Study of adsorbate interactions by electron spectroscopies

Physlca 127B (1984) 240--245 North-Holland, Amsterdam

STUDY OF ADSORBATE ~ I ' E R A C I ~ O N S BY E I . ~ C q ~ O N SPECTRO,SCO]?'EF~ Eberhard U M B . A C H Fal~ulffJfrite Physik E 20, TecttnisdJe Univerxffiit MiJr~chen, D-8046 Garcfiing, Fc:d..Rcp. Germany

The investigationof adsnrbate interactions has lately attracted considerable interesl. It is shown by a few examples how electron spcctroscopiescan contribute to the solution oI some topical problems, Foe b~stanco,the quantitative utilization of XPS makes il possible,to study a~ extremely slow dissociation reaction in situ. The combined u~c of various spectroscopies gives insight into the reEati'~elystrong electronic changes of adsorbed molecules izlduced by coadsorbed alkali promoters, Low ~empcraturc {30 K) surfit¢csare ideally stlitcd to ~tudy we~k interactions between physisorbed, weakly and ~tront~ly ehemisorbed eo'adsorbates.For irtos! of such studies ~t is very important or useful to understand th,,"physicsof. the electron emission processes such as the screening mechatllsm and the nature of 1~atcllltes.

1. Introduction For two decades the main object of basic research in surface science has been the development of various experimental methods and the investigation of 'sinlple' model systems. Even now the understanding of the mechanisms of probing 'techniques al~d of their influence on the interpretation of experimental results require de.. vo~ed work in order to utilise the obtainable information completely and correctly. Such q u e s tions are usually very inviting because they are connected with interesting physics. O n the other hand, most surface problems, in particular in applied research, are much more complicated than simple model systems and thus d e m a n d well-understood standard teclltniques. W e show in the following by a few recent examples how electron speelroseopies which are generally accepted as some of the major sources of surface informatkm can contribute to the solution of s o m e w h a t more complicated questions. These examples clearly demonstrate that a detailed u n derstanding of the mechanisms of electron emission is absolutely necessary for a correct evaluation and interpretation of the results. Until some few years back most surface studies were confined to well-defined and very simple adsorbate sy,~tems, i.e, they concentrated on the investigation of the geometric and]or elcclronic properties of single adsorbate species on smoolh crystal ,~;urfaces [1], Only a few

studies attempted to analyse surface reactions and the interaction between different adsorbates in s o m e detail; the interpretation of such experimental results often needed some i9tuition. This situation has changed partly because suitable surface techniques have been developed and now appear to be reasonably well understood, and partly because m a n y chemical and physica! surface problems urgently call for answer ~: which should now be possible based on our knowledge of simple systems. From ~he large number of i n t e r e s t i n g - a n d still relatively s i m p l e - cases of interacting adsorhates, three examples have been chosen: the dissociation reaction of C O on W ( l l 0 ) (section 2), the interaction of molecular C O with coadsorbed potassium on a Ru(001) surface (section 3), and the inters;orion of weakly chemisorbed N 2 with physisorbed N z and noble gases or with strongly ehemisorbed C O or oxygen on a N i ( l l l ) surface (section 4). In addition, the influence of the screening mechanism in photoemission is briefly discussed (section 5),

2. Disso¢~ation reaction o[ CO on W(t!O) Thermal desc~,rption spectroscopy (TDS) is probably the h,est technique for quantitative studies, e.g. for ~he a,.curate determination of the kinetic parameters of desorption. Its major disadvantage is, however, that it is confined to the

U378-4363/84/$03,00 ~ Elsevier Science Publishers B,V. (North-Holland Physics Publishing Division)

241

E. Umbach I Study of adsorbate interactions

det~etioa o~ desorbing particles, and that thus the details of reactions on the surface usually rerm~m invisible, For ir,stanoz, if the reaction involves m o r e than one step, or if the rate constunt of desorption of the reaction product~ i~ a b o u t equal or smaller tiaan that of the reaction itself (e.g. for dissociation) T D S must be replaced b~ a proper ia-s!itu technique. Unfortunately all in-situ mC'rthOds suffer from severe drawbacks which are the major reason for the lack of information on s=lzfface reactions. A m o n g such mothods X-ray photoemi~sion (XPS) appears to be the most suitable in-situ technique for m a n y applications I2] because it is non-. destructive, and the ioniI~ation cross section of z c~t-e level is independenll of the chemical state of the at¢,m under consid=cration. Therefore, the XPS signal is directly proportional to the n u m b e r of' atoms in a particular State if the intensities of all possible final stazes of the photoionization process are s u m m e d up, and if angular and shadowing effects are taken into account [2]. Provided that the XPS peaks of reactants and products are separable (z~BE~=I eV) the reaction can be followed stepwise, and reaction parameters can be ex~.raeted. This is shown in fig. 1 for the example C O on W ( l l 0 ) . One can clearly see that heating, of a saturated layer of molecuZarly adsorbed C O to 3£10 K causes some desorption which can be derived from the decrease of the O l s peak at 531.7eV. Upon ~urther heating desorpfion is accompanied by dissociation which !s indicated by the fl~cr,ease of a second peak at 530.3 eW. The latter r~mains constant between 400 arltd 900 K and then disappears between 900 a:ad t209 K. The assignment of ~he two peaks to a molecular and a dissociated CO-state, respectively, ~:ould unambi~;uously be performed by the combined use of XPS, XPSsatel.5tes, UPS and X,~ray induced AES [2]. From XPS results ilk e tJaose of fig. f it could be derived that the maj0rity of the C O mo ecules of a saturated C O laye: dl~sorbs between 250 and 400 K in two steps while the remaining 37% simultaneously dissociate. At low coverages (below 1/3 of saturation) dissociation occurs at much lower temperatures, and no desorption can be observed below 900 K. Thiis indicates that at

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Fig. l. L,fft: Typical O Is-photoemi.~sion spectra of CO/W(11.0) taken after stcpwlse heating tts the temperatures indicated, in the figure. Mixed peaks are ;'.ecomposed into their two compone~ts, molecular V-CO and dissociated f~CO. Right: Arrhenius plots of the rezetlon constants /%1,, derived from step dir~soeiationmeasurer~ents for a low cover. a~e layer. For detaits see ref. 2. higher coverages di~ociation is hindered by site blocking of the molecular species. XPS results like those of fig. 1 can be further evaluated. If the XPS spectra were taken after stepwise heating o r after isothermal heating (i.e. iheating to the same temperature for different time intervals) ratte constants can be derived from the decrease of the molecular ('V-CO) or the increase of the dissociated (/3-CO) C O state. This is also shown in fig. I (rlght-hand side) for the conversion of a dilute layer (only dissocia, tion) upon stepwise heating. From these ~;ad similar plots, which were independently obtaint~d by isothermal experiments, average rate parameters could be deduced: While the activation energy of dissociation (21 kJ/mol) appears t o be reasonable, the (first order) preexponentiaI turned out to be extremely sinai/ (1.0Zs-~) in agreement with other experimental evidence [2], i.e. it is about ten orders of magnitude smalh:r than expected from tra~si..ion state theory. %~aus it is probably the smalle,;t preexponential ever found for a surface reaction, Although some plausibility arguments could t,e given for the occurrence of a very unlikely transition state in the dissociation reaction [2] a detailed explanation is still missing at the present. In conclusion,

242

E. Umbach / Study of ad~orbate interactioas

we note that this example clearly demonstrates the potential of XPS for quantitative in-situ studies of surface reactions. ~30

e,

3. Coadsorptiou of C O and K o n Rtt(001)

Recently, several investigations have been concerned with the interaction between alkali atoms and simple molecules eoadsorbed on transition metal surfaces. One reaso~ for this remarkable iaterest is that alkali atoms o~ten act as promoters of catalytic reactions such as in the Fischer-Tropsch synthesis, Of course, in such cases se,veral questions arise concerning the influence o~ the alkalis on the reaction mechanism and on the reaction parameters. In parCeular, one may ask for the details of the interaction of alkali atoms and coadsorbed molecules on a microscopic scale, and then one can even hope to learn more about surface bonding in general. Most basic studies are confined to singlecrystal surfaces, such as the following example, C O + K/R,~f(XH ), which has been investigated in some detail utilizing several different surl'aee techniques [3]: TDS measurements show that coadsorbed CO and K both desorb at a higher temperature ( - 6 7 0 K) than each single adsorbate (CO/Ru(001): 40()~530K, K/Ru(O01.): ~ 3 7 0 K ) , indicating a higher chemical binding energy of the coadsorbed species by strong attractive interaction. This interaction also leads to long range ordering, which can be observed as coverage dependent overstructnres 6y LEED (e.g. (3 x 3)). In addition, work Iunction changes, photoeraission and HRIEELS results clearly show that the interaction with coadsorbed K drastically changes the electronic structure of CO]Ru(O(tl t. Some of the photoemission results are dis~ played in fig. 2 as illustration. In the lower left corner O i s spectra are plotted representing a saturated C O layer on a clean and on a Kpredosed surlace, respectively. The given coverages refer lo the number of substrate surface atoms and are derived from XPS and TDS data. tn |he upper left corner K 2p spectra are shown for different K co're,ages" ,m a clean surface and

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Fig. 2. Coadsorplion of C O + K/Ru(001), Left: XPS-~peclr;~ of CO {O Is, bottom,t and K (K 2p, top} for the tend,ilium given in the figure. Right; Binding ea~ergy shifts of the K 2p levels as a function of CO coverage (top) and as a functioll of K coverage without CO (middle) as well zs UPS spectra ~'Hc Ii f r o m CO on a clean ~nd on u K-ptedose.d Ru(00l ) surf'Jee. respectively (bouom). Binding em~rgies ,are referred to Ihe Fermi level.

for a ( - ~ x x/3)K + CO coadsorption layer. It can be observed (see fig. 2, middle right) that the K 2p doublet shifts to lower binding energies with increasing K coverage~ and that the formatior~ of K multilayers is indicated by tile appearance of additional peaks at higher BE. The BE decrease for the growing first layer can be easily understood as an initial state effecTI. The adsorbed alkati atoms donate charge into the Ru valence band which leads to an enhancemm~t of eleetrou density near the surface. This density increases with alkali coverage causing an i~crease of electronic repulsion and thus a reduction of the cllarge donation from the alk',di atoms at higher coverages. This leads to a decrease ot' the K 2p binding energy in agreeme,~t with the ex0erimental results (fig, 2). The O ls BE shift of

24-3

E, Ombach I Stud). ~sf a,#~,.rbate interactions

coadsorbed C O has apparent;~ I, the same origin, in that the enhanced substratc~ DOS gives rise to enhanced ~-bonding betwee~ ithe metal and the CO-2~'," orbital causing an O ~ls BE decrease, a strengthening of the chemic'41 blond, and a reduction ot! the ( 2 - 0 stretch :!requency by about 400crn -t. Until recently the reason why coadsorbed CO also cav.ses a redudtion of the K 2p binding energy (see fig. 2, rig~l-hand side, top) was unclear since the charge accepting C O molecv:le should reduce the electronic repulsion. However, the reason can be derived from the recent reinterpretation of CO bonding by Bagus et al. [41!. These authors show that the main contribution to the C O - M e bored is provided by the 7r-back-bonding rather ~han the 5o'-donation which was initially conside%d '1:o be the major contributor. The 5~r overlap witl!, the metal actually ca:~tses a strong ¢r-repulsion which polarises substrate D O S ,way from the CO and leads to a further reduction of the alkali deflation. This effect is apparently the reason foi: the decrease of the K 2p binding energy upon CIO coadsorption, This explanation is further corroborated by the UPS results which are shown in the lower right corner of fig. 2. The curve frc,m ithe coadsorbate layer clearly shows drastic pe~tk ~:hifts, and there could be even a splitting of the :asually overlapping 5¢r- and l~r-derived peak i~ito two separate peaks. Highty resolved Auger spectra [5] can be ~,tilized to assign the peak at 8 eV to a predominantly 1 ~--derived level. The lcl,eak at 6.3 eV, or at least a part of it. could ~:heln~represent the 50- lewd which is shifted to Io,~vef BE because of the en~az~ced o--repulsion cau.,ted by the K charge donation. This new interpretation [4], on en¢ hand, enables us to understand all experimentlal ohservations, and, on the ether hand, is itself convincingly co]roborated by the pre,seat ~esults. Furthermore, these results sugges( that tlhe coadsorbed CO molecule is rehybfidized !because of the :~trong M e - C O - w - b o n d i n g cor,.~pared to CO on the clean Ru surface. Note tha'.: tl~i~s,rehybridizadon does not lead to a tiltin[~ elI ',Ihe molecule witlh respect to the surface normal~ as can unambiguously be derived from angte dependent Auger measarements [5].

4. Interactions in iowitemperature Myers The study of low temperature adsorption (T.o < 4 0 K) has recently attracted much interest. For instance, the mechanisms of interactions and the electronic properties of physisorbed or w~zakly ehemisorbed adsorbates are m~rkedly different from those of strongly chemisorbed species. Unusual sticking coefficients [6], the possibility of direct observation of precursor state,,~ or reaction intermediates, and the growth of muhilayers and exchange reactions between them result as interesting phenomena. In partitular, two-dimensional phase transitions in physi,~;orbed layers are nowadays in the centre of interest of both theoretieians and experimentalists. In the following some experiments with weakly ehemisorbed and physisorbed nitrogen on Ni(11.1) are presented [7] as example. N z is a particularly interesting molecule because it usually ehemisorbs only at very low temperatures if it chemisorbs at all Its photoemission spectra slhow very pronounced structures, in particular a giant satellite can be observed [7, 8]. This is shown on the right-hand side of fig. 3 where some N ls spectra taken under different conditions :tre plotted, After exposure to 1.2 or 3.7 L

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Fig. 3. P,dsorpti~n of N~ on Nil|ll) at 30 K. Thermal desorption spectra (mass 14. leJ~t) and XPS-N ls spectra aher difTe~'cntexposures and partly after heating to 70 K a~ ind'cared, The inset shows Ar TD spectra from a clean and from a CS-N2 predoscd Ni(l l 1) surface.

244

E. Urnbaoh / Study of ad~'orbate interactioas

N2 at 30 K a multipeak structure can be 3bserved (spectra b and d). Heating to 70 K removes the dominating peak at 403.5 eV a~,d a two-peak structure remains (spectra a and c) which then disappears after further l~eating to 100 K. it can unambiguously be shown that this two-peak structure belongs to only one weakly chemisorbed state, CS-N z [7]. By subtraction of spectrum c (heated monolayer of CS-N2) from spectrum d (mixed N., monoiayer) a very narrow and rather symmetric single peak can be produced (e) which is attributed to ,.l physiso~bed N 2 state, PSI-N~ [7]. There is some evidence that this physisorbed molecule lies on the surface between verlieally adsorbed CS-N: molecules, and that PS1-N, is stabilized by attractive interaction with CS-Nz. The TDS spectra on the left: of fig. 3 show that heating of a low-coverage layer only yields one peak around 80 K which is attributed to the CS-Nz-state; i.e. the initially coadsorbed PSI state (see XPS data) entirely converts to CS;-N,. After exposing the crystal to higher doses of Nz additional peaks appear in the TDS spectra (fig. 3). One can clearly distinguish between 3 different physisorbed N2-states, labelled PSI to PS3, where the number of the state represents the layer in which the molecule was adsorbed with respect to the surface. The proof that PSI is directly adsorbed on the metal surface can be derived from Ar eoadsorplion experiments (see inset in fig. 3). The TDS spectrum of Ar adsorbed on a clean N i ( l l l ) surface shows ~wo peaks which are attributed to Ar in the first (48K, A r l ) and in the second layer (42 K, Ar2). respectively. Preadsorptlon of a CS-Ne layer causes a shift of the A r i peak to 53 K indicating a relatively strong attractive interaction between Ar and C S - N 2 in the first layer. This A r l state entirely blocks PSI desorpti(~n if high doses of N= are postadsorbed onto an Arl + C S - N , layer thus indicating that PSI competes with Art for the same adsorption sites on the metal surface between the CS-N, molecules. These Ar coadsorptJon experiments can also be used to litrate the number of PSI sites. Hence the absolute number of adsorbed molecules or atoms in any state can be derived by using "i"DS ~md quantitative XPS. For instance, the absolute

coverages of CS-N2 and PSI-N2 are equal: O(CS) = 0(PS1) = 0.25 (with respect to subs~rate surface atoms), It can also be shown that the total sticking coefficient continuously increases with coverage from 0.~35 to 0.9 during the build-up of the first N2-1ayer, and that it is independent of the substrate ~emperature between 27 and 50 K. The eompositlon of the adsorbed layer, however, changes drastically; the ratio PS1/CS reduces from 2 to 0. Other int,~resting observations [7] concern the displacement of adsorbed N~ by postadsorbed molecules or atorr~. While the displacement of PS1 or CS-N2 by strongly chemisorbed CO or oxygen appears to be expected it is not selfevideat that post-adsorbed A~r replaces PS1-N z after slight annealing since both species have about the same chemical binding energy. Coadsorbe:! C O or oxygen cause drastic changes in the Nz-TDS and XPS spectra: a variety of less and more strongly bound N2-species could be found sorne of which show ve~T interesting new features in their XPS spectra [7]. In conclusion, it can be stated that the combined use of photoemission arid thermal desorption, and the variation of coads~rbates can give some new insight into the interactions between low temperature adsorbates.

5. The in~e~preta|ion o~ XP$ spectra The examples of the p r e c e d i ~ sections indicate that it is of great importance to u~derstand the details of the observed photoemission spectra. Th,~ key is the understanding o( the screen;ng process, i.e. the ~response of the m:dtielectron system to the sudden creation of a hole. For many adsorb~-:es this screening mechanism can be described as charge trac, sfer from the metal to an-initially almost unoccupied-adsnrbate level (e.g. the 2~r leveI for CO, NO or Nz) which has been pulled down below the Fermi level by Coulomb attraction o1' the hole [8, '9]. The probability for such a charge transfer mechanism depends on tlhe bonding strength, i.e. on the overlap of metal density el states with the adsorbate 2~r level. In general,

E. Umbach : Study o f adsorbale inleraetion~"

this leads to well-screened final states at lower I~inding energy, a n d poorly-screened states Csatellites') at higher binding energy, tn the ease of stro~g,~y chemisorbed CO a relatively weak satellite at 538 eV is observed (see fig. 2) which even disappears if alkali atoJ~rls are coadsorbed. This is expected by' theery [8, 9] because the e n h a n c e d #r-bonding shou;d ljacilitate the charge transfer screening and should1 thus reduce the intensity of the satellite. I~ both eases, CO]Ru(00~) and CO+K/R:u(O01), the main peaks represent well-screene,i:l final states, and the observed peak shifts are only due to initial state (i.e, bonding) effects, in Ithe case of weakly ehemisorbed Nz the bondin~l strength is relatively s i n a i / a n d thus the satellite gains considerable ir~ensity (see fig. 3). EVeJll the details of the observed structures (the sp~etira of CS-N z actually contain 3--4 peaks or shola]ders) are understandable within this mode~; they depend on the banding a n d the projected metl~I density of states [,8, 9]. It is obvious that for quantitative studies it i~: important to know which slatellites (i.e. final sl~Lte channels) belong to a particular chemical st~te because they have to be taken into account

[2i. 6. C o n d e s i o n Guided by a few recent examples it has been sh~'}wn that electron spectroseo~:ies can very well be utilized to sttutly interactions on surfaces because of their quantitative putential and because of :he obtainable electronic h£:ormation, Howe ,f~r, a detailed ur~derstandir~g Of the underlying mechanisms if often useful or even absolutely

245

necessary to avoid en6rely incorrect interpretations,

Acknowledgements [ would llke to thank sdl those who have contributed to the presented results and in* terpretations by interesting d~scussions or technical assistance, or by providing experimental resuits prior to publication, in particular M, Breitschafter, E. I[-ludeczek, D. Menzel, K, Sch6nhammer, J.J, Weimar and W, Wurth. "]'his work has been s u p p o : e d by the Deutsche F o r s c h m : ~ g e m e i n s c h a f t (SFB 128).

References [1] See e.g.D.A. King and D.P. Woodruff, ads.. Chemical Physi~ of Solid Surfaces and Heterogeneous Cata]~,'sis~ vols. I to IV (Elsevier, Amsterdam). [2] E. Umbach and D, Mental. Surf, Sci. I35 (1983) 199: and referer~cestherein, [3] J.J. Weimar and E, Umb•ch, 9bys. Re'~. ~-;, submil'ted; J.J, We~mer, E. Umbaeh and D. Mental. Surf. Sc~.. submitted, [4] P.S. Bagus. CJ'. Nelin and C.W. Bausehlicher.Phys, Ray. 1328 {1983) 5423. [5] E. Umbach and Z. Hussain, Phys. l~v. Left, 57, (1984) 457; W. Wurth, E. I'~[udeez.ek,J.J'. Weimer and E. Urnbach, to be published, [6] P. Feulner and D. Menzal, Phi. Rev. B25 (1982) 4,,.95. [7] M. Br¢ilschnfter and E. Umbaeh, to be published, [8] J.C. Fuggte, ]E. Umbzch. D. Menzel. K. Waadelt and C.R. Brundle. Solid State Cerumen. 27 (19"78) 65; ~. Umbaeh. Suf£. Sei. 117 {19~;~) 484; S~id State Commun, 51 (1984)365, 1~9] t<, SehSnhammer and O. Gunnarsson, Solid State Cornrnun. 23 (1977) 691; O, Gunnar.S~oa and K. Seh6nhammer, Phys. Ray. Lett. 4t (19783 1608.