Interpretation of satellite structures in XPS for CO adsorbed on metal surfaces

Surface Science 143 (1984) L405-L410 North-Holland, A m s t e r d a m

L405

S U R F A C E SCIENCE LETTERS I N T E R P R E T A T I O N O F S A T E L L I T E S T R U C T U R E S IN XPS FOR CO A D S O R B E D ON M E T A L SURFACES Shm-lchl ISHI and Yulchl O H N O Research Institute for Catalysts, Hokkatdo Umverstty, Sapporo 060, Japan Received 5 March 1984, accepted for publication 26 Aprd 1984

The satellite peak - 6 eV away (lugher m binding energy) from the mare core peak m XPS for CO adsorbed on metal surfaces is assigned to shake up due to the mtra-adsorbate valence excltauon (So(or 1 ¢ r ) ~ 2~r) on the basis of recent experimental results by near-edge X-ray absorption free structure and reverse ultrawolet photoelectron spectroscopy

Core electron spectra of CO adsorbed on transition metal surfaces have been studied extensively by X-ray photoelectron spectroscopy (XPS) [1-13]. Data for the binding energy near the carbon ls (C(ls)) and the oxygen ls (O(ls)) core region for adsorbed CO on single crystal surfaces have been summarized in table 1. The data for polycrystalline samples are known to be almost ~dentical to those obtained w~th single crystals. From here on, we will discuss the C(ls) core region in the spectrum rather than the O(ls) regmn. The features in the core spectra may be convemently classified into the following three cases; (A) The core spectrum can be characterized as only one peak, whose binding energy is - 286 eV referred to the Fernu level (EF). The peak at - 286 eV referred to E~ is commonly observed on all metal surfaces as seen from the values given in table 1. (B) The spectrum can also be charactenzed as being composed of two peaks whose centers of gravity are represented by ~ 6 eV [4]. (C) The spectrum for CO on Cu [11-13] (a weaker adsorption system than CO/N1 system) extubxts a charactenstic three-peak structure. Theoretical considerations about the C(ls) spectrum for CO on Cu ((C) in our classlficatmn) were made by means of the Anderson-type model Hamiltoman [15] and molecular orbital (MO) cluster calculations [16,17] These interpretations, however, were subtly different from each other. Freund and Plummet [18] have taken a general discussion of the photoelectron spectra of transmon metal carbonyl compounds and CO adsorbed on transition metal surfaces. Their mterpretatmn of the core spectra of adsorbed CO is nearly identical to that of Messmer et al. [17]. The purpose of this letter is to give the 0039-6028/84/$03.00 © Elsevier Science Pubhshers B.V. (North-Holland Physics Publishing Division)

L406

S lshl, Y Ohno / Satelhte structures m X P S

Table 1 Peak positions m core regions m XPS on a variety of single crystal surfaces

Substrate

Peak posmons referred to Ev(eV ) 20 (C(ls))

lo (O(ls))

Ref

Fe(100) Ru(0001)

284 8

[11 [2]

Rh(100)

285 285 290 285 285 285 286 285 285

531 531 538 531 531 537 531 530 531 532 531 531 538 531

N~(100) N~(100) N~(lll) Pt(lll) Pt(100) W(100)

4 6 8 8 6 4 8 4 5

Mo(100)

Cu(100)

Cu(poly)

COgas(Ev)

286 288 292 286 4 289 4 294 0 296 2

3 2 7 3 5 0 6 9 1 6 9 6 5 2

533 7 536 0 541 0 5,1.2 3

[3] [4] [5] [6] [7] [8] [9]

[10] [11,12] [13]

[14]

assignment of the C(ls) spectra on metal surfaces talong into account recent experimental results by the so-caUed near-edge X-ray absorption free structure (NEXAFS or surface absorption fine structure (SAFS)) [19], and inverse ultraviolet photoelectron spectroscopy (I-UPS, or bremsstrahlung lsochromat spectroscopy (BIS)) [20]. Before discussing the assignment of the C(ls) core spectra for CO adsorbed on metal surfaces, we will explain briefly the electronic structures and their energy levels for gaseous CO. The ground state for gaseous CO is a 1Z+ state with the electromc configuration (lo)2(2o )2(3o )2(40 )2(50 )2(l~r

)4,

(I)

where the l~r orbltals are doubly degenerate Here and hereafter, the ahgnment with configuration is not in order of increasing energy The character of the lo and 20 molecular orbltals is primardy O(ls) and COs) atonuc orbltals, respectively The electromc configurations for ionic and neutral excited states are described only by a single electron configuration, hereafter. The electromc configurations of interest for CO + (positive Ion), C O - (negative ion) and CO*

L407

S Isht, Y Ohno / Satelhte structures m X P S

(neutral exoted state) are CO + .

(lo) 2

(2o)'

(30) 2

(40) 2

(5o) 2

(17r) 4

(II)

CO-

(lo) 2

(20) 2

(l~r)'

(2 ~'~)',

(III)

CO*.

(lo) 2

(2o)'

(l'n') 4

(2~r.)'

(IV)

The 2o (C(ls)) binding energy is - 2 9 6 eV from XPS [14] The 2~r~ (2¢r-affimty level) hes 1 5 - 2 0 eV above the vacuum level ( E v) from resonance electron scattering [20]. The e x o t a u o n energy from the neutral ground state (I) to the CO* state (IV) (C(ls) ~ 2¢rn exotatlon energy) is - 287 eV as obtained from K-shell electron energy loss spectroscopy [22]. The electronic configuration o f the ground state for the adsorption system ( C O / m e t a l ) is denoted by .. CO, where the dots and tdde indicate somewhat perturbed MO's of the metal and CO components, respecuvely The configuration of interest for CO i s ('i~ff)2(~)2(~)2(4"oa)2(5"~a)2(1"~) 4 The electronic confIguraUon of the ground state is given as

.c-6, c-6. The 5o and l~r orbitals are almost degenerate on transmon metal surfaces as seen from UPS measurements [23] The electronic configurations of both ionic and neutral excited states for adsorbed CO of interest are represented as

.co_,

co_,

(To)

...C*-O,

C*-'~,

(~)2

...co,

co;

(Vo): ( ~ ) 2 ...

...

(1~)4(2~,)a,

(~'~)

(N)

The 2o(C(ls)) binding energy is - 2 8 6 eV referred to E F. Tlus peak corresponds to the mare core peak, or normal hole state, m XPS terminology. It should be noted that the peak at ~ 286 eV is not assigned to the mare core peak in the previous theoretical mterpretatxons [15,16]. As we want to obtain the value referred to Ev, we add ~ + A,# to the value referred to EF, where ~ is the work function of a clean surface and A,# the varlatxon due to adsorption. Then, the ~ ' s ) binding energy referred to E v is - 292 eV, where we assume that ~ Is - 5 eV, A~ - 1.0 eV for the C O / N I system [24]. The 2~r, (2~r-affimty level) hes ~ 3 eV above E F from I-UPS [20]. The excitation energy on a d s o r p t i o n s y s t e m from the neutral ground state (I) to the . . . C O * state (IV) (C(ls) -~ 2~r, excitation energy) is - 287 eV from N E X A F S [19], whose value is sirmlar to that of gaseous CO. A schematic descnptlon of the C(ls) binding energy and the C ( l s ) ~ 2~r, excitation energy for gaseous and adsorbed CO, respecUvely, Is shown m fig. l a

L408

S lsht, Y Ohno

/

Satelhte structures m X P S

It is considered from the previous theoretical mterpretaUons [15-18] on the core spectra that a C(ls) electron ts ejected from the adsorbed C O and that the C(ls) core hole is screene...dd by the transfer of a metal electron to a low-lying unoccupied orbital, 1 e , 2~r orbital This configuration of the screening state can be represented as

( )+~*,

C"~*, (~)2(~) 1

('i~W)4(~'~)1

(~'~")

However, we can tdentffy the ( ) + C O * state (IV') wtth the CO* state (IV) because the difference between the n u m b e r of metal electrons N with (IV) and N - 1 with (IV') is negligible, where the value of N m a metal ~s large The p o s m o n of the 2~r energy level measured with N E X A F S hes - 1 eV above E v as seen from fig 1 Energy level dmgrams of the 2~ra and the 2¢r. for gaseous and adsorbed CO are shown m fig lb. A~.ccordlng to the previous theorettcal interpretations [15-18], the energy of the 2~r level is pushed down below E F o..~n creation of a core hole. The experimental result about the p o s m o n of the 2¢r energy level does not seem to support thts theoreUcal conclusion. Hlmpsel and Fauster have already pointed out that with respect to the energy of the 2~r~ (2~r-afftmty level), the theoretical descnptton ~s inconsistent w~th the experimental result [28]. Thus, ~t appears that a re-asstgnment of core spectra ~s warranted

{o) COgos CO/N)(IO0)

(b)

COoos COINi

Ev

~9

~

2T. ~287

e- 2"trn ,,-287

~I05 ~

'-T-" 2Ta

~2 , ¼ ,"

....

C(Is)i

~4

2"

4 '0"° -I~--

~

"~'

~ n (C~)hole)

EF

"2 n (C(Is)hole)

or~

hole

~6

Clls] ...a

5 0 " + IT

Fig 1 C(ls) binding energy and C(ls) ---, 2~r, excitation energy (e electron, (3 hole) (a), and the 2rra (2~r-afhmty level) and the 2~r excited energy levels (b), for both gaseous and adsorbed CO

S lsht, Y Ohno / Satelhte structures m X P S

L409

The mtra-adsorbate (mtramolecular) valence excitation is more important than the electron transfer from a metal to adsorbed CO to interpret the satellite peak, which is due to shake up, i.e., valence electron excitation which occurs concurrently with the ejection of photoelectrons. A slrmlar suggestion has also been given by Freund and Plummer [18]. The valence excitation energy from the 50 (or 1,r) in the neutral ground state toga 2~r in the neutral excited state (5o (or l~r) ~ 2~rn excitation) for both gaseous and adsorbed CO is known to be 6 - 8 eV from electron energy loss spectroscopy [25-27]. Even when a C(ls) hole state exists, the 50 (or l~r)---, 2~rn excitation energy is expected to be about 6 eV In summary, we assign the peak at - 286 eV referred to E F to the main core peak ( C ~ ) binding energy) as previously pubhshed The satellite peak - 6 eV away.~(hlghe~r m binding energy) from the main peak is due to shake up: the 5o (or l~r)---, 2~r excitation occurs with the ejection of photoelectrons Another satellite peak between the main peak and the satelhte peak - 6 eV apart from the main one for CO and Cu cannot be assigned to intrinsic excitation of adsorbed CO. It may be possible that this is attributable to transitions due to the band structure of Cu [26]

References [1] J Benrager and R J Ma&x, Surface Sct 94 (1980) 119 [2] J C Fuggle, T E Madey, M Stemkdberg and D Menzel, Chem Phys Letters 33 (1975) 233, cited in ref [9] [3] R J Baird, R C Ku and P Wynblatt, Surface Sct 97 (1980) 346 [4] C R Brundle, P S Bagus, D Menzel and K Hermann, Phys Rev B24 (1981) 7041 [5] B E Koel, D E Peebles and J M Wlute, Surface Scl 125 (1983) 739 [6] Y Jugnet, J C Bertohnl, J Massar&er, B Tardy, Tran Mmh Duc and J C Vedrme, Surface Scl 107 (1981) L320 [7] P R Norton, J M Goodale and E B Selklrk, Surface Scl 83 (1979) 189 [8] H P Bonzel and G Plrug, Surface Sol 62 (1977) 45 [9] E Umbach, J C Fuggle and D Menzel, J Electron Spectros Related Phenomena 10 (1977) 15 [10] S Semanclk and P J Estrup, J Vacuum Scl Technol 17 (1980) 233 [11] C R Brundle and K Wandelt, m Proc IVC-7, ICSS-3, ECOSS-2, Vienna, 1977, Eds R Dobrozemsky et al, p 1173 [12] J C Fuggle, E Umbach, D Menzei, K Wandelt and C R Brundle, Sohd State Commun 27 (1978) 65 [13] P R Norton, R L Tapping and J M Goodale, 72 (1978) 33 [14] T D Thomas, J Chem Phys 53 (1970)1744 [15] K Sch6nhammer and O Gunnarsson, Sohd State Commun 23 (1977) 691, O Gunnarsson and K Seh6nharnmer, Phys Rev Letters 41 (1978) 1603 [16] P S Bagus and M Seel, Phys Rev B23 (1981) 2065, P S Bagus, K Hermann and M Seel, J Vacuum Sol Technol 18 (1981) 435 [17] R P Messmer, S H Lamson and D R Salahub, Sohd State Commun 36 (1980) 263, Phys Rev B25 (1982) 3576

L410 [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28]

S Isht, Y Ohno / Satelhte structures m XPS

H -J Freund and E W Plummer, Phys Rev B23 (1981) 4859 J Stohr and R Jaeger, Phys Rev B26 (1982) 4111 Th Fauster and F J Hlmpsel, Phys Rev B27 (1983) 1390 G J Schulz, Rev Mod Phys 45 (1973) 423 G R Wight, C E Bnon and M J van der Wlel, J Electron Spectros Related Phenomena 1 (1972/73) 457 For example, E W Plummer and W Eberhardt, Advan Chem Phys 49 (1982) 533 K Chnstmann, O Schober and G Ertl, J Chem Phys 60 (1974) 4719 M A Chesters, B J Hopkins and R I Wmton, Surface Sc~ 59 (1976) 46 Ph Avouns, N J DzNardo and J E Demuth, J Chem Phys 80 (1984) 491 S Ishl and Y Ohno, Surface Scl 139 (1984) L219 F J Hlmpsel and Th Fauster, Phys Rev Letters 49 (1982)1583