loss studies of uranium, uranium oxidation, UO2, U308, and UF4

Surface Science 61 (1976) 37-59 0 North-Holland Publishing Company

AUGER/LOSS and UF, * WaIton

STUDIES OF URANIUM, URANIUM OXIDATION,

UO,, U,O,,

P. ELLIS

University of California, Los Alamos Scientific Laboratory, New Mexico 87545, USA

Los Alamos.

Received 8 June 1976

Auger/loss spectra were obtained from uranium metal free of detectable impurities in the 01, p and y phases. Loss excitations for the metal are in agreement with XPS levels of Fuggle et al. except for the 04 loss peak which appears to couple to a transient state ca. 7.3 eV more energetic than in XPS. Good agreement is obtained with Auger spectra if experimental XPS valence bonding bands are used, and chemical shifts upon oxidation are accounted for accurately. Loss spectra for uranium compounds do not agree in general with XPS. Relaxation differences in the excited states appear to be the reason for this discrepancy. UO was identified as a stable surface compound on U metal under specific oxidizing conditions.

1. Introduction Apart from their practical applications in atomic energy, the actinide materials with their chemical and physical complexities are fascinating subjects to study. As a group they .have the highest atomic numbers in the periodic table, they include as many as three principal quantum shells in the valence band, e.g. the ground-state uranium atom is listed as 5f36d17s2, they have both localized and non-localized valence electrons and form many structurally complex, pure elemental phases, and form non-stoichiometric compounds, e.g., UO,,,. An immense volume of literature has been written on these materials, but in common with other classes of elements and compounds, much remains unanswered and several hundred new reports appear each year. The study of Th(OO1) is indicative of an increasing interest in surface processes on actinides [ 11. In areas related to surface studies on uranium, the recently revised core-level designations by Fuggle et al. [2] indicated that our earlier Auger/loss studies [3] with uranium needed systematic re-examination with high purity material. First, the possible Auger/loss designations given in ref. [3] needed to be revised. Second,

* Work performed under the auspices of the US Energy Research and Development Administration. 37

38

W.P. Ellis /Auger/loss

studies of uranium

valence-band, VB, effects for example in a comparison of U and U + 02 needed re-inspection in the light of X-ray photoelectron spectroscopy, XPS, studies by Fuggle et al. [2] and the interpretations of Veal and Lam [4]. Third, one core loss And, in particular, the O,, remained a paradox that needed closer examination. fourth, comparisons with other Auger/XPS studies [5,6] of U by Allen and his colleagues were indicated. A direct correlation of inner-shell loss excitations with XPS levels involves descriptions of the two probes and considerations of final-state coupling in the solid. Numerous recent articles include discussions on final-state coupling, relaxation and many-body effects in the electron spectroscopy of solids [7-91 and do not need an in-depth repetition here. But brief, general descriptions of the central features, especially those of a problematical nature, will be of value in specific comparisons of Auger/loss results with XPS observations. Fig. 1 indicates various conceptual processes in XPS. Fig. 1A represents the ground-state atom in the solid with core levels X, Y, Z in order of increasing binding energy. The valence band, indicated schematically as VB, has a maximum at S below the fermi level, E,. In XPS, incident monoenergetic photons interact with the solid with the ejection of photoelectrons into the vacuum: from the VB near the fermi edge as in lB, or from a core level Y as in 1C. The solid gains one positive

0

x

Et

Y

2

Energy Fig. 1. Various conceptual processes in XPS of solids. Energy abscissa: 0 vacuum level; Ef fermi level; VB valence band; X, Y, Z core levels in order of increasing binding energy. A ground state solid atom; B unrelaxed VB vacancy with one photoejected electron; C unrelaxed core vacancy; D partially relaxed core vacancy; E fully relaxed core vacancy. Drawings here are very schematic.

W.P. Ellis f Auger/loss studies of uranium

39

charge and relaxation occurs as in 1D in which the localized outer-shell electrons, X, rearrange to greater binding energy. That something of this sort occurs is well recognized: calculated binding energies exceed the XPS values by an amount that is greater for deeper shells. There are also valence-electron relaxation effects as indicated schematically in figs. 1D and 1E. In ID the valence electrons are indicated to become more tightly bound with the increased core charges, and in 1E conduction electrons flow in to neutralize the charge so that, by screening, the point charges of lC, D become spread over the crystal. Fig. 1E then represents very schematically one conceivable final relaxed state of the core-vacancy atom. An exact representation is not intended, and the two VB* levels are shown separately solely to emphasize that a perturbation occurs. Particularly pertinent to uranium is the article by Ley et al. [7] in which they concluded for conduction-band relaxation that measured relative energies in photoemission are independent of whether the vacancy is localized or non-localized. According to their model there would be negligible differential relaxation across the 5f36d17s2 valence band of metallic U and the relative VB energies as measured by XPS would not differ from the relative ground state energies in the metal by more than a fraction of an eV. They calculated for the hypothetical Na 3s localized hole, for (example, a relaxation energy of 2.93 eV, and 3.05 eV for the free-electron delocalized hole state. Certainly this subject and the related question fermi-level pinning (location), deserve closer theoretical scrutiny for uranium with its partial s, d, and f valence bands, localized 5f hole states (see below), and “delocalized” 7s, 6d electrons. This is especially true for the semiconducting oxides for which there is no reservoir of freely conducting electrons. For compounds, e.g. Fe(CO)s{C(CH,)s}, Conner et al. [9] calculated with a SCF MO model relaxation energies that vary from 0.7 eV for the C2p level to 6.2 eV for Fe3d and concluded that Koopman’s theorem inaccurately predicted relative positions of the low-energy photoelectron bands. For UOa there is a sharp, intense peak near Ef [2,4,10] which Veal and Lam [4] have attributed to localized non-bonding 5f electrons. An intriguing question that remains, one that has strong implications in chemisorption and catalysis on uranium oxides, is whether or not relaxation differences have significantly modified the shape and relative position of this sharp UO, peak near E, If so, then it may or may not be true in the ground-state oxide that the 5f level is as sharp as XPS indicates or that it is close to the metal EF The discussions above on VB relaxation effects serve to emphasize the complexities implied by fig. 1. Since the ejected photoelectron is in the vacuum continuum, a common assumption in XPS is that emission distributions reflect initial states in the solid rather than joint densities of initial and final states [8]. But as Connor et al. have demonstrated, relaxation variations can play a prominent role. Another point worth mentioning here is that in XPS the experimentally derived fermi level of the sample is customarily taken as the reference point rather than the vacuum are reported as differences in measured photoeleclevel. Thus XPS energies, E,,,, tron energies between the two distinctly separate excitations, with the implicit

40

WY. Ellis /Auger/loss

0

studies of uranium

I

X

Et

Y

Z

Energy Fig. 2. Core losses in solids. A ground-state atom with empty band y above Ef; B primary tation event with promotion of Y-core electron into empty band above Ef; C fully relaxed excited state. identical to 1E.

excifinal

understanding that the location of Ef in the experimental VB N(L) versus 6‘ distribution is somewhat uncertain within 1 eV or so. The advantage of reporting XPS energies as differences of course is in the avoidance of such uncertain factors as instrumental work functions. The actual excitation energy between figs. 1A and 1E is [EXPS (Y) + $1 w h ere 4 is the work function for the specimen at ground potential. Core-loss excitation differs from photoemission. In the loss process incident electrons at energy E, promote a core-level electron to the empty states above Ef as shown in fig. 2. The loss is detected at an energy A below E,, and in contrast with XPS the solid remains neutral. If relaxation occurs during the lifetime of the loss transition and promotion is equivalent to one directly from 2A to 2C, A is simply the energy difference between 2C and 2A and all information on empty bands above I!z’~is lost from the loss data. The excited electron in higher bands in fig. 2B, the transient nature of the excited state and the relaxation processes all suggest difficulties in precise, direct comparisons of loss spectroscopy with XPS. Indeed for some transitions in uranium and its compounds this appears to be the case. Only where the final, excited states are the same, e.g., as in 1E and 2C, would the correlation appear to be unambiguous in which case the energy difference is simply the photoelectric work function, 4. For this particular excitation the shape of the empty band above Bf is lost and no amount of loss-peak deconvolution can retrieve it. A general guideline can be proposed here: since XPS core-level energies do not involve levels above Ef except as minor shakeup satellites, it follows that if A(Y) = E,,, (Y) + q5 density-of-state information on the empty bands above E, cannot be expected from loss data. In

W.P. ISis /Auger/loss studies of uranium

41

this expression A(Y) is the loss energy for core level Y, E,,, (Y) is the XPS energy for level Y relative to E,, and it should further be noted that this expression is valid only for crystals at ground potential. The quantity, $, enters into such comparisons because of the format in which XPS energies are customarily reported. As stated above, tabulated XPS energies are relative to E, and have to be corrected for Cpif absolute excitation values are needed. in loss measurements this consideration does not enter since @cancels: the incident electron beam gains # in energy upon striking the crystal but loses r#, both elastic and inelastic components, upon emission. As indicated in fig. 3, for the WV Auger process one electron from the valence band falls into the core vacancy, Y*, with the simultaneous ejection of another valence electron leaving a net charge gain of +I on the solid and two holes in the VB, fig. 3C. In comparing energies of the Auger process with XPS and loss levels it is pertinent to following sections to note that differences in relaxation configurations of the excited valence states affect the available energies. Thus the excited valence configuration of fig. 2B will produce a more energetic YVV Auger electron than from the fully relaxed VB* of 2C, 3B. Another complication is added for transitions of the type YXV in which an electron from level X falls into Y with the simultaneous ejection of a VB electron. In this latter case, the final state is excited with a core vacancy, X*, and the valence band, VB*, is missing one electron. Energetics for the YXV Auger transitions have been approximated and tabulated [l I], but the unknown @ and 6 for VB* add to the question of nuclear charge discussed in ref. [ 11 J, Even with Auger transitions of the type YXX valence band effects are a consideration through VB* relaxation effects.

Fig. 3. The YVV valence-band Auger process. A ground-state solid atom; B the fully relaxed excited state with one VB* electron falling into core vacancy Y and the simuitaneous ejection of another VB electron; C final state following Auger emission. Drawings are schematic, with relaxation of VB in C and distribution of VB vacancies not indicated.

In addition to the above comparisons of some specific uranium core-loss, XPS, and Auger energies, in the present report are several other topics, including (1) cleaning procedures for uranium, (2) Auger/loss spectra of clean U free of detectable impurities, (3) Auger/loss studies of the 01,/3, and y phases of metallic U, (4) UO was identified as a separate surface phase independent of carbon, i.e., it was not stabilized as an oxycarbide as may have been the case with Allen and Tucker [5], (5) IJ308 and two types of UF, were examined, (6) 0, adsorption onto clean U was monitored for comparison with Riviere’s workfunction observations [ 121, and (7) spectra were taken at higher resolution than in ref. [3].

2. experimental Samples were examined in a modified Varian 4-grid LEED/Auger unit at base pressures in the mid-lo-l1 Torr (IO-* Pa) range. Data were taken in either (1) the conventional modulated retarding mode (grids 1,4 grounded and 2,3 ramped with an ac audio sinewave) or (2) in a higher-resolution retarding mode by grounding grid 1, and ramping 2, 4 with pure dc at 1.5 V positive relative to grid 3, to which a SO-500 mV ac signal was applied. For the features discussed here identical results were obtained, and data taken in the conventional mode are shown. Sample centering was checked three ways: (1) by inspection of the LEED array when examining single crystals, (2) by measurement of the elastic peak width, and (3) by observation of background uniformity in the ffuores~ent-screen display as the retarding grids were dc ramped through the elastic peak. Approximately 1000 spectral scans were analysed in preparing this report. The Auger energies were measured with a calibrated digital voltmeter and checked periodically against the elastic peak position. Instrumental contact potentials were not measured, but as discussed in following sections they do affect the measured absolute Auger energies. Loss energies were measured as differences and thus are independent of instrumental/specimen work functions. Extreme care was taken to prevent surface contamination of the uranium metal samples. High-purity polycrystalline metal low in S and C, the two principal surface contaminants, was obtained from G.L. Powell, Oak Ridge Y-12. Samples ca. 2 X 10 X 10 mm were polished to a mirror finish, etched in 6N HNO,, and rinsed ultrasonically. They were then mounted onto an indirectly heated molybdenum holder and inserted into the vacuum .system. Before *taking data the system was first pumped 1 week without baking. It was then baked 1 week at pressures <10m7 Torr (lOus Pa), initially for short periods at low temperatures. The final bake at 220°C lasted 72 h. The sample was then Arf bombarded 2d, 500 eV and 40 PA, without heating followed by 3d of repeated 30 mm bombardment - 30 min anneals at 800°C. With this treatment the uranium surfaces could be cycled to the (Y--Ptransition [ 131 at 663°C and yet remain free of detectable C, 0, S, Cl, P, etc., and metallic impurities. Although this treatment was somewhat tedious, previous experience [3,

WY. Ellis / Augerfloss studies

of uranium

43

141 with U and Th had demonstrated that such handling with initially clean metal was essential. The UO,, U30s, and UF, samples were not given such extended treatment. Although the same sequence of steps was taken, only 1 week was spent. The UO, single crystals from BPNL have been described previously [ 151. The U,Oa samples were prepared by S. Stoddard at LASL, massive UF, was obtained from MC Tinkle at LASL, and thin UF, films of optical thickness were prepared on the UO, crystals by hydro~uorination.

3. Results and discussion 3.1, Uranium metal 3.1.1 Auger spectra In fig. 4 are retarding-grid Auger spectra for very clean ar- and P-phase polycrystalline ur~ium metal free of detectable impurities. The two traces are identical except for 7% greater peak heights, h of the #?-phase. These results demonstrate that

Q1

o0 *-

,“-

z_

.cLa_

k W-

s W_

z-no-

t

Fig. 4. Uranium Auger spectrum in the ol and p phases, E = 1000 eV, 6 V p-p modulation. Metal free of detectable surface impurities. Only the region 8 5-l 10 eV discussed in text. Inset: energies reported as Kc,, in text. Data taken consecutively on same spec.

44 Table 1 Major uranium VB Auger peaks

iMetal

---~ ~. ..__^._ ^. ._ .._.. Econ a Designation b, E,,I~ -I__-_._--_-..--. _.._-_-_- ._.-.. 103.8 94.6 86 70

See text 04VV, 95.6 0s vv, 87

.” . . Oxide

_

.

~ -.-. - ..___~~.__________.

EC,,, a

Designation b, ~~~~~

101.5 90.5 81.7

See text OqVV, 90.4 0s W, 82.6

_^-

(68.8) c

Observed peak, E,,,, as per fig. 4, i 1 eV. Energies in eV. Based upon Fuggle et al. [ 21. Bonding bands used except for note d. Eqs. (1) and (2) used with r# from ref. [12] and 6 from ref. [2] as follows: metal, # = 3.2 and 6 = 2; oxide, @= 2.5 and 6 = 6. With non-bonding bands poor agreement results and chemical shifts are inaccurately estimated. Intrusion of slow inelastic tail distorts peak; see fig. 7. Non~onding bands used as follows: metal, @= 3.2 and 6 = 0.5; oxide, # = 2.5 and 5 = 2. Much weaker peaks and shoulders at Emin: 186,61,53?, 44,34,and 17 eV. Not discussed in text.

there are no major differences in the VB electronic structure between the a- and P-phases that affect the VB Auger excitation processes. Except for 20-30% increased intensity the same spectrum was observed for y-phase uranium. The y-phase was not examined as thoroughly, however, because bulk interstitial impurities returned to the surface in a few minutes at 800°C. The increased intensity in going from (Yto fl to y undoubtedly results from changes in crystal structure, in particular at the surface. But after having observed complicated c~stallographic effects with other materials [16] and with no knowledge whatever of the surface crystallography of these metallic U specimens, we are reluctant to propose a model for the emission differences. Table 1 lists the low-energy Auger transitions and their possibfe, revised designations based upon Fuggle’s latest tabulation [2]. The use of excitation thresholds [3] and chemical shifts has drastically reduced the number of conceivable YVV and YXV transitions. Excitation of deeper levels with higher beam energies was considered in ref. [6] but is not included here. Also listed in table 1 are energies estimated from the expression

where EAug(YVV) is the energy of the WV Auger peak,E.&Y) is the tabulated XPS Y level, [EXps(Y) + (61 is the energy stored in the XPS Y-core level vacancy, @ is the photoelectric work function, and 2(# + 6) is the experiment~ly derived

W.P. Ellis / Augerfloss studies of uranium

45

energy required to remove two electrons from the center of the VB of fig. 3A to the vacuum zero level. Inclusion of these factors, bonding bands, and use of thresholds result in tables that differ from refs. [3] and [6]. As discussed in the introduction, for transitions of the type YXV an uncertainty exists because of the vacancy in VB *. Measured XPS binding energies of levels X and Y yield an approximation to EAug( YXV), but because 6 and @in this case refer to a relaxed VB* around a core hole, X*, an uncertainty is added. The energy of the YXV Auger transition is taken here as &@XV)

= -Q,,(WX)

= &S(Y)

- E,,,(X)

- (4 + s>* 3

(2)

where ($ t 6)” refers to VB* of core-vacancy, X*. As an approximation, (4~+ 6) for U was substituted for (4 t S)* in arriving at the values in table 1. One would get a better estimate with (# + 6) of element (2 t 1) impurity. In the case of uranium this approach isn’t much help since nothing of this nature is known for Np impurity in uranium, or for pure Np metal. About all that can be said is that by using (#J + 6) of U in table 1, a value estimated to be ca. 2 eV too high will be derived from eq. (2). An additional source of error in estimating what these Auger energies should be is the use of measured @from one source, VB spectra and thus 6 from another, and E,, levels from yet another. Unless all three sources agree on Ef a spread of several eV will result. The error is doubled in eq. (1) through the term 26, but if 6 and EXPs are derived from the same source the error goes as 6 since E,,, - 6 is fixed for a given measurement. The same is true for eq. (2). Thus the data of Fuggle et al. [2] were used in estimating Auger energies. Finally, of an instrumental nature, there are at least three readily identi~able sources of error. (1) Contact potential differences and calibrations in the LEED/ Auger unit can shift the absolute measured energies as measured by 1-2 eV (estimated). (2) The window of the retarding grids could shift the energies to a lower value, but this effect was checked in the higher resolution mode and estimated to less than 0.5 eV. (3) In our unit it makes a difference of ca. 0.5 eV whether we ramp up or down in retarding voltage regardless of scanning speed, time constant of the amplifier, calibration, etc. We convention~ly scan down in voltage, i.e., in fig. 4 from right to left, and thus read voltages lower by ca. 0.5 V than if we scanned up. Charging of the specimen is no problem with metals or semiconducting oxides but does have profound effects with insulating UF,, (later section). The above experimental errors all cancel in loss spectroscopy, and presumably in XPS as well, since differences in energies are reported. But in Auger analysis where absolute energies are measured, or postulated, the cumulative effect may be several eV. In table 1 agreement between estimated and observed energies is 1-2 eV, which is better than justifiable on the basis of possible errors. Very good agreement is obtained for the 86 eV peak with an 0,VV estimation based on bonding electrons, whereas the O,O,V calculation of ref. [3] is several eV off and did not yield the

46

W.P. Ellis / Augerfloss studies of uranium

observed chemical shift on oxidation. The 0,VV estimation is based on the relaxed 102.8 eV XPS level, (next section), and the 103.8 eV Auger peak is an anomaly also to be discussed in the next section on losses. For the intense 70 eV peak there are several possibilities based on Fuggle’s tabulation. Most certainly this peak is not the PIW as listed in ref. [3] since the revised compilation lists PI at 43.9 eV and there is no XPS level in the vicinity of 70 eV [2]. Our brief XPS measurements [lo] on UO, agree with Fuggle et al. and we have no reason to question the levels they reported for U metal. Disagreements do appear in direct comparisons of XPS levels with core-loss excitations. 3.1.2. Loss spectra Fig. 5 illustrates loss peaks observed from clean polycrystalline a-U, free of detectable impurities, at E, = 450 eV. An intense surface plasmon is observed at 13.9 eV, a weak uncertain shoulder at ca. 22 eV, peaks at 29 and 64 eV, but of special interest here are the two well defined peaks at 98.3 and 114.2 eV. It is toward an interpretation of these two peaks that the introductory comments on excited-state coupling were directed. If the final, excited states are the same, A(Y) = Exps(Y) + 4. As seen in table 2 for the 05 level, for example, there is no probiem: A(05) = 98.3 eV = E,,,(O,) +#

I

400

350

Energy,E,eV

Fig. 5. Uranium loss spectrum, Metal free of detectable surface

room temperature impurities.

LYphase, Ep = 450 eV, 3 V p-p modulation.

W.P.Ellis /Auger/loss Table 2 Uranium

studies of uranium

41

metal losses

Level a

Mann b

XPS a

xps+C$

Observed

03 01 05

5p312, 220.33 5d3”, 120.69 5d5’2, 109.32

194.8 * 0.4 102.8 + 0.4 94.2 ? 0.6

198 106 91.4

PI

6~“~

, 58.79

43.9 ? 0.8

47.1

P2

6~ 1’2,

38.07

26.8 * 0.5

30.0

p3

6~ 3’2,

27.05

16.8 + 0.5

20.0

199 114.2 98.3 (-65 48 37 29.0 (22.3 19.9 13.9

a Level designations and XPS energies from Fuggle Energies in eV. b Relativistic Hartree-Fock free-atom, ground-state Mann, Los Alamos Scientific Laboratory (1969). c See text. d Ep 2 800 eV. e E = 200 eV, see fig. 6. f ,e strong, wk = weak, sh = shoulder, v = very.

2 94.2

k 3,vwkshd + 1, wk c f 1, wk f 3?), v wk sh r2,vwke +_2,wke r 1, wk e f 1.5?) wk sh + 0.4 s c f 0.1 vs c

et al. [2]. @ = 3.2 eV from calculations.

loss, A f

Unpublished

Riviere work

[12].

by J.B.

t 3.2 = 97.4 eV. The difference of 0.9 eV is readily explained by the somewhat arbitrary choice of E,, within 1 eV or so, in the XPS measurements. For UO2 we chose [lo]. . . arbitrarily . . . an E, 1 eV higher than Fuggle et al. and 2.5 eV higher than Veal and Lam. The work function 4 = 3.2 from ref. [12] was the most reliable, but it is 0.5 eV lower than the handbook photoelectric value [17]. Also, uncertainties in XPS and loss measurements are ca. 0.5-I eV each. Thus, XPS and loss values for the 0, level, and most of the other levels as well in table 2, are in substantial agreement, from which it is concluded that coupling is to the same final states in the solid. Similar agreement is not obtained for the 0, level. With identical final states in both XPS and loss excitations, a peak at A” 102.8 + 3.2 = 106.0 eV would be observed (106.9 with the 0.9 eV correction from 05). A thorough search failed to yield even so much as a suggestion of a loss at A - 106 eV. Instead, a broad peak at A = 114.2 eV is observed at an energy 114.2 - 106.9 = 7.3 eV higher than expected. The disagreement of 7.3 eV is completely outside the range of experimental error. The 102.8 eV XPS level is not out of line with the other levels in a comparison with Mann’s calculations in table 2. Thus, the 114.2 eV loss peak appears to be the anomaly. One possible explanation of this 0, discrepancy is that excitation is to different final states, and since A is -7.3 eV higher than predicted by XPS, that the excited loss state during the lifetime of the transition is to a more energetic transient configuration such as fig. 2B, whereas in XPS the excited state is less energetic, e.g., as

48

W.P. Ellis /Auger/loss

studies of uranium

in fig. 1E = fig. 2C. Support for this possibility comes from three sources. (1) The 0, loss band is much broader than the 0, with part of this breadth presumably resulting from a short lifetime in the transient state and part from the densities of empty states. (2) Although the proposed promotion of VB electrons in arsenic to a level 9 eV above Ef has been questioned [ 181, at least for what it is worth one internally consistent interpretation of electron spectra from As(0001) involved related excitation to higher, unrelaxed conduction bands [ 191. And, (3) the U Auger spectrum appears to involve a more energetic excited state. From eq. (1) there simply is not enough energy in the “normal” 102.8 XPS level to excite the 103.8 eV Auger transition. The fact that this Auger peak is observed with EP = 140 eV [3] excludes levels deeper than 04. Eq. (1) can be rewritten in terms of loss energies E,,,(YVV)

= A(Y) - 2($ + 6).

(3)

This expression applies only to the relaxed final state of fig. 3c in which the promoted electron is delocalized and is not involved in the Auger process thus leaving two vacancies in VB. If the electron above E, remains localized and is involved in the Auger process, e.g. by falling back into level Y, then only one VB vancancy will be created and E,,,(YVV)

= A(Y) - (@ + 6).

(4)

The most energetic Auger electron from the “normal” 0, XPS level involves non= 102.8 - 3.2 - 2 X 0.5 = 98.6 bonding electrons, and from eq. (1) is E &O,VV) eV. From eq. (4) for the 114.2 eV loss, EAug(04VV) = 114.2 - 3.2 - 0.5 = 110.5 eV. The observed value of 103.8 eV, which is between these two extremes, indicates that combination band and/or relaxation processes we can only guess about are operating. Clearly there is a limit to how far this argument should be carried. But our earlier point that the 102.8 eV XPS level is not energetic enough to account for a 103.8 eV Auger transition, and that the more energetic 114.2 eV loss is somehow involved, appears to be supported. The energy requirement of the 103.8 eV Auger peak also tends to discount a double-loss interpretation of the 114.2 eV peak. If there is any validity to this proposed excitation, then it appears that the 114.2 eV 0, loss excited state can also relax the 7.3 eV to yield an intermediate, “normal” state which then decays with the emission of a “normal” 0,VV 94.6 eV Auger electron, and possibly an 04P2V peak, as listed in table 1. Agreement between XPS and loss values in table 2 is good except for 0,. Several weak peaks are seen in the loss spectrum which are not listed by Fuggle et al. These peaks may represent multiple losses or additional examples of relaxation variants. But since they were not studied extensively and we have no background literature for comparison, they are simply noted but not discussed here. Of additional interest in the near-elastic loss spectrum of U are two sharp peaks at A = 19.9 + 0.4 eV and 13.9 + 0.1 eV. Perhaps it is fortuitous, but these two

W.P. Ellis /Auger/loss ED

=60 eV

200

studies

110

160

250

450

BOO

I

’

contominatod

I

‘iLIi!L II

N(E)

49

100

II I X

of uranium

x’

XI0

1

XI

x IO

Energy.E.eV

Fig. 6. Uranium metal loss spectra, 60 < E,, < 800 eV, 3 V p-p modulation. Except for dashed curve E = 800 eV, all data are for clean metal free of detectable surface impurities. Solid vertical line P 3.9 eV below elastic peak, dashed vertical line 19.9 eV below elastic peak.

losses are almost exactly what one calculates for bulk and surface plasmon energies: Aw,, = h(4rrne2/m)’ = 19.94 eV and ho, ho&/2 = 14.10 eV for room temperature (Y-U. These calculations are based upon 6 e-/U atom, an electron mass, m equal to its free electron value, and a density of 19.06 g/cm3 [13]. Fig. 6 illustrates these two peaks at different primary beam energies between 60 and 800 eV. The 13.9 eV loss is excited at all energies in this range and is very sensitive to surface contamination, i.e., C and 0. The strong indication then is that this peak is a surface plasmon involving all six 7s, 6d, 5f electrons oscillating collectively rather than in individual band modes. In contrast, the 19.9 eV loss is at maximum intensity in the range 110
50

W.P. Ellis /Auger/loss

studies oj’uranium

U by changing phases. The density of /3-U is 18.11 g/cm3 [13] for which hw, = 19.4 eV and Aw, = 13.7 eV. These differences of a fraction of an eV with a-U are at the edge of our limit of precision. But, for what it is worth, the P-U loss energy for the 13.9 peak was consistently 0.2-0.5 eV lower than in the a-phase. Unfortunately we did not take data for a-phase U in the range I?‘:‘,- 110 eV and thus have no indication for the 19.9 eV peak. The other loss peaks with E, = 450 eV did not shift detectably. Although the experimental scope of this article does not include Pu, it may be worth a few words here to point out that Pu with large density changes among its many phases should prove to be fascinating in its near-elastic plasmon loss structure. 3.2. Uranium plus oxygen 3.2. I. Auger spectra The Auger study of U t 0, has been reported by Allen and Wild [6]. Since our observations are in broad agreement with theirs the subject does not need a thorough

n

Uranium

Oxygen

U,o-phase

~~~~~111111111111111, I”““““’

0

50

100

150

450

500

550

Energy,E,eV Fig. 7. Auger spectral changes upon oxidation of uranium metal, Ep, 7 1000 eV, 6 V p-p modulation. With the near-surface bulk saturated with oxygen, heating stablhzes the UO phase.

W.P. Ellis /Auger/loss

studies of uranium

51

exposition here. Our starting surfaces were very clean however with no detectable impurities, we followed the Auger spectrum as a function of 0, exposure and sample treatment, and there are Auger/loss transitions that relate to discussions in section 3.1. These topics are summarized below. Fig. 7 shows spectral changes in U with 0, exposure. The bottom curve for thoroughly cleaned (Y-Uis the same as fig. 4 with the solid arrows pointing up to the four major U VB Auger transitions. With the vacuum system on the mid-lo-l 1 Torr range (lo-* Pa) and CO as the major residual, the spectrum would remain unchanged for several days. Starting with a freshly prepared, well annealed room temperature specimen, 0, was admitted to the system in controlled doses. After an initial exposure of 10 L 0, the U Auger spectrum was unaltered, the oxygen Auger signal was not detected, but the near-elastic loss spectrum was altered. Further exposure to 50 L 0, at room temperature produced only minor changes in the U spectrum, less than l/4 monolayer of oxygen, but again there were further changes in the loss spectrum. All traces of oxygen effects in the Auger/loss spectra disappeared from this sample in a few minutes at 700°C. These results suggest that the sticking coefficient of 0, on a clean uranium surface at RT may be lower than unity, and that the 0, which does stick diffuses rapidly into the near-surface bulk, in agreement with Riviere’s work-function observations [12]. The sample was then heated to 400°C and exposed to 60 L O,, cooled to room temperature in 30 L 0, and examined. There were significant changes in the loss structure and the U VB spectrum, and a symmetrical oxygen peak developed. Furthermore, subsequent heating in the P-phase did not deplete surface oxygen to zero but the intensity continued to diminish with time, i.e., a factor of 3 in 1 h. At this stage the near-surface region was nearing saturation with oxygen, and further exposure to 150 L 0, saturated the surface in the UO, phase (top curve fig. 7). The sample was then cycled to 7OO’C upon which the oxygen Auger peak diminished to l/2 its value of the top UO, curve of fig. 7, and the U VB Auger spectrum became intermediate between U and UO, (center dashed curves, fig. 7). This phase, because of its stability and Auger spectra, is identified as [JO. It could be cycled into the bulk U P-phase region at 700°C several times for short periods, i.e., 5 min each, without diminution of the oxygen signal. Prolonged heating at 700°C for l/2 h periods as expected did decrease the oxygen signal in the P-phase a few percent but, surprisingly, upon cooling to ambient temperature the oxygen signal reproducibly returned to its previous UO value. Heating at 800°C caused an irreversible reduction in oxygen presumably as it diffused farther into the bulk. These results are significant in that they demonstrate (1) that UO is a stable surface species, (2) that carbon is not necessary for stability, (3) near-surface saturation of the bulk with dissolved oxygen leads to enhanced stability, and (4) without near-surface saturation and stabilization with heat, UO exists only as a transient species. We also observed that exposure of the stabilized UO to 3 L 0, caused a 60% increase in the oxygen Auger signal, i.e., from 3.3 to 5.3 units. Thus oxygen sticks and remains at the UO surface more than with clean U, as is also the case with residual CO. We also

exposed clean U to initially pure H2, and observed only oxidized U with a strong oxygen peak. Apparently, in a mechanism described by Beavis [20], Hz reacted with surface oxides on the inner walls of the all-metal vacuum chamber and produced small but detectable amounts of H,O, which then reacted with U to form the oxide. Changes in Auger spectra of U upon oxidation agree generally with those reported by Allen and Wild [6], but we do see some differences and have interpreted the results according to the general outline in the Introduction. Table I lists the observed Auger energiesE, (fig. 4), and includes the shift between the metal and its oxide. A chemical shift of ca. 4 eV is observed for the 90.5 and 8 1.7 eV peaks from which with eq. (1) it is concluded that these two peaks involve bonding electrons, the 68.8 eV peak may or may not, and the 101.5 eV peak is uncertain. As with the metal, from eq. (1) even using non-bonding electrons there is not enough energy in the “normal” 0, XPS level to produce the 101.5 eV Auger electron although the case is not as clearcut: 104.9 - 2.5 -2 X 2 = 98.4 eV is the most energy available. 3.2.2 Lctss spectra Loss spectra from single crystal UO, are shown in figs. 8 and 9. A thorough discussion here is not warranted but four features are worth noting. (1) First, the nearelastic structure differs from the case of o-U. Compare fig. 8 with fig. 5. (2) The 04 and 0, levels are not shifted drasticaliy, and the possibility remains of an 0, loss excitation to a higher, unrelaxed band as postulated for the metal. Also, the 0, peak width is comparable to the metal, but the 0, width at half-maximum has been

300

350

400

450

Energy,E ,eV Fig. 8. UO2(111)

loss spectrum,

Ep = 450 eV, 3 V p-p modulation.

Energy,

E,eV

Fig. 9. UOz(l11) loss spectrum, Ep = 110, 200, and 800 eV, 3 V p-p modulation.

reduced from 13 eV for the metal to 9 eV for U02. (3) Intensity changes with oxygen exposure were not anticipated. Although the curves in figs. 8 and 5 as drawn at different gains suggest the opposite, measurements of the ratio, R, of 0, loss to elastic peak heights shows that Roxide = 3.2 X 10e3 andR,,,,ti = 4.1 X 10-3. On a simplistic basis with valence-band depletion upon oxidation one might expect the opposite as was the case with thorium [IO] where the intensity ratio increased 70100%. But this effect only illustrates the difference in chemistry between U and Th. Thorium, which has no 5 f electrons, might be expected to behave in a simplistic manner since all of its 7s26d2 valence electrons are consumed in forming ThO,, whereas U with two remaining unbonded 5f electrons (assumed), near E, in UO, would require promotion of core electrons at least above these remaining 5f states. Scattering by oxygen, emission effects, and structure could account for the observed decrease of R. Thus, while not anticipated, the comparison with Th seems quite reasonable. And (4), as illustrated by the short vertical dashed line in fig. 9, a loss at 20.5 + 0.7 eV is resolved in U02 at 110
54 Table 3 Uranium oxide losses - ----_---. Level a

XPS a

xPs+c+eb

03

198.4 +_0.4

200.9

04

OS

104.9 +_0.4 97.1 + 0.6

107.4 99.6

pi

44.2 f 0.8

48.7

PZ

29.0 f 0.5

31.5

23.2 F 0.5 18.4 f 0.5

25.7 20.9

Ll

(oxy)

p3 --“.

.-_ _.....

.

Observed loss,

A f

199 r 3,vwkc 145 r4,vwkcte 113.2* 1,wke 99.2 f 1, wk (-73 * 4?), v wk sh e (-56 * 3), wk sh d,e 36 i-1,sc _ 28 i 1,se _ 20.5 f 0.7, wk e 16.0 t 0.5, VS e

.---.

Level designations and XPS energies from Fuggle et al. [ 21. Energies in eV. b Work function @= 2.5 eV from Riviere [ 121. ’ u308, fig. 11. d Ave. figs. 9 and 11, u308. e See text. f s = strong, wk = weak, v = very, sh = shoulder. a

three peaks occur at the estimated energies: OS(U), O,(U), and perhaps P$.U). The P2(U) and PI(U) loss peaks were not detected at their anticipated energies, and the 04(U) loss was shifted 5.8 eV. Other peaks at 28,36,56 and 73 eV do not correlate directly with XPS. It appears that final-state couplings are quite different between XPS and loss processes in the semiconducting oxide. In the loss process core electrons conceivably are promoted above the band gap and, except for 03, 0,) and P3, apparently are frozen there during the excitation event, i.e., part of the narrowing of the 0, level in UO, could result from increased lifetime. Perhaps the full picture will not emerge until continuing band studies [21] are complete and low-temperature, electron-stimulated luminescence studies are performed. Further discussion here is not warranted, but it is instructive to compare these results with those from u30g and UF,

The semiconducting properties of UO, with its partially covalent bonds and unbonded 5f electrons indicated that we look at higher oxides and more ionic compounds. For U+& the Auger spectrum of fresh material, untreated in vacuum, was very similar to UO2 but less intense. Upon heating 1% h at 500°C in vacuum, the spectrum became more intense but remained similar to that of UO, with the same peaks at the same energies within 0.5-l eV for the four U VB peaks. Not much

W.P. Ellis /Auger/loss

Fig. 10. Fresh U3Oa. untreated

in vacuum.

studies of uranium

Loss spectrum

at Ep = 450 eV, 3 V p-p modulation.

36

300

350

400

450

Enerav.E.eV Fig. 11. UsOa loss spectrum, spec. heated in vacuum at 500°C, 1% h, Ep = 450 and 470 eV, 3 V p-p modulation. Varying Ep identifies the 145 and 199 eV peaks as losses.

W.P. Ellis /Auger/loss studies of uranium

56

new was gained here, but the loss spectrum was interesting. Fig. 10 shows the loss spectrum for fresh U,O,. Absent is the 16 eV peak, but sharper than their equivalents with UOZ are two peaks at 28.7 and 35.4 eV, and a clearly resolved peak at 52 eV. Heating at 500°C in vacuum, resulted in the spectrum of fig. 11: the 16 eV peak became very intense (surface plasmon indicated), the two peaks at 29 and 36 eV became more similar to UO, of fig. 8, and a peak at 145 eV previously not detected with UO,(l 11) at this 1”;‘,under these conditions was developed. Of the nine loss peaks shown in fig. 11) only three correlated with XPS levels: the 0, at 199, 0, at 113.5 and 0, at 99.0 eV. And, as indicated earlier, the 0, peak presumably is ca. 6 eV higher than would be estimated from XPS tables. Thus IJ,O, in its Auger/loss spectrum is not radically different from UO,. As expected, however, R = 4.6 X 1O-3 which is greater than for UO, or the metal.

In contrast

with semiconducting

UO, which has partially covalent bonds, UF, is

an insulator with ionic bonds, and we expected the Auger/loss spectra to reflect this difference in chemistry. UF, is the “green salt” resulting from the 4HF + UO, -+ UF, + 2H201‘ reaction, and in production applications is reduced by Ca to yield the metal. Thus UF, is readily available and for this study was examined in two forms: (1) as a massive slug of fused polycrystalline UF, and (2) as thin optical films on UO,. In both materials there was extensive damage by the electron beam, and data for UF, could be taken only from the massive slug. To minimize damage the beam was reduced in intensity, defocussed, and swept across the surface. Damage by the electron beam is illustrated in the Auger spectra of fig. 12. In curve a, a weak beam was defocussed and swept across the UF, surface. The secondary-electron emission ratio, SEER, is less than 1 for UF, and, with an insulator, charges the surface negatively which in turn pushes the Auger peaks up in energy. The fascinating observation is that only two U VB Auger peaks shifted 16 eV(?) are detected. If the beam is sharply focussed but the specimen translated as in curve b, three U peaks are detected. These tr~sitions are the same as in the oxide except that the 81.7 eV peak is missing. The fluorine Auger peak was very weak. Another fascination in curve b is that the peaks occur ca. 3 eV lower in energy than for curves c or d, presumably a chemical shift complicated by charging. In curve c the sharply focussed beam was held stationary for 1 min, and 30 min for curved. Curves c and d are similar to UO, fig. 7, which suggests a lower fluoride or mixed U and UF,. With a 2-h exposure the spectrum was identical to the one from metallic U, fig. 4. With thin optical films of UF, on UO,, the damage was instantaneous. Except for dispersion effects, i.e. changes with incident energy, the loss spectrum is not affected by charging, as shown in fig. 13. The same major peaks are observed from UFq as from the oxide with only small differences in energy and intensity. For UF,, R z 4.5 X10w3 which reflects the increased ionicity relative

I1

1

’

1

‘1

”

1”

100

50

1 150

Energy,E,eV Fig. 12. UF4 uranium VB Auger spectra, massive fused polycrystalline sample. Ep = 1000 eV, 6 V p-p modulation. Sequence shows charging and decomposition of compound with incident electron beam. (a) beam intensity reduce, defocussed, and swept across surface. Sample charges negatively and Auger peaks pushed to higher E. Note only 2 peaks seen. (b) beam intensity reduced, sharp focus, beam swept across sample. Note only 3 peaks seen. (c) Sharply focussed stationary beam exposed I min, and 30 min for (d).

0, E : 2 ,D .g ?I @ z

300

350

400

450

Eneray,E,eV Fig. 13. UF4 loss spectrum, Ep = 450 eV, 3 V p-p modulation. surface.

Defocussed beam swept across

W.P. Ellis /Auger/loss

58

studies

of uranium

to UOZ. But the difficulty of loss excitations and coupling with final states apparently exist for UF, the same as with the oxide. Further discussion here is not appropriate.

4. Concluding

remarks

A comparison of core-loss excitations with their corresponding XPS values has indicated for U metal that final-state coupling in the solid for the two processes is the same except for the 0, loss. The 7.3 eV excess energy of the 04 loss, its anomalously large half-width, and apparent necessity in accounting for the 103.8 eV Auger peak all point toward excitation to an energetic, transient state. Estimated Auger transitions are in very good agreement with observation if bonding electrons from experimentally derived XPS valence bands are used. Loss excitations for uranium compounds however are not in general agreement with XPS. The divergent results indicate a need of further experimental valenceband studies, of UO, in particular. High-resolution UV photoemission data are needed as well as low-temperature electron-stimulated luminescence measurements. But especially needed are first-principles, theoretical estimates on excitation configurations and relaxation energies both in the metal and in the se~niconducting oxide.

References [ 1) T.N. Taylor, C.A. Colmenares, R.L. Smith and G.A. Somorjai, Surface Sci. 45 (1976) 317. 121 J.C. Fuggle, A.F. Burr, L.M. Watson, D.J. Fabian and W. Lang, J. Phys. F (Metal Phys.) 4 (1974) 335. [3] W.P. Ellis and B.D. Campbell, J. Appl. Phys. 41 (1970) 1859. [4] B.W. Veal and D.J. Lam, Phys. Rev. BlO (1974) 4902; also Phys. Letters 49A (1974) 466. [5] G.C. Allen and P.M. Tucker, J. Chem. Sot. Dalton (1973) 470; also G.C. Alien, J.A. Crofts, M.T. Curtis, P.M. Tucker, D.Chadwick and P.J. Hampson, J. Chem. Sot. Dalton (1974) 1296. [6J G.C. Allen and R.K. Wild, J. Chem. Sot. Dalton (1974) 493; also Chem. Phys. Letters 15 (1972) 279. [7J L. Ley, F.R. McFeely, S.P. Kowalczyk, J.C. Jenkin and D.A. Shirley, Phys. Rev. Bll (197.5) 600. [S] R. Gamer, Electron Spectroscopy of Chemisorption on Metals, in: Advances in Chemical Physics: Aspects of the Study of Surfaces, Vol. 27, Eds. I. Prigogine and S.A. Rice (Wiley, 1974) p. 211. See also PC. Kemeny and N. J. Shevchik, Solid State Commun. 17 (1975) 25.5. [9] J.A. Connor, L.M.R. Derrick, I.H. Hillier, M.F. Guest and D.R. Lloyd, Mol. Phys. 31 (1976) 23. [ 101 W.P. Ellis, in: The Actinides: Electronic Structure and Related Properties, Eds. A.J. Freeman and J.B. Darby, Jr. (Academic Press, 1974) Vol. 2, ch. 8, p. 345. [ 11 f W.A. Coghtan and R.E. Clausing, Surface Sci. 33 (1972) 411.

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studies of uranium

59

[ 121 J.C. Riviere, The Surface Potential of Oxygen on Uranium, U.K. Atomic Energy Authority, AERE, Harwell, U.K., Report No. AERE-R4631 (1964). [ 131 A.N. Holden, Physical Metallurgy of Uranium (Addison-Wesley, Reading, Mass., 1958) p. 36. [14] W.P. Ellis, J. Vacuum Sci. Technol. 9 (1972) 1027. [15] W.P. Ellis, Surface Sci. 45 (1974) 569. [16] T.W. Rusch and W.P. Ellis, Appl. Phys. Letters 26 (1975) 44. [17] American Institute of Physics Handbook (McGraw-Hill, New York, 1963) p. 9-151. [18] M.K. Bahl and R.L. Watson, Surface Sci. 54 (1976) 540. [19] W.P. Ellis, Surface Sci. 41 (1974) 125. [20] L.C. Beavis, J. Vacuum Sci. Technol. 10 (1973) 286. [21] J. Naegele and L. Manes, European Institute for Transuranium Elements, Karlsruhe, Germany; Presentation at 5th Intern. Conf. on Pu and Other Actinides, Baden-Baden, 1975.