

dramatically reveal the change of character of the valence band (VB) states

between CuO and 123.

The basic electronic structure of the HTSC's can be described by an

extended Hubbard model, characterized by the transfer or hopping integral t,

the Cu and 0 orbital energies ca and cv, the core polarization energy Qi, the

intra-site Coulomb repulsion energies U, and Up, and the inter-site repulsion

energies U., and U, •* (i.e. between neighboring Cu-O and 0-0 atoms). The

extended Hubbard model is most appropriate when the U's are large relative to

the band widths [3], i.e. when correlation effects dominate covalent or

hybridization effects. A CuO.( a4-1 ) cluster model, which is also reasonably

valid when U >) t, simplifies the model [3). Both La and CuO contain CuO.

groups (41, having 4 short and 2 long Cu-a bonds. The 123 HTSC contains

CuOs and planar CuO4 groups [4]. The different n may alter the relative

intensities of various features, but similar features are present in each case.

The different bond lengths may increase the widths of the spectral features,

but little else since correlation dominates.

The CuO.(20- 2 )- cluster has one hole shared between the Cu 3d and 0

2p shells in the ground state, which we term the v (valence) states. The

spectroscopic final states reflect multi-hole states, e.g. v I , cv (c = core) etc.

3

We indicate the location of the v holes by d (Cu 3d) or p (0 2p). In the case

of two holes on the oxygens, we distinguish two holes on the same 0 (ps), on

ortho neighboring 0 stoms (pp'), or on pars 0 atoms (ppv) of the cluster.

Furthermore, neighboring pla holes can dimerize [51, so we distinguish

between two holes in bonded (pp%) and antibonded (pp*.) 0 pairs. Most of

the 0 atoms actually participate in two CuOs clusters. Consistent with

previous work [6], we account for this by defining the effective parameter, ¢p

= cp' + UP,, where U,. includes the interaction of a hole in an 0 p orbital

with its environment. In general U,. will be less than U.p due to

polarization.

The v states, as reflected by the theoretical DOS [1], can be described

as having the Cu-0 bonding (*b) and antibonding (+&) orbital@ centered at 4

and 0 eV and the nonbonding Cu and 0 orbitals at 2 eV. The 0 features each

have a width 21- = 4 eV due to the 0-0 bonding and antibonding character

and the Cu-0 dispersion. The +b and +a wavefunctions can be expressed as

[31,

+. = d cosO, - p sindt (la)

+b = d sind, + p cosO,, (Ib)

where 0, = 0.5 tan-'(2t/A). We also define the Cu-0 hybridization shift 61

0.5 sqrt(A2+4t2) - A/2, which is utilized in Table I to give the energies. In

this picture, the ground state of an average CuOs cluster is located at 1 eV

having the energy c4 -8 1 +r 2 = cs-, which we use as a reference energy for 3r

the excited v2, v3, and cv states reflected in the core level spectra. In CuO,

the hybridization shift r is smaller, and we shall see below that Atc p-cd has C1

increased to I eV. C1

Recently (8] we consistently interpreted the VB photoelectron spectra

(UPS and XPS). Most of the features in the UPS and XPS are also reflected

.dbLzlty Codes

Avail and/orSpecial

• ~ ~ ~ ~ ~ ~ ~ ~ ~ 0 man +ED niml miN• -

4

in the AES and XES, so that we review these assignments here. In 123, the

states were assigned as indicated in Table I [8,101. Calculated photoemission

intensities, their variation with 6, and photon energy dependencies confirm

these assignments (8]. In CuO [9], we have previously assigned a feature at

5.5 eV to ppea and ppo and at 3 eV to dp. The character switch of state 1

from mostly dp to pp, and vice versa for state 2 between CuO and 123 arises

because A decreases from I eV in CuO to 0 eV in 123. The reduction in a as

indicated by the UPS data is consistent with the Cu 2p XPS data and with the

XES data to be discussed below.

The "shakeup" or "many-particle" features at 9.5, 12.5, and 16 eV have

also been assigned as Indicated in Table 1 [8]. The pp% state has been

assigned to the "mystery" feature at 9.5 eV. Such a feature also appears for

CuO [9,11] so that this feature is not unique to the HTSC's. This feature

cannot arise from the p2 final state because U, is around 12-13 eV, much too

large to cause a feature at 9.5 eV. In fact we have found evidence (8] for

the existence of the p2 feature around 16 eV in 123. Finally the d2 state is

known to cause the 12.5 eV feature [12).

Cu_2p and 0 Is core level XPS. In order to understand the XES and

AES data, we first characterize the initial state, which is reflected directly in

the Cu 2p and 0 Is XPS data. The primary and satellite features seen in the

Cu 2p XPS spectrum for CuO [131 and 123 or La [1,14] are known to arise

from the cp and cd states, respectively (3], having the energies given in

Table 1. The relative satellite intensity, I(cp)/I(cd) decreases from 0.55 in

CuO to 0.37 in 123 as determined from the experimental data [1). The energy

separation, E(cd) - E(cp) increases from 8.7 eV in CuO to 9.2 in 123 (1].

The primary (cp) and satellite (cd) wavefunctions can be written similar

to eq. (1), with hybridization angle 6 = 0.5 tan-t(2t/(-Qd)) [3]. In the

i

5

sudden approximation, the Intensities are proportional to the overlap between

the ground state wavefunction, #, and the final states, so that I(dp) -

cos(,&-3) and I(cd) - sins(O.-O) (3). Thus the satellite intensity increases

with change in the hybridizhtion angles between the v and cv states. In the

ground v state, the hole is shared equally in the p and d orbitals since J1 ?

450, in the primary cv state it is mostly in the p orbital since jc ? 780. The

changes between CuO and 123 noted above are just that expected for a

decrease in a and reflect an increased covlency in 123 (151.

The large width of the primary cp peak is believed to arise from the

mixing with the cd state [3,15). The cd state has a large width due to the

large core-hole, valence-hole interaction, indeed, the satellite actually reveals

the cd multiplet structure. Evidence that the primary cp peak width arises

from the cd interaction comes from the Cu halide data [3], which show a direct

correlation of the primary cp peak width with the satellite cd peak intensity.

We do not believe that the primary peak width arises from the 0 p band width

as proposed by others (161.

The 0 Is spectra have been reported by many authors; however, it is

seriously altered by impurities such as 011- and CO" on the sample surface

[171. Recent data (181 from single crystal samples of the La material cleaved

in-situ are expected to be reasonably free of impurity effects. The cp- and

cpP states listed in Table 1 are believed to account for the tailing off of the

spectra seen in these spectra (this will be positively identified upon

examination of the XES data). Consistent with the sudden approximation, the

cp state is not seen in the 0 Is XPS because now both the v and cv states

have similar hybridization angles, i.e. the valence hole is mostly in the d

orbital in both cases.

We will find below that the ISSU process, which is responsible for the

satellites in the XPS noted above, does not produce satellites in the AES or

XES data, because the ISSU states generally "relax" to the primary states of

the same symmetry before the core level decay. Such a relaxation is expected

when the ISSU excitation energy is larger than the core level width (191.

Previously, vanderLaan et al t31 suggested the intensity of these ISSU

states in the XPS should be quantitatively reflected in the intensity of the

Auger satellites found in the LsVV lineshapes for the Cu halides. The data

do not indicate this however. While I(cd)/I(cp) increases from 0.45 for CuBra

to 0.8 for CuFa, the Auger satellites do not increase (3]. We previously [1]

indicated that a fraction of these ISSU states probably resulted in Auger

satellites for the HTSC's, and that this fraction increased with the increasing

covalency of the HITSC material. Evidence presented here indicates rather that

the ISSU states relax before the core level decay to states of the same

symmetry, provided they have a ISSU excitation energy much greater than the

core level width. We believe this to be a general result, at least in the Cu*'

materials.

The Cu L2 3 and 0 K XES data. The 0 K XES data (21 in Figure la confirms

our assignment of the 0 XPS, and clearly shows the dependency of the rSSU

state relaxation on the excitation energy and symmetry. The principal XPS

peak arises from the cd state, and it decays to the dp state since the x-ray

emission process is intra-atomic in nature. Therefore the principal 0 XES

peak aligns with the dp feature in the UPS as shown in Fig. 1. The cp- state

does not mix with the primary cd state; therefore, it does not relax before the

decay, but decays directly to the ppe% (and perhaps a little also to the ppa)

state. This accounts for the feature around 6.5 eV in the XES, just 3 eV

above the ppo feature in the UPS. The shift of 3 eV matches the energy

difference between the cps and cd core hole states. The cpv state can m x

with the cd state, therefore it can relax to the cd state, but it does this

slowly because of the small excitation energy of 0.5 eV. Therefore, the cpP

state decays either directly to the ppP state, or relaxes to the cd state, which

then decays to the dp state. This explains the photon energy dependence

seen (21 in the data of Fig. Is. At high photon energy, the sudden

approximation is more valid, creating a larger intensity for the cpr state, and

consequently a larger ppo contribution around 2.5 eV in the XES.

The Cu Lt3 XES data t2,201 shown In Figure lb dramatically reveals the

switch in character of the I and 2 v2 states between CuO and 123. Again, the

satellite cd initial state relaxes to the cp state before the decay so that the

XES reflects primarily the dp DOS. In CuO the XES spectrum peaks at 3 eV,

in 123 it falls around 4.2 eV, very near where we indicated the dp states fall

in the UPS data. The large intensity in the CuO XES extending above the

Fermi level is believed to be an experimental artifact [20].

The Cu L23VV and L33M23V Auger data. Comparison of the L3VV data

for CuO [11] end 123 [11 are shown in Fig. 2. The data reveal features at 7

(the two-center feature), 15, and 19 eV, which we previously [l] attributed to

dp, d2, and d3 final states, utilizing a vi final state model. The d3 states

were attributed to a combination of 3 different processes; I) initial state

shakeoff (ISSO) followed by Auger decay (g.e. + hu - L3V - d3), 2) Coster-

Kronig (CK) decay followed by Auger decay (g.e. + hv - Lu - Liv - di), and

3) ISSU followed by Auger decay (g.s. + hv - Live -. Liv -. d3, where e

denotes the excited electron). The ISSO and CK processes accounted for all of

the ds component in CuO, and the ISSU process was believed, as mentioned

above,to account for the increasing d component in La and 123 [1].

We report and interpret here, for the first time, the LzsMI:V Auger

lneshapes for the 123 SC. The sample preparation, treatment, and instrument

utilized were described previously (1). Fig. 2 compares the LtsMlgV spectra

for CuO [241 and 123 , and Identifies the various features. The L23Ms3V

lineshapes reflect the cv2 DOS# the main features arising from the cdp final

state, and the satellite from the cd'p state apparently resulting from the

similar ISSO, CK, and ISSU processes defined above. However, Fig. 2 reveals

a most interesting point; although 123 shows an increased satellite in the

L23VV relative to CuO, It is not increased in the LssMfV. This Indicates

strongly that the ISSU process is not responsible for the increased satellite in

the L3VV, because then it should Increase the satellite in both 123 lineshapes.

Since only the primary cp core-hole state Auger decays, and this

process is also known to be strictly intra-atomic, the L2WV lineshape in our

current v3 final state model, reflects the d'p DOS, as it is distributed among

the vs states listed in Table 1. Thus the features at 7, 15, and 19 eV arise

naturally from the dppP, d2p, snd dpZ final states. The ISSO and CK

processes also contribute to the "satellite" contribution at 19 eV just as in

CuO. The dppo state does not appear in the LVV lineshape because it does

not have the same symmetry possessed by all the other vs final states and

the cv initial state. The increased "satellite" feature at 19 eV in the HTSC's

arises apparently because of increased configuration mixing between the dZp

and dp2 states. Its intensity is increased in 123 relative to CuO because the

energy separation (before hybridization) between d2p and dp2 has decreased

from 3.8 eV in CuO to 2.5 eV in 123. We have indicated this mixing in Table I

by adding the hybridization shifts 62 to the energy expressions for these two

states.

The L,3M 23V lineshape reflects the cdp DOS. The mixing of the other

states (cd', cppP, cpp%, and cpp%; the latter three are not listed in Table 1)

9

with the cdp state is small because of the large energy separations involved.

The cp' state Is close to cdp; however, it falls in between the 3L and IL

multiplets of the cdp state. Although it may have some intensity, It surely

does not contribute to the CK + SU satellite around 25 eV in either CuO or

123. The exchange splitting (2K) between the 3p and d holes is known to be

very large [3), so we include it explicitly in Table I to account for the haL

multiplets.

The 0 KVV lineshape is severely altered by impurities on the sample

surfaces, and no single crystal lineshape data have been reported. The 0 KVV

lineshapes for CuO and CuaO have been reported (111, and they have the

primary dp2 or pt features, respectively, around 19 eV. A very small satellite

appears around 7 eV in CuaO which we attribute to the ppP state. A much

larger and broader satellite around 7 to 14 eV in CuO appears, which we

attribute to the dp state around 14 eV as well as a smaller amount to the

dpp state around 7 eV. Thus the d~p and dps states appear in both the Cu

L2jVV and 0 Auger lineshapes for Cu"* oxides, except their primary and

satellite roles are reversed.

In summary, we have interpreted XES and AES data utilzing a highly

correlated CuO. cluster model. Both the XES data and the previously

interpreted UPS data reveal the reversal in character of the VB states

between CuO and the ITSC's. We have also shown that the initial-state shake-

up states evident in core level XPS, do not generally produce satellites in the

core emission spectra, because they relax to the primary core states of the

same symmetry, provided the ISSU excitation energy is greater than the core

level width.

TABLE I Summary of hole states revealed in the spectroscopicdata, and estimated energies using the following optimal valuesfor the Hubbard parameters in eVa:

6, = 2 to = 2 Up - 12, 13 UP =9.5,10.26a = 0.5,0.8 to = 2, 3 UP,* = 4.5, 4 U4 , =1" = 2 Upp : 0. Uo z 2 Q4 =9a a 1, 0.5 A a 0, 1. K: 4

Stateb Energy expression Calc. E. Exp. I. RemarkeVe.4 eVe

G.S. and IPRS. v*.) d ce - a, * r 0 : 2 - heavily+6) p cp + a, * r 4 : 2 - mixed

UPS and XES. vs1)0 ppP c + A - 62 + a 2.5 2.5 heavily2)" dp c, UP, +dr +a 4.5 4.2 mixed3) pp*. c,+ A +Ut,, -r'+a 5.5 5.4) ppeb tp+ A *U,," +fla 9.5 9.5 mystery peak5) d' to + U, + a 12.5 12.5 Cu sat.6) pt C + A + Up + ,a 15 16

Cu 2p IPS, evCP cc + A + a cc, +1 ES maincd C. + Q4 + a C. + 10 BP+9.2 sat.

0 In IPS, CvCd cc + a cc4 I Eta maincpP cc + A + a cc + I BE. maincpO cc + A + Ue +a c, + 3 Ej.+2 ? tailcp c. + A + Q, + a ? not obs

Cu LIVV ARS. v2dppP 2c, + 2UdP * a 7 7 2 cent. featuredppo 2cp+Uepo+2U.p+ 11.5 - no mixingdip td+c,+U+2U.p-6,+a 16 15.5 main featuredpi 2cp+Up+2Up+6+a 19.5 18-25 sat. feature

Cu LIMisV ARS. cvs

cdp cC+cp+Q+U.p.X+a Ce +9 E3,+l0 main, ILc.+17 Ejp+18 main, 3L

CpR cC +Cp +A +Up + a c. + 15 - not observedcdi cc"cd+U*+2Qe +a cc+30.5 - not observed

*PSrameters for 123 indicated first, those for CuO serond.The dominant character in the hybridized states is given.

eThe Calc. E and Exp. E columns indicate the results for 123.dThe calculated E is defined relative to the ground vi (d) state energy = C -a. The vIld) energy defines the Fermi level relative to the vacuum level atzero.*The dominant character switches an described in the text, and thus the signin front of 82 in the opposite for CuO.

Figure Captions

Fig. 181 Comparison of 0 K XES data for 123 taken at the indicated

photon excitation energies (from Ref.2).

lb) Comparison of Cu Lu XIS data for 123 (Ref. 2) and CuO

(Ref. 20).

Ic) UPS data for 123 (h' z 74 eV from Ref. 10).

Fig. 2) Comparison of Auger data for the materials indicated. Cu

L2VV data for CuO and 123 from refs. 25 and 1. Cu

Lz3 M2aV data for CuO from ref. 24 and for 123, this work.

The Ls2VV data is on a 2-hole binding energy scale = ELZ-

Ek,, and the L:3M22V on a 1-hole scale = ELi-EI-EN,, where

E.3 = 933.4 and Em3 = 77.3 eV (9,11].

0OK (a)XES

123

*570hog=535

CuCuO (b)

.0 1231I'

65

zI

z

. . . . . . . . . . . . .

15 10 5 0BINDING ENERGY (eV)

sat. L3

co

123

c/z/

CuO

sat. L 3 LL

3228 2420 16 128 4 0 -4 -8BINDING ENERGY (eV rel. L,3)

References

1. D.E. Ramaker at &l., Phys. Rev. 36, 5672 (1987).

2. K.L. Tuang at &l., Phys. Rev. 337, 2293 (1988).

3. G. vanderLean et al., Phys. Rev. 24, 4369 (1981); J.C. Fuggle et al.,

Phys. Rev. B37, 1123 (1988).

4. J.E. Greedan at al., Phys. Rev. B35, 8770 (1987).

5. R.A. de Groot, H. Gutfreund, and M. Weger, Sol. State Commun. 63,

451 (1987); W. Folkerts et al., J. Phys. C: Solid State Phys. 20,

4135 (1987); A. Manthiram, X.X. Tang, and J.B. Goodenough, Phys.

Rev. B37, 3734 (1988).

6. J.E. Hirsch et al., Phys. Rev. Letters 60, 1168 (1988).

7. J. Redinger et al., Phys. Lett. 124, 463 and 469 (1987).

8. D.E. Ramaker, N.H. Turner, and F.L. Hutson, submitted.

9. M.R. Thuler, R.L. Benbow, and Z. Hurych, Phys. Rev. B26, 669

(1982).

10. N.G. Stoffel et al., Phys. Rev. B37, 7952 (1988); also preprint.

11. C. Benndorf et al., J. Electron. Spectrosc. Related Phenom. 19, 77

(1980).

12. R. Kurtz et al., Phys. Rev. B35, 8818 (1987).

13. A. Rosencwaig and G.K. Wertheim, J. Elect. Spectrosc. Related

Phenom. 1, 493 (1972/73).

14. P. Steiner et al., 2. Phys. B- Condensed Matter 67, 497 (1987).

15. D.E. Ramaker et al., Phys. Rev. 36, 5672 (1987).

16. D.D. Sarma, Phys. Rev. B37, 7948 (1988).

17. S.L. Qiu et al., Phys. Rev. B37, 3747 (1988); W.K. Ford et al., Phys.

Rev. B37, 7924 (1988); D.E. Ramaker, N.H. Turner, and F.L. Hutson,

In Thin Film Processing and Characterization of Hibh Temperature

Superconductors, J.M.Harper, J.H. Colton, and L.C. Feldman ads.,

AVS Series No. 3 (American Institute Physics, New York, 1988), p

284.

18. T. Takahashi et ml., Phys. Rev. D37, 9788 (1988).

19. J.W. Gadzuk and M. Sunjic, Phys. Rev. 312, 524 (1975).

20. A.S. Koeter, Mole. Phys. 26, 625 (1973).

21. D. van der Marel et &l., Phys. Rev. B37, 5136 (1988).

22. Y. Chang et &l., Phys. Rev. B (In press).

23. N. Nucker et al., Z. Phys. B: Cond. Matter 67, 9 (1987); Phys. Rev.

37, 5158 (1988).

24. L. Fiermans, R. Hoogewijs, and J. Vennik, Surf. Sci. 47, 1 (1975).

25. P.E. Larson, J. Electron Spectroec. Related Phenom. 4, 213 (1974).

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Athens, Georgia 30602 Berkeley, California 94720

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Dr. F. Carter Dr. John T. YatesCode 6170 Department of ChemistryNaval Research Laboratory University of PittsburghWashington, D.C. 20375-5000 Pittsburgh, Pennsylvania 15260

Dr. Richard Colton Dr. R. Stanley WilliamsCode 6170 Department of ChemistryNaval Research Laboratory University of CaliforniaWashington, D.C. 20375-5000 Los Angeles, California 90024

Dr. Dan Pierce Dr. R. P. MessmerNational Bureau of Standards Materials Characterization Lab.Optical Physics Division General Electric ConpanyWashington, Q.C. 20234 Schenectady, New York 22217

Dr. R. G. Wallis Dr. J. T. KeiserDepartment of Physics Department of ChemistryUniversity of California University of RichmondIrvine, California 92664 Richmond, Virginia 23173

Dr. D0ak Dr. R. W. PlummerChem(itry Department Department of PhysicsyPge Washington University University of Pennsylvania#Adashington, D.C. 20052 Philadelphia, Pennsylvania 19104Dr. J. C. Hemminger Dr. E. YeagerChemistry Department Department of ChemistryUniversity of California Case Western Reserve UniversityIrvine, California 92717 Cleveland, Ohio 41106

Dr. T. F. George Dr. N. WinogradChemistry Department Department of ChemistryUniversity of Rochester Pennsylvania Sate UniversityRochester, New York 14627 University Park, Pennsylvania 16802

Dr. G. Rubloff Dr. Roald HoffmannIBM Department of ChemistryThomas J. Watson Research Center Cornell UniversityP.O. Box 218 Ithaca, New York 14853Yorktown Heights, New York 10598

Dr. J. Baldeschwleler Dr. Robert L. WhettenDepartment of Chemistry and Department of Chemistry

Chemical Engineering University of CaliforniaCalifornia Institute of Technology Los Angeles, CA 90024Pasadena, California 91125

Dr. Galen D. Stucky Dr. Daniel N. NeumarkChemistry Department Department of ChemistryUniversity of California University of CaliforniaSanta Barbara, CA 93106 Berkeley, CA 94720

Dr. A. Steckl Dr. G. H. MorrisonDepartment of Electrical andSystems Engineering Department of ChemistrySysem EginerngCornell UniversityRensselaer Polytechnic Institute .. Ithaca, New York 14853

Troy, New York 12181

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Dr. G. A. Somorjat Dr. L. KesmodelDepartment of Chemistry Department of PhysicsUniversity of California Irdiana UniversityBerkeley, California 94720 Bloomington, Indiana 47403

Dr. J. Murday Dr. K. C. JandaNaval Research Laboratory University of PittsburgCode 6170 Chemistry BuildingWashington, D.C. 20375-5000 Pittsburg, PA 15260

Or. W. T. Peria Dr. E. A. IreneElectrical Engineering Department Department of ChemistryUniversity of Minnesota University of North CarolinaMinneapolis, Minnesota 55455 Chapel Hill, North Carolina 27514

Dr. Keith H. Johnson Dr. Adam HellerDepartment of Metallurgy and Bell Laboratories

Materials Science Murray Hill, New Jersey 07974Massachusetts Institute of TechnologyCambridge, Massachusetts '02139 Dr. Martin Fleischmann

Department of ChemistryDr. S. Sibener University of SouthamptonDepartment of Chemistry Southampton S09 5NHJames Franck Institute UNITED KINGDOM5640 Ellis AvenueChicago, Illinois 60637 Dr. H. Tachikawa

Chemistry DepartmentOr. Arold Green Jackson State UniversityQuantum Surface Dynamics Branch Jackson, Mississippi 39217Code 3817Naval Weapons Center Dr. John W. WilkinsChina Lake, California 93555 Cornell University

Laboratory of Atomic andDr. A. Wold Solid State PhysicsDepartment of Chemistry Ithaca, New York 14853Brown UniversityProvidence, Rhode Island 02912 Dr. Ronald Lee

R301Dr. S. L. Bernasek Naval Surface Weapons CenterDepartment of Chemistry White OakPrinceton University Silver Spring, Maryland 20910Princeton, New Jersey 08544

Dr. Robert GomerDr. W. Kohn Department of ChemistryDepartment of Physics James Franck InstituteUniversity of California, San Diego 5640 Ellis AvenueLa Jolla, California 92037 Chicago, Illinois 60637

Dr. Stephen 0. Kevan Dr. Horia MetfuPhysics Department Chemistry DepartmentUniversity Of Oregan University of CaliforniaEugene, Oregon 97403 Santa Barbara, California 93106

Dr. David M. Walba Dr. W. GoddardDepartment of Chemistry Department of Chemistry and ChemicalUniversity of Colorado EngineeringBoulder, CO 80309-0215 California Institute of Technology

Pasadena, California 91125