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Ni and related systems
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Surface Science 296 (1993) 87-96 North-Holland
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On the resonantly core excited states of the NJNi and related systems M. Ohno and P. Decleva Dipartimento
di Scienze Chimiche,
Universitri di Trieste, via A. Valerio 38, I-34127 Trieste, Italy
Received 22 March 1993; accepted for publication 6 July 1993
The 1s to n* resonant excitation oscillator strengths and energies of the inner and outer N atomic sites of NiN, have been calculated by the 2h2p/2h2p CI scheme, using the core excited state SCF relaxed orbitals. A comparison is made with previous theoretical results obtained by using the same CI scheme but with the neutral ground state orbitals or the core ionized state SCF relaxed orbitals. The oscillator strength ratio for the inner and outer N atoms is reversed in comparison to previous results. The strength reflects the empty N2p occupation of the n* orbital at the excitation atomic site in the ground state. Use of the core ionized state relaxed orbitals fails because of the larger core relaxation in comparison to the core excited state. The influence of the core relaxation in the resonantly excited state on the spectral features of the DES spectra with variations of adsorbate/substrate systems is also discussed.
1. Introduction
Recently we have performed ab initio lhlp/ lhlp (one hole one particle excitations for both ground and excited states) and 2h2p/2h2p CI (configuration interaction) calculations of 1s NEXAFS (near edge X-ray absorption fine structure spectroscopy) spectra of NiCO and NiN,, using an extended basis set [l-3]. Our many-body calculation gives a fairly detailed description of the low lying excitation spectral features, including the Rydberg-derived levels of NEXAFS spectra of CO/Ni and N,/Ni systems. Recently we proposed the cooperative core hole screening mechanism [4-81 to give a systematic description of the strong changes in XPS core spectral features with different adsorbate/ substrate systems. By this mechanism the screening is associated not only with the r metal-ligand charge transfer (CT) but also with s-da promotion and hybridization in the local metal u and u* orbitals. So the XPS core hole spectral features such as the presence or disappearance of the giant shakeup satellite depend on the s-d population of the local metal ground state. We explained the 0039-6028/93/$06.00
absence of the giant shakeup satellites in the NEXAFS spectra in terms of the hindrance of the cooperative core hole screening mechanism by the resonantly excited r* electron [l-3]. Recently the Uppsala group has studied DES (deexcitation electron spectroscopy) spectra of the N,/Ni system with excitation energy variations [9,10]. They have deconvoluted the NEXAFS Is to r* spectrum of N,/Ni as a superposition of N, and N, spectra (N, and N, denote the inner and outer N atoms, respectively). They obtained a relative spectral intensity ratio of 0.7: 1.0 for N, and N,, respectively. This disagrees with our previous prediction of 1.0 : 0.8 [l-3]. For NiN, use of the core ionized SCF relaxed orbitals employed in the former work may have lead to an inaccurate f-value (oscillator strength) because of different core relaxation between the core ionized and core excited states. The large difference in the results obtained even at the 2h2p CI level for the resonant excitation in NiN, points out the importance of an optimal choice of the orbital set if converged results are to be obtained at practical excitation levels. Therefore in the present work we set out to obtain an accurate estimate of
0 1993 - Elsevier Science Publishers B.V. All rights reserved
88
h4. Ohno, P. Decleca / Resonantly core excited states of N2 /Ni and related systems
f-value and excitation energy for the Is to rr* excitation in NiN,. We employ the lhlp core excited state SCF relaxed orbitals instead of the neutral ground state and the core ionized state SCF relaxed orbitals, and push the CI excitation level to convergence. In the case of NEXAFS spectra of adsorbates, the screening mechanism is different from the XPS core hole spectra. The excess of r* charge by the resonantly excited electron hinders the increase in drr backdonation. Because of the increased electron-electron repulsion not only the x CT but also the s-da hybridization and promotion (local metal (T relaxation) are hampered. The excess of r charge could even give rise to “reversed” s-da hybridization and a promotion mechanism by which the s-population increases and the d-population decreases. This could give rise to a significant u to cr* shakeup satellite. However, in the resonantly excited state there seems to be no efficient mechanism which reverses the drr-r* CT, increasing the d-population so as to promote the reversed s-da mechanism. So the (T local metal shakeup excitation is much suppressed in the NEXAFS spectrum [l-3]. In order to study how the shakeup satellite features in the NEXAFS spectrum of adsorbate depend on the local metal s-d population, in previous work we used both the neutral ground state and the core ionized state SCF relaxed orbitals. The latter, particularly in the case of the N,/Ni system, mimic the initial state where the s-population is significantly depleted. We can then show that not only the shakeup satellite feature of the XPS core hole spectrum but also that of the NEXAFS spectrum depends on the s-d population. The smaller the s-population, the more enhanced are the u to u* shakeup satellites. In the present work we show that the relaxation of the occupied orbitals in the resonantly excited state is small enough that the one-electron picture is valid and the shakeup satellite intensity is negligible. Furthermore we shall shed some light on the relaxation in the resonantly excited state and its influence on the spectral features of the DES spectra. Finally we discuss the spectral behaviours of the DES spectra of CO/Ni and related systems with variation of the photon excita-
tion energies. This is necessary for the understanding of the more complicated spectral behaviours of N,/Ni.
2. Theoretical
Method
Finite basis set LCAO and nhnp CI schemes, amounting to a maximum of n excitations out of the ground state configuration, are employed as in previous work [l-3], to which we refer for computational details, mentioning briefly some points relevant to the present work. To obtain Is-rr * SCF orbitals proves particularly difficult for this system. We start from the converged Is- ’ core hole orbitals, and optimize first the rr* singly occupied level, keeping all the doubly occupied orbitals frozen. We then proceed with relaxing the metal u shell, which immediately produces a large change in the metal u orbital and loses d-participation towards the metal 4s. Then the full metal d-shell and N, 17~ and 5u orbitals are alternatively allowed to relax, while the remaining occupied orbitals are always kept frozen. In addition to the lhlp and 2h2p CI schemes defined in our previous work, to test convergence of the CI expansion for the single resonantly excited state, we employ a larger space, comprising all single and double excitations with respect to the resonantly excited configuration. This amounts to including a selection of 3h3p excitations with respect to the ground state reference. Also for the ground state, which is described by the same set of orbitals, we employ larger 3h3p spaces, with perturbative selection from the lhlp zeroth-order vector. The largest spaces comprise about 56000 configurations for the resonantly exTable 1 Theoretical [2] and experimental 1s to 7 * excitation [11,12] (in eV) and f-values for the N, /Ni system Excitation
Experiment Method A Method B Method C Method D
energy
f-values
N,
N,
N,
401.0 391.2 400.3 401.4 401.1
400.4 389.2 400.4 400.4 400.0
0.171 0.148 0.026 0.057
N, _ 0.205 0.029 0.024 0.041
energies
N, IN, ratio 1.42 1.20 0.20 0.92 0.71
M. Ohno, P. Decleua / Resonantly core excited states of N, / Ni and related systems
cited state and 25000 for the ground state. The ground state energy and populations are then taken from the 3h3p results relative to N, ls-r* SCF orbitals. Let us remark that lhlp CI, with a fixed lh core hole, is completely equivalent to a frozen core ASCF calculation.
3. Results and discussion 3.1. Resonant excitations for NiN,
In table 1 we give a summary of the previous results for NiN,. The lhlp CI scheme using the neutral ground state orbitals (method A) neglects the relaxation of the core hole and excited electron pair and the screening of the bare hole-particle Coulomb interaction. In this case the f-value reflects the ground state empty r* charge distribution on the excitation atomic sites. According to the final state rule [ll] the spectral line profile should be accurately obtained by a one-electron theory, provided that the dipole matrix elements are calculated from valence wavefunctions corresponding to the final state. Following this line, the lhlp/lhlp CI scheme using the core ionized relaxed orbitals (method C) was used. By the 2h2p/2h2p CI method, using the ground state orbitals (method B) one treats the hole and particle relaxation, the screening of the bare holeparticle Coulomb interaction and the lhlp shakeup excitations, in terms of the lhlp excitations in the presence of the lhlp primary excitation. For NiCO inclusion of such many-body effects by method B, or by employing relaxed orbitals (method C (lhlp/lhlp CI> and method D (2h2p/2h2p CD), significantly improves the f-values, obtaining good agreement with experiment. Table 2 Theoretical and experimental 1s to rr* excitation energies [11,12] (in eV) and f-values for the N, /Ni system Excitation energy
Experiment
SCF 2h2p/2h2p CI
f-values
N,
N,
N,
N,
401.0 401.8 400.8
400.4 401.8 399.8
0.0936 0.1018
0.1002 0.1067
Nb 1% ratio
1.42 1.07 1.05
89
Table 3 Mulliken population analysis of NiN, State
SCF ground state 2h2p ground state N, 1s --t rr * (SCF) N, 1s + rr * (SCF) N, 1s + rr * (2h2p) N, 1s + ,rr* (2h2p) N,ls-’ (SCF) N, Is-’ (SCF)
NIP
Ni
NE!
N,
3d
4s
2.088 2.067 3.120 1.918 3.063 2.OOQ 2.786 1.732
1.853 1.941 1.775 3.037 1.842 3.002 1.614 2.801
8.713 9.030 8.657 9.051 8.747 9.034 9.343 9.215
1.487 0.880 1.666 1.000 1.484 0.920 0.170 0.158
However, for NiN,, the relaxation causes a large f-value reduction, inducing large shakeup satellite intensities (which are not observed). It also reverses the f-value ratio between the N, and N, to 1: 0.8, with the exception of method B, where the ratio is much diminished. In table 2 we summarize the 1s to r* resonant excitation energies and f-values of NiN, obtained by the ASCF (lhlp CI) method and the largest CI scheme (which we will call for brevity 2h2p/2h2p CI>, using the resonantly core excited state SCF relaxed orbitals. In table 3 we summarize the Mulliken population analysis of the ground state, the core ionized states and the core excited states. In table 4 we summarize the orbital overlaps between the ground state and core ionized (or excited) states. At the ASCF level the N, and N, resonant excitation energies are almost identical because of the near degeneracy of the Koopmans energies in the ground state (about 0.1 eV difference). The same is true with the previous results obtained by
Table 4 Overlaps between the ground state and core ionized or resonantly excited relaxed orbitals Overlaps (air> (ula*> (1irllP) (nlw) (rrlrr*)
N, (Is-‘)
;; -+ P*)
N, (Is-‘)
N, (1s + %-*I
0.7078 0.6799 0.9740 0.9608 0.2272
0.9984 0.9712 0.9943 -
0.6983 0.6890 0.9732 0.9171 0.3516
0.9810 0.9622 0.9794 -
(r, r are metal orbitals.
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h4. Ohno, P. Decleva / Resonantly core excited states of N,/ Ni and related systems
method B (see table 1) which is quite similar to the ASCF method. With introduction of the higher-order valence excitations by the 2h2p CI scheme, the experimental excitation energies are well reproduced, including the N,-N, excitation energy difference of 0.6 eV (which is due to the inequivalent N atoms), while the f-values obtained by the ASCF method and the 2h2p CI method are similar, except for a slight increase in the latter. The ratios N,/N, are 1.08 and 1.05, respectively, which values are slightly reduced from the empty N2p population ratios in the ground state (1.12 and 1.07, respectively). The experimental value proposed by the Uppsala group is 1.42 [9,10]. The increase of the N2p population at the excitation site from the ground to the core excited states obtained by the ASCF (2h2p CI> method is 1.032 (0.997) for N, and 1.184 (1.061) for N,, respectively. This gives an N,/N, ratio of 1.15 (1.06) which is close to the empty N2p ground state population ratio. The relaxation of the occupied orbitals in the resonantly excited states is small enough that the one-electron picture is valid and the shakeup satellite intensity is negligible. So the reduction of the f-value due to the shakeup excitations is negligible. In comparison to the previous result obtained by using the core ionized state SCF relaxed orbitals, there is a dramatic change not only in the f-values but also in their N,/N, ratios. The fvalue ratio is reversed now and the present result indicates that the f-value represents the N2p population of the empty r* orbital in the ground state (the sum rule [ll]). The main cause of this large discrepancy is the use of the core ionized orbitals instead of the core excited orbitals. The N, 2p population in the N, core ionized state is slightly larger than the N, 2p population in the N, core ionized state, in contrast to the reversed situation in the neutral ground state. So one can expect a smaller f-value for the N, excitation when using the core ionized relaxed orbitals. Furthermore, the core ionization leads to a dramatic change in the local metal u orbital (as seen in both the local metal s-d population change and the orbital overlap of the (+ orbital). Consequently, this leads to a large shakeup satellite
Table 5 The estimated adsorbate/substrate resonantly excited states (in eV)
Cls 01s N,ls N, 1s
binding
energies
CO/CU(100)
CO/Ag(110)
CO/Ni(100)
CO/Pd(100)
0.5 1.1
( - 0.25) (0.15)
1.2 2.0
1.0 2.3
in the
N,/Ni(loo)
0.4 1.0
intensity and an f-value reduction. So at the 2h2p CI level, use of the core ionized state relaxed orbitals reproduces well the resonant excitation energies but not the f-values. In the resonantly core excited state, there is a small change in the occupied rr and u orbitals. However, the change is smaller in the N, excited state than in the N, one because the adsorbate/ substrate coupling strength in the former state is smaller than in the latter state (by 0.6 eV, see table 5 and later discussion). The adsorbate/ substrate coupling in the N, excited state is the same as that in the ground state so the changes in the occupied orbitals should be small. The s-d population in the N, resonantly excited state shows a small s-population increase and d-population decrease in comparison to the ground state. This possibly induces the reversed s-da promotion (hybridization) mechanism, increasing slightly the u repulsion and reducing the rr backdonation (weaker coupling). On the other hand the s-d population in the N, resonantly excited state shows a substantial decrease in s-population and increase in d-population. This still indicates the presence of the s-da promotion mechanism which slightly increases the dr backdonation (stronger coupling). The increased electron-electron repulsion due to the presence of the resonantly excited r* electron hinders the dr backdonation and consequently the s-da promotion mechanism. ,So the dr backdonation in the resonantly excited state is much smaller than in the core ionized state. In other words the adsorbate/ substrate coupling in the resonantly excited state is much weaker than in the core ionized state (see also section 3.4.3).
M. Ohno, P. Decleua / Resonantlycore excitedstatesof N2/Ni and relatedsystems
3.2, Extra-molecular relaxation The 1s to r* resonant excitation energy shifts between the free and adsorbed molecules are small. Following the discussion given by Jugnet et al. [12], we consider the resonant excitation energy shift within the framework of the present surface molecule approach. The 1s to r* resonant excitation energy for a free molecule is given by E,,,( free molecule) = E, - E,, - U( Is, a).
(1)
Here EIS, E, and U&a) are the Is core ionization energy, the r* affinity energy and the effective Coulomb interaction between the 1s hole and r* excited electron for a free molecule such as CO, respectively. For the resonant excitation energy of the adsorbate, we make the following assumptions: (i) The initial state energy shifts (chemical shifts) from a free molecule to a chemisorbed molecule are small. (ii) The intraligand relaxation energy shifts do not change from a free molecule to a chemisorbed molecule. (iii) The extra-molecular screening such as metalligand CT screening is localized at the core excited atomic site. Then, the excitation energy for a chemisorbed molecule is approximately given by the following equation, E,,,(adsorbate)
= (E, - A,) - (E,, + Al,) -[u(Is,
a) -Ais-Aa].
(2)
Here Ais and Aa are the extra-molecular relaxation energy shifts of the 1s and r* levels due to the rr metal-ligand CT screening. U will be reduced also by the extra-molecular screening, namely the drr to rr* CT hole-particle pair excitations induced by the r* excited electron and the 1s core hole which can be approximated by Aa and Als, respectively. (See ref. [13] for discussion on the case where the same kind of formalism is used to determine the effective hole-hole interaction in double hole excitations in atomic-like localized systems.) The closeness between the resonant energies of free and adsorbed molecules indicates that the approximation used for eq. (2) is valid. As an example we
91
consider the case of the CO/Ni sytem. The Als and Aa can be determined from free and adsorbed molecular data (see references and data in refs. [9,10]>. They are 4.1 (4.2) and 4.8 (4.8) eV for the carbon (oxygen), respectively. Then we obtain an U of 1.5 and 1.4 eV for the C and 0 1s to 7 * excitations, respectively. The experimental values are 1.4 and 1.7 eV, respectively. This shows that the shifts in the XPS (and IPES) are dominantly due to the relaxation effect and the chemical shifts are small. Riihl and Hitchcock [14] also reached the same conclusion in a different way. 3.3. Binding energy of the resonantly excited state The small core excitation energy shifts between the adsorbed and free molecules correspond to the adsorbate/substrate binding energy shifts between the resonantly core excited state and the neutral ground state. For a comparison of the resonant excitation energy between the adsorbed and free molecules, the Uppsala group [1.5] recently recommended the configuration average excitation energy of a free molecule rather than the singlet resonant excitation energy. As the dipole forbidden C 1s to 2~* triplet state transition of free CO was observed at 1.3 eV lower by EELS [16], they suggested 286.4 and 533.8 eV as the appropriate configuration average excitation energy for the carbon and oxygen of free CO, respectively. They argued that in adsorbates the exchange splitting is quenched due to the hybridization of the empty adsorbate orbital (r*) with the substrate band and this exchange splitting quenching is seen in the lack of any exchange splitting in the XPS core hole spectra of NO on Ni(100). Recent studies of NO on Ni(100) by XPS (together with XPD) and UPS [171 show one atomic and two molecular adsorption forms. NO adsorbs molecularly into two different geometries: at low coverage, the molecular one is lying down or highly tilted, while at saturation coverage, the molecular axes are oriented perpendicular to the surface. This agrees with theoretical predictions by Bauschlicher and Bagus [181. They studied the geometries and binding mechanism of NiNO by the CASSCF method and showed that there are two mechanisms for the
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M. Ohno, P. Decleva / Resonantly core excited states of N2/ Ni and related system
bonding of NO to Ni: one leads to a linear system and the other to a bent system. For both the linear and bent models of the bonding, they showed that there is no open shell character on the NO and, consequently, there will be no multiplet splitting in the NO derived XPS spectra of chemisorbed NO. So the lack of exchange splitting in the XPS spectra of NO/Ni does not indicate the quenching of the exchange splitting in adsorbates. What one can expect upon chemisorption is that the mixing of adsorbate and substrate states delocalizes the free molecular orbitals. As a result, the splitting decreases because the smaller the spatial overlap of 1s and ?r * is, the smaller is the singlet-triplet splitting. Therefore, the splitting is larger for the C 1s excitation than for the 0 1s excitation [18]. We use the singlet excitation energy of a free molecule as reference energy. In table 5 we list the adsorbate/ substrate binding energies of resonantly excited states determined by the resonant excitation energy shifts. As already mentioned, the coupling strength in the N, core excited state is smaller than that in the N, core excited state. For adsorbed CO the coupling strength in the 0 excited state is larger than that in the C excited state. 3.4. DES spectra of adsorbates 3.4.1. Spectral behaviours with excitation energy variation
As there is a significant difference in the core relaxation between the core ionized and resonantly excited states, one may expect significant spectral feature differences between the Auger electron spectroscopy @ES) spectrum and the DES spectrum because their spectral features are expected to be governed by the valence wavefunctions in the core ionized state and the resonantly excited state, respectively. However, the DES spectra of adsorbates resemble the AES spectra much. All the autoionization features are found to move with the photon excitation energy, i.e. have a constant kinetic energy [9,10,19,201. As discussed already by several authors, the delocalization of the excited electron from the adsorbate to the substrate results in a state that could be
considered as locally identical to the XPS lowest energy state before the Auger core hole decay starts [9,10,19,211. This may explain the constant kinetic energy and similarity between the AES and DES spectra. However, Wurth et al. [22] observed a major spectral intensity change in the 0 Is DES spectra of different adsorbate/ substrate systems when the photon excitation energy is increased 6eV above the resonance energy, namely the spectral intensity decrease of the ?r backbonding peak, which they interpreted as due to the hampering of the 7~ CT from the metal by the electron resonantly excited to the Rydberg level. Recently, the Uppsala group [9,10] also studied carefully the DES spectra of several adsorbate/substrate systems with variations of the photon excitation energy. In the 0 1s DES spectra of the CO/Ni(lOO) system they observed notable spectral intensity variations depending on the excitation energy, already at the rr resonance maximum in comparison to the XPS ionization limit. However, no corresponding changes are observed in the Cls DES spectra except for the enhancement of a small structure at 263 eV which occurs at the excitation energy of 291 eV. The major 01s spectral intensity changes observed were the relative intensity variations of the two prominent peaks at the KE of 510 and 513.5 eV. They are interpreted mainly as 40~ ‘lY1 and respectively. The extra l?r -2 configurations, structure which appears at 512 eV at the excitation energy of 535.9 eV seems to remain in all spectra measured at larger excitation energies. Riihl and Hitchcock [14] determined the fvalue for the 1s to r* excitation in transition metal carbonyls. The reduction of the 1s to r* f-value from a free CO molecule to the transition metal carbonyls is smaller for 0 1s than for C 1s. This reflects the smaller coupling with the metal substrate for the oxygen excitation than for the carbon excitation. The Uppsala group reached the same conclusion by considering only the spatial closeness to the substrate of the carbon site in comparison to the oxygen site [9,101. Consequently, there are more significant spectral intensity variations in the 0 1s DES spectrum than in the C 1s DES spectrum, because a localized reso-
M. Ohno, P. Decleua / Resonantly core excited states of N2 /Ni and related system
nantly excited electron may influence the spectral intensity before it delocalizes to the substrate. Before we proceed further in the analysis of the DES spectra of the CO/Ni system, we would like to discuss the DES spectra of the CO/H/Ni(lOO) system. 3.4.2. DES spectrum of the CO/H/ Ni(lO0) system In contrast to the CO/Ni system, the 1s to r* resonant excitation energy of the CO/H/Ni system shows significant variations with different geometries, namely top, bridge and hollow sites [91. The C 1s to r* resonance energy shifts between adsorbed and free molecules are 0.0, -0.2 and -0.8 eV, respectively, for the top, bridge and hollow sites. For the 0 1s they are -0.4, - 1.3 and - 2.3 eV, respectively. To estimate the chemisorption energies of the CO/H/Ni(lOO) system in different geometries, we study the spectral behaviours of the DES spectra of the CO/H/Ni system with different geometries. The 0 1s DES spectrum of CO/H/Ni shows much more significant spectral feature changes than the C 1s DES spectrum with different geometries [93. The 0 1s DES spectra of the CO/H/Ni system of top, bridge and hollow sites resemble those of CO/Cu, CO/Pd and CO/K/Ni systems, respectively. As the chemisorption energies of the CO&u and CO/Pd are known (0.6 and 1.6 eV, respectively [23,24]), the adsorbate/ substrate binding energy of the 0 1s core resonantly excited state can be determined from the available spectroscopic data (for the CO/K/Ni system the chemisorption energy is unknown so we estimate it to be 2.0 eV). They are 1.1, 2.3 and 4.0 eV, respectively. Then, the chemisorption energies for CO/H/Ni top, bridge and hollow sites will be 0.7, 1.0 and 1.7 eV, respectively. Using the C 1s data we obtain 0.5, 0.8 and 1.9 eV, respectively. These results agree with the observation that the chemisorption energy increases with an increase of the coordination number and that coadsorption of H decreases the backdonation (the chemisorption energy of CO/Ni (top or bridge) is 1.3 eV 1251).With a stronger adsorbate/substrate coupling in the order of top, bridge and hollow sites, the bonding orbital is more polarized to-
93
ward the ligand (larger r backdonation). Consequently, the backbonding peak spectral intensity increases. This is noted also by the Uppsala group [9]. We note that with an increase of the adsorbate/substrate coupling the 1~ orbital (which is orthogonal to 2~ orbital) will be polarized more toward the oxygen and that the spectral intensity of the lrr-’ two hole state peak increases dramatically in the 0 1s DES spectra. So the adsorbate/substrate coupling strength in the resonantly excited state (or the ground state) can be monitored by observing the spectral intensity changes of the backbonding peak and the 1~~ peak in the 0 1s DES spectrum. 3.4.3. DES spectrum of the CO/Ni(lOO) system The Uppsala group [9] pointed out a similarity in the spectral intensity changes between 0 1s DES spectra of CO/Ni(lOO) with a photon energy increase from 531.8 to 533.4 eV and those of the CO/H/Ni(lOO) system with a decrease of the adsorbate coordination. The spectral intensity of the l,rr-2 peak decreases relative to that of the 4a- ‘1~~ ’ peak. As also pointed out by them, this similarity indicates that the resonantly excited electron tends to be more localized with the photon energy increase. The presence of localized electrons resonantly excited to the r* or Rydberg levels will hamper the dr to rr* CT from the substrate so that the polarization of the bonding orbital towards the ligand is hindered. Consequently, the spectral intensity for the backbonding peak and the 1Y2 peak will be reduced. In other words, the adsorbate/ substrate coupling strength in the core excited state is weaker than that in the core ionized state. The origin of the 512 eV peak in the 0 1s spectrum and the 263 eV peak in the Cls spectrum of CO/Ni(lOO) is considered as the excitation to the Rydberg level [91. However, it seems to be difficult to. explain why these peaks remain even at the excitation energies far above the Rydberg excitation energy. The two prominent peaks at 513.5 and 510 eV in the 0 1s spectrum are mainly due to the 1~~ and 4(5)a-‘1~~’ double hole configurations, respectively. Their BE is 18.7 and 22.2 eV, respectively. In the Cls spectrum there is a small peak at 271.8 eV (14.1
94
M. Ohno, P. Decleva / Resonantly core excited states of N2 / Ni and related system
eV BE) for the CO/Ni system which is not seen for CO/Pd and CO/Cu systems. The Uppsala group interpreted this peak as the dv--r* CT shakeup satellite of 4e-1~;1 [9]. The BE of 4~ ‘rr;i is 11.3 eV (prominent peak seen around 275 eV). Then the r shakeup energy in the presence of 4u-‘7rG1 is 2.8 eV. The origin of the 262 and 512 eV peaks is probably due to the dr-a* shakeup core hole states. Their energies are 288 and 533.8 eV for C and 0, respectively (see the next section). In that case the expected final states are lW27r,‘27r* for 0 and 4(5)(+-l l~-%r,‘27r for C. The BE will be 21.5 and 25.0 eV, respectively; the KE 512 and 263 eV. This agrees well with experiment. These peaks are observed to remain in the AES spectra. This indicates that the origin of these peaks is the incomplete core hole relaxation, namely the drr to v* CT shakeup excitation associated with the main line 1s core ionization. 3.4.4. 0 2s XES spectrum of the CO / NitlOO) system
The recently measured 0 Is X-ray emission spectrum of the CO/Ni(lOO) system shows an interesting spectral feature in comparison to that of free CO [26]. The spectrum shows a strong satellite feature at 1.8 eV at the high energy side of the main line. The main line is observed at 524.8 eV. The satellite intensity is comparable to the main line intensity. The main line is associated with the transition from the 0 1s core hole state (BE is 532.2 eV) to the final 1~ hole state (BE is 7.5 eV). This gives a transition energy of 524.7 eV which agrees well with the experimental value. The authors of ref. [26] suggested that the 1.8 eV satellite is due to the Is core hole shakeup excitation. If the final state for the satellite is the same as that of the main line, the initial shakeup state must be 1.8 eV larger than the initial core hole main line state. The high resolution XPS Cls core hole spectrum of the CO/Ni system shows the satellite at 2.1 eV at the high energy side of the main line, while the 0 Is core hole spectrum does not show such a satellite [271. However, our recent many-body calculation predicts the presence of a rr to rr* shakeup satellite of a small intensity at 1.6 eV at the high energy
side of the main line [4,5]. For the shakeup state to be the initial state, the final state of both main line and satellite transitions must be the same. Many-body calculations show that the one electron picture of the 1~ ionization breaks down due to the strong configuration interaction between the 1~~’ and 1~-‘~;‘2’rr* configurations [28]. The final state of the X-ray emission process which involves the 11~.level is not the single hole state but most likely the 2hlp state, So the main line is due to the transition from Is-’ to 171’-lak127r* and the satellite is from Is-~~;‘27r* to the same final state. This explains at least the energy separation of 1.8 eV between the main line and the satellite line. Despite of a small XPS satellite intensity, how can it be that the satellite originating from the XPS shakeup satellite takes such a large intensity? If the interpretation of the final “lr-“’ state in terms of the lrr-‘x;‘2~* “screened state” is correct, we may observe that transition intensity will be much enhanced from the Is-%T;~~~* initial state (1.8 eV satellite), because of the single electron transition, in contrast to the two electron transition required for transitions from the initial Is-’ state. A different interpretation may be addressed in terms of final state effects, due to a breakdown of the one particle picture for the 1r-l level, as predicted by the calculation [28]. In this case the transition moment from the dominant Is-’ initial state will divide over different final states according to the weight of the la- ’ configuration. Although the calculations clearly show such effects, the quantitative prediction does not appear to fit precisely both the valence photoemission and the XES data. This might be due to the difficulty in obtaining precise numerical values in such a complex system. In any case a decisive choice between the two possible mechanisms could be afforded by XES measurements below and above the XPS shakeup energy. In the initial state mechanism the XES satellite is suppressed below the threshold, while it is not affected in the case of a final state effect. 3.4.5. DES spectrum of the N,/Ni system The spectral features of the N,/Ni system change with an increase of the excitation energy
M. Ohno, P. Decleva / Resonantly core excited states of N2 / Ni and related systems
[93 because of a superposition of the N, and N, DES spectra obtained at different excitation energies. So an analysis of the spectra becomes difficult, although the Uppsala group used the DES spectra to deconvolute the N 1s XAS spectrum into the N, and N, contributions [9,10]. (i) The AES spectrum obtained at the excitation energy far above the N, and N, ionization limit appears to be a superposition of the AES spectrum obtained at the N, and N, ionization limit, respectively, although there may be some extra contributions due to the incomplete core hole relaxation. (ii) With an increase of the excitation energy from the N, XPS ionization limit to the N, 7r* resonance maximum, except for the r backbonding peak (389 eV), the new spectral feature appears at the larger kinetic energy. Here probably the initial state is the resonantly excited state, but the final state is the two hole state. So the kinetic energy increases by the XPS-XAS energy shift for N, (1.0 eV). The spectral intensity of the r backbonding peak must be reduced in comparison to the AES spectrum because of the smaller polarization of the r bonding orbital in the resonantly excited state than in the core ionized state. This may result in the non-shifting of the 389 eV peak (dominance of the AES peak). (iii> With further increase of the excitation energy to the N, XPS ionization limit, the new spectral feature appears at the larger kinetic energy side, except for the rr backbonding peak. The energy shift is most likely due to the N, - N, XPS ionization limit difference (1.3 eV>. The adsorbate/substrate coupling in the N, core ionized state is weaker than that in the N, core ionized state. So the polarization of the bonding peak is less in the N, core ionized state than in the N, core ionized state. So the spectral intensity of the 7 backbonding peak of the N, AES spectrum is smaller than that of the N, AES spectrum. (iv) With a further increase of the excitation energy to the N, r* resonance maximum, a prominent spectral feature appears at smaller kinetic energy (379 eV>. This spectral feature may be interpreted as the autoionization spectrum. The autoionization kinetic energy is shifted from
95
the Auger energy by Uvra + Uvz, - UC,_+. Here U is the effective hole-particle Coulomb interaction, Vl, V2, c and a denote the valence hole Vl, V2, core hole and adsorbate level, respectively. The spectral intensity of the rr backbonding peak decreases because of a weaker adsorbate/ substrate coupling in the N, resonantly excited state than the N, core ionized state. The Uppsala group deconvoluted the N 1s XAS spectrum by monitoring the change of the peak height at 389 eV (for the N, atom) and that at 379 eV (for the N, atom) [9,10]. However, as mentioned above, there seem to be notable spectral feature changes within the r resonant excitation energy region. The Uppsala group assumed that the spectral feature does not change in the r resonant excitation energy region. This may have caused some errors in their deconvolution procedure.
4. Conclusion The 1s to IT* resonant excitation energies and f-values of NiN, are calculated by the lhlp or 2h2p CI method, using the core excited state SCF relaxed orbitals. The excitation energies are well reproduced by the present calculations, including the N, - N, energy difference of 0.6 eV which is due to the inequivalent N atoms. The excitation energy shift between the free and adsorbed molecules shows that the adsorbate/ substrate binding energy in the N, core excited state is the same as that in the ground state and smaller than in the N, core excited state by 0.6 eV. The f-values obtained follow closely the N2p population of the empty r* orbital in the ground state, as expected from the sum rule for the absorption process. The f-value ratio for the outer to the inner N atom excitation is 1.05, while the experimentally determined value is 1.42. This large discrepancy seems to be due to the error associated with the deconvolution procedure of the N 1s XAS spectrum of the NJNi system into the N, and N, contributions, by monitoring the spectral intensity changes in the DES spectra of N,/Ni measured at different excitation energies. The DES spectral intensities of the structures, the
96
M. Ohno, P. Decleua / Resonanily core excited states of N2 /Ni and related systems
dominant configurations of which involve the orbitals of the R symmetry, are sensitive to the variations of the photon excitation energies because of the hindrance of the r backdonation due to the presence of the resonantly excited electrons to the V* or Rydberg derived levels. The DES spectra1 intensity changes by different adsorbate/ substrate systems can be used to study the adsorbate/ substrate coupling strength. As such an example, we considered the case of CO/H/Ni(lOO> system in different geometries. Finally, two alternative mechanisms for the appearance of the strong satellite in the XES spectra of the CO/Ni system are proposed, and a discriminating experiment is suggested.
The authors are grateful to Dr. 0. BjGrneholm for a copy of his PhD Thesis. References [l] M. Ohno and P. Decleva, J. Chem, Phys. 98 (1993) 8070. [Z] M. Ohno and P. Decleva, Surf. Sci. 284 (1993) 372. [3] M. Ohno and P. Decleva, Chem. Phys. 171 (1993) 9. [4] M. Ohno and P. Decleva, Surf. Sci. 258 (1991) 91; 269/270 (1992) 264. 151 M. Ohno and P. Decleva, Chem. Phys. 156 (1991) 309. f6] P. Decleva and M. Ohno, Chem. Phys. 160 (1992) 341, 353; 164 (1992) 73. [7] P. Decleva and M. Ohno, J. Chem. Phys. 96 (1992) 8120. [8] M. Ohno and P. Decleva, Chem. Phys. 169 (1993) 173. 191 0, ~j~rneholm, A. Sandell, A. Nilsson, N. MIrtensson and J.N. Andersen, Phys. Ser. T41 (1992) 217, A. Sandell, 0. Bjiimeholm, A. Nilsson, J.N. Andersen, B. HernnIs and N. M&-tensson, Phys. Rev. B, submitted.
1101A. Sandell, 0. Bjiirneholm, A. Nilsson, E.O.F. Zdansky, H. Tillborg, J.N. Andersen and N. MCtensson, Phys. Rev. Lett., submitted; also published in: 0. Bj~rneholm, PhD Thesis, Uppsala University, 1992, Acta Univ. Ups. 367 (Almqvist and Wiksell, Stockholm). 1111 U. von Barth and G. Grossmann, Solid State Commun. 32 (1979) 645, Phys. Rev. B 25 (1981) 5150. D21 Y. Jugnet, F.J. Himpsel, Ph. Avouris and E.E. Koch, Phys. Rev. Lett. 53 (1984) 198. [131G. Wendin and M. Ohno, Phys. Ser. 14 (1976) 148. I141 E. Riihl and A.P. Hitchcock, J. Am. Chem. Sot. 111 (1989) 2614. (1510. Bjiirneholm, A. Nilsson, E.O.F. Zdansky, A. Sandell, B. Hernnis, H. Tillborg, J.N. Andersen and N. Mlrtensson, Phys. Rev. B, submitted; also published in: 0. Bjiirneholm, PhD Thesis, Uppsala University, 1992, Acta Univ. Ups. 367 (Almqvist and Wiksell, Stockholm). Ml L. Ungier and T.D. Thomas, Chem. Phys. Lett. 46 (1983) 247. Surf. Sci. 1171A. Sandell, A. Nilsson and N. Mlrtensson, Lett. 241 (1991) Ll. ll81C.W. Bauschlicher, Jr. and P.S. Bagus, J. Chem. Phys 80 (1984) 944. 1191W. Wurth, D. Coulman, A. Puschmann and D. Menzel, Phys. Rev. B 41 (1990) 12933, and references therein. l201G. Illing, T. Porwol, I. Hemmer~ch, G. Dijmijtijr, H. Kuhlenbeck, H.-J. Freund, C.-M. Liegener and W. von Niessen, J. Electron Spectrosc. Relat. Phenom. 51 (1990) 149. 1211M. Ohno, Phys. Rev B 45 (1992) 3865. 6% W. Wurth, C. Schneider, R. Treichler, E. Umbach and D. Menzel, Phys. Rev. B 35 (1987) 7741. [231J.C. Tracy and P.W. Palmberg, J. Chem. Phys. 51 (1969) 4852. 1241J.C. Tracy, J. Chem. Phys. 56 (1972) 2748. D51 J.C. Tracy, J. Chem. Phys. 56 (1972) 2736. P61 N. Wassdahl, A. Nilsson, T. Wiell, H. Tillborg, L.-C. Duda, J.H. Guo, N. Martensson and J. Nordgren, Phys. Rev. Lett. 69 (1992) 812. D71 A. Nilsson and N. Mgrtensson, Phys. Rev. B 40 (1989) 10249. B81 M. Ohno and W. von Niessen, Phys. Rev. B 42 (1990) 7370; B 45 (1992) 9382.
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On the resonantly core excited states of the NJNi and related systems M. Ohno and P. Decleva Dipartimento
di Scienze Chimiche,
Universitri di Trieste, via A. Valerio 38, I-34127 Trieste, Italy
Received 22 March 1993; accepted for publication 6 July 1993
The 1s to n* resonant excitation oscillator strengths and energies of the inner and outer N atomic sites of NiN, have been calculated by the 2h2p/2h2p CI scheme, using the core excited state SCF relaxed orbitals. A comparison is made with previous theoretical results obtained by using the same CI scheme but with the neutral ground state orbitals or the core ionized state SCF relaxed orbitals. The oscillator strength ratio for the inner and outer N atoms is reversed in comparison to previous results. The strength reflects the empty N2p occupation of the n* orbital at the excitation atomic site in the ground state. Use of the core ionized state relaxed orbitals fails because of the larger core relaxation in comparison to the core excited state. The influence of the core relaxation in the resonantly excited state on the spectral features of the DES spectra with variations of adsorbate/substrate systems is also discussed.
1. Introduction
Recently we have performed ab initio lhlp/ lhlp (one hole one particle excitations for both ground and excited states) and 2h2p/2h2p CI (configuration interaction) calculations of 1s NEXAFS (near edge X-ray absorption fine structure spectroscopy) spectra of NiCO and NiN,, using an extended basis set [l-3]. Our many-body calculation gives a fairly detailed description of the low lying excitation spectral features, including the Rydberg-derived levels of NEXAFS spectra of CO/Ni and N,/Ni systems. Recently we proposed the cooperative core hole screening mechanism [4-81 to give a systematic description of the strong changes in XPS core spectral features with different adsorbate/ substrate systems. By this mechanism the screening is associated not only with the r metal-ligand charge transfer (CT) but also with s-da promotion and hybridization in the local metal u and u* orbitals. So the XPS core hole spectral features such as the presence or disappearance of the giant shakeup satellite depend on the s-d population of the local metal ground state. We explained the 0039-6028/93/$06.00
absence of the giant shakeup satellites in the NEXAFS spectra in terms of the hindrance of the cooperative core hole screening mechanism by the resonantly excited r* electron [l-3]. Recently the Uppsala group has studied DES (deexcitation electron spectroscopy) spectra of the N,/Ni system with excitation energy variations [9,10]. They have deconvoluted the NEXAFS Is to r* spectrum of N,/Ni as a superposition of N, and N, spectra (N, and N, denote the inner and outer N atoms, respectively). They obtained a relative spectral intensity ratio of 0.7: 1.0 for N, and N,, respectively. This disagrees with our previous prediction of 1.0 : 0.8 [l-3]. For NiN, use of the core ionized SCF relaxed orbitals employed in the former work may have lead to an inaccurate f-value (oscillator strength) because of different core relaxation between the core ionized and core excited states. The large difference in the results obtained even at the 2h2p CI level for the resonant excitation in NiN, points out the importance of an optimal choice of the orbital set if converged results are to be obtained at practical excitation levels. Therefore in the present work we set out to obtain an accurate estimate of
0 1993 - Elsevier Science Publishers B.V. All rights reserved
88
h4. Ohno, P. Decleca / Resonantly core excited states of N2 /Ni and related systems
f-value and excitation energy for the Is to rr* excitation in NiN,. We employ the lhlp core excited state SCF relaxed orbitals instead of the neutral ground state and the core ionized state SCF relaxed orbitals, and push the CI excitation level to convergence. In the case of NEXAFS spectra of adsorbates, the screening mechanism is different from the XPS core hole spectra. The excess of r* charge by the resonantly excited electron hinders the increase in drr backdonation. Because of the increased electron-electron repulsion not only the x CT but also the s-da hybridization and promotion (local metal (T relaxation) are hampered. The excess of r charge could even give rise to “reversed” s-da hybridization and a promotion mechanism by which the s-population increases and the d-population decreases. This could give rise to a significant u to cr* shakeup satellite. However, in the resonantly excited state there seems to be no efficient mechanism which reverses the drr-r* CT, increasing the d-population so as to promote the reversed s-da mechanism. So the (T local metal shakeup excitation is much suppressed in the NEXAFS spectrum [l-3]. In order to study how the shakeup satellite features in the NEXAFS spectrum of adsorbate depend on the local metal s-d population, in previous work we used both the neutral ground state and the core ionized state SCF relaxed orbitals. The latter, particularly in the case of the N,/Ni system, mimic the initial state where the s-population is significantly depleted. We can then show that not only the shakeup satellite feature of the XPS core hole spectrum but also that of the NEXAFS spectrum depends on the s-d population. The smaller the s-population, the more enhanced are the u to u* shakeup satellites. In the present work we show that the relaxation of the occupied orbitals in the resonantly excited state is small enough that the one-electron picture is valid and the shakeup satellite intensity is negligible. Furthermore we shall shed some light on the relaxation in the resonantly excited state and its influence on the spectral features of the DES spectra. Finally we discuss the spectral behaviours of the DES spectra of CO/Ni and related systems with variation of the photon excita-
tion energies. This is necessary for the understanding of the more complicated spectral behaviours of N,/Ni.
2. Theoretical
Method
Finite basis set LCAO and nhnp CI schemes, amounting to a maximum of n excitations out of the ground state configuration, are employed as in previous work [l-3], to which we refer for computational details, mentioning briefly some points relevant to the present work. To obtain Is-rr * SCF orbitals proves particularly difficult for this system. We start from the converged Is- ’ core hole orbitals, and optimize first the rr* singly occupied level, keeping all the doubly occupied orbitals frozen. We then proceed with relaxing the metal u shell, which immediately produces a large change in the metal u orbital and loses d-participation towards the metal 4s. Then the full metal d-shell and N, 17~ and 5u orbitals are alternatively allowed to relax, while the remaining occupied orbitals are always kept frozen. In addition to the lhlp and 2h2p CI schemes defined in our previous work, to test convergence of the CI expansion for the single resonantly excited state, we employ a larger space, comprising all single and double excitations with respect to the resonantly excited configuration. This amounts to including a selection of 3h3p excitations with respect to the ground state reference. Also for the ground state, which is described by the same set of orbitals, we employ larger 3h3p spaces, with perturbative selection from the lhlp zeroth-order vector. The largest spaces comprise about 56000 configurations for the resonantly exTable 1 Theoretical [2] and experimental 1s to 7 * excitation [11,12] (in eV) and f-values for the N, /Ni system Excitation
Experiment Method A Method B Method C Method D
energy
f-values
N,
N,
N,
401.0 391.2 400.3 401.4 401.1
400.4 389.2 400.4 400.4 400.0
0.171 0.148 0.026 0.057
N, _ 0.205 0.029 0.024 0.041
energies
N, IN, ratio 1.42 1.20 0.20 0.92 0.71
M. Ohno, P. Decleua / Resonantly core excited states of N, / Ni and related systems
cited state and 25000 for the ground state. The ground state energy and populations are then taken from the 3h3p results relative to N, ls-r* SCF orbitals. Let us remark that lhlp CI, with a fixed lh core hole, is completely equivalent to a frozen core ASCF calculation.
3. Results and discussion 3.1. Resonant excitations for NiN,
In table 1 we give a summary of the previous results for NiN,. The lhlp CI scheme using the neutral ground state orbitals (method A) neglects the relaxation of the core hole and excited electron pair and the screening of the bare hole-particle Coulomb interaction. In this case the f-value reflects the ground state empty r* charge distribution on the excitation atomic sites. According to the final state rule [ll] the spectral line profile should be accurately obtained by a one-electron theory, provided that the dipole matrix elements are calculated from valence wavefunctions corresponding to the final state. Following this line, the lhlp/lhlp CI scheme using the core ionized relaxed orbitals (method C) was used. By the 2h2p/2h2p CI method, using the ground state orbitals (method B) one treats the hole and particle relaxation, the screening of the bare holeparticle Coulomb interaction and the lhlp shakeup excitations, in terms of the lhlp excitations in the presence of the lhlp primary excitation. For NiCO inclusion of such many-body effects by method B, or by employing relaxed orbitals (method C (lhlp/lhlp CI> and method D (2h2p/2h2p CD), significantly improves the f-values, obtaining good agreement with experiment. Table 2 Theoretical and experimental 1s to rr* excitation energies [11,12] (in eV) and f-values for the N, /Ni system Excitation energy
Experiment
SCF 2h2p/2h2p CI
f-values
N,
N,
N,
N,
401.0 401.8 400.8
400.4 401.8 399.8
0.0936 0.1018
0.1002 0.1067
Nb 1% ratio
1.42 1.07 1.05
89
Table 3 Mulliken population analysis of NiN, State
SCF ground state 2h2p ground state N, 1s --t rr * (SCF) N, 1s + rr * (SCF) N, 1s + rr * (2h2p) N, 1s + ,rr* (2h2p) N,ls-’ (SCF) N, Is-’ (SCF)
NIP
Ni
NE!
N,
3d
4s
2.088 2.067 3.120 1.918 3.063 2.OOQ 2.786 1.732
1.853 1.941 1.775 3.037 1.842 3.002 1.614 2.801
8.713 9.030 8.657 9.051 8.747 9.034 9.343 9.215
1.487 0.880 1.666 1.000 1.484 0.920 0.170 0.158
However, for NiN,, the relaxation causes a large f-value reduction, inducing large shakeup satellite intensities (which are not observed). It also reverses the f-value ratio between the N, and N, to 1: 0.8, with the exception of method B, where the ratio is much diminished. In table 2 we summarize the 1s to r* resonant excitation energies and f-values of NiN, obtained by the ASCF (lhlp CI) method and the largest CI scheme (which we will call for brevity 2h2p/2h2p CI>, using the resonantly core excited state SCF relaxed orbitals. In table 3 we summarize the Mulliken population analysis of the ground state, the core ionized states and the core excited states. In table 4 we summarize the orbital overlaps between the ground state and core ionized (or excited) states. At the ASCF level the N, and N, resonant excitation energies are almost identical because of the near degeneracy of the Koopmans energies in the ground state (about 0.1 eV difference). The same is true with the previous results obtained by
Table 4 Overlaps between the ground state and core ionized or resonantly excited relaxed orbitals Overlaps (air> (ula*> (1irllP) (nlw) (rrlrr*)
N, (Is-‘)
;; -+ P*)
N, (Is-‘)
N, (1s + %-*I
0.7078 0.6799 0.9740 0.9608 0.2272
0.9984 0.9712 0.9943 -
0.6983 0.6890 0.9732 0.9171 0.3516
0.9810 0.9622 0.9794 -
(r, r are metal orbitals.
90
h4. Ohno, P. Decleva / Resonantly core excited states of N,/ Ni and related systems
method B (see table 1) which is quite similar to the ASCF method. With introduction of the higher-order valence excitations by the 2h2p CI scheme, the experimental excitation energies are well reproduced, including the N,-N, excitation energy difference of 0.6 eV (which is due to the inequivalent N atoms), while the f-values obtained by the ASCF method and the 2h2p CI method are similar, except for a slight increase in the latter. The ratios N,/N, are 1.08 and 1.05, respectively, which values are slightly reduced from the empty N2p population ratios in the ground state (1.12 and 1.07, respectively). The experimental value proposed by the Uppsala group is 1.42 [9,10]. The increase of the N2p population at the excitation site from the ground to the core excited states obtained by the ASCF (2h2p CI> method is 1.032 (0.997) for N, and 1.184 (1.061) for N,, respectively. This gives an N,/N, ratio of 1.15 (1.06) which is close to the empty N2p ground state population ratio. The relaxation of the occupied orbitals in the resonantly excited states is small enough that the one-electron picture is valid and the shakeup satellite intensity is negligible. So the reduction of the f-value due to the shakeup excitations is negligible. In comparison to the previous result obtained by using the core ionized state SCF relaxed orbitals, there is a dramatic change not only in the f-values but also in their N,/N, ratios. The fvalue ratio is reversed now and the present result indicates that the f-value represents the N2p population of the empty r* orbital in the ground state (the sum rule [ll]). The main cause of this large discrepancy is the use of the core ionized orbitals instead of the core excited orbitals. The N, 2p population in the N, core ionized state is slightly larger than the N, 2p population in the N, core ionized state, in contrast to the reversed situation in the neutral ground state. So one can expect a smaller f-value for the N, excitation when using the core ionized relaxed orbitals. Furthermore, the core ionization leads to a dramatic change in the local metal u orbital (as seen in both the local metal s-d population change and the orbital overlap of the (+ orbital). Consequently, this leads to a large shakeup satellite
Table 5 The estimated adsorbate/substrate resonantly excited states (in eV)
Cls 01s N,ls N, 1s
binding
energies
CO/CU(100)
CO/Ag(110)
CO/Ni(100)
CO/Pd(100)
0.5 1.1
( - 0.25) (0.15)
1.2 2.0
1.0 2.3
in the
N,/Ni(loo)
0.4 1.0
intensity and an f-value reduction. So at the 2h2p CI level, use of the core ionized state relaxed orbitals reproduces well the resonant excitation energies but not the f-values. In the resonantly core excited state, there is a small change in the occupied rr and u orbitals. However, the change is smaller in the N, excited state than in the N, one because the adsorbate/ substrate coupling strength in the former state is smaller than in the latter state (by 0.6 eV, see table 5 and later discussion). The adsorbate/ substrate coupling in the N, excited state is the same as that in the ground state so the changes in the occupied orbitals should be small. The s-d population in the N, resonantly excited state shows a small s-population increase and d-population decrease in comparison to the ground state. This possibly induces the reversed s-da promotion (hybridization) mechanism, increasing slightly the u repulsion and reducing the rr backdonation (weaker coupling). On the other hand the s-d population in the N, resonantly excited state shows a substantial decrease in s-population and increase in d-population. This still indicates the presence of the s-da promotion mechanism which slightly increases the dr backdonation (stronger coupling). The increased electron-electron repulsion due to the presence of the resonantly excited r* electron hinders the dr backdonation and consequently the s-da promotion mechanism. ,So the dr backdonation in the resonantly excited state is much smaller than in the core ionized state. In other words the adsorbate/ substrate coupling in the resonantly excited state is much weaker than in the core ionized state (see also section 3.4.3).
M. Ohno, P. Decleua / Resonantlycore excitedstatesof N2/Ni and relatedsystems
3.2, Extra-molecular relaxation The 1s to r* resonant excitation energy shifts between the free and adsorbed molecules are small. Following the discussion given by Jugnet et al. [12], we consider the resonant excitation energy shift within the framework of the present surface molecule approach. The 1s to r* resonant excitation energy for a free molecule is given by E,,,( free molecule) = E, - E,, - U( Is, a).
(1)
Here EIS, E, and U&a) are the Is core ionization energy, the r* affinity energy and the effective Coulomb interaction between the 1s hole and r* excited electron for a free molecule such as CO, respectively. For the resonant excitation energy of the adsorbate, we make the following assumptions: (i) The initial state energy shifts (chemical shifts) from a free molecule to a chemisorbed molecule are small. (ii) The intraligand relaxation energy shifts do not change from a free molecule to a chemisorbed molecule. (iii) The extra-molecular screening such as metalligand CT screening is localized at the core excited atomic site. Then, the excitation energy for a chemisorbed molecule is approximately given by the following equation, E,,,(adsorbate)
= (E, - A,) - (E,, + Al,) -[u(Is,
a) -Ais-Aa].
(2)
Here Ais and Aa are the extra-molecular relaxation energy shifts of the 1s and r* levels due to the rr metal-ligand CT screening. U will be reduced also by the extra-molecular screening, namely the drr to rr* CT hole-particle pair excitations induced by the r* excited electron and the 1s core hole which can be approximated by Aa and Als, respectively. (See ref. [13] for discussion on the case where the same kind of formalism is used to determine the effective hole-hole interaction in double hole excitations in atomic-like localized systems.) The closeness between the resonant energies of free and adsorbed molecules indicates that the approximation used for eq. (2) is valid. As an example we
91
consider the case of the CO/Ni sytem. The Als and Aa can be determined from free and adsorbed molecular data (see references and data in refs. [9,10]>. They are 4.1 (4.2) and 4.8 (4.8) eV for the carbon (oxygen), respectively. Then we obtain an U of 1.5 and 1.4 eV for the C and 0 1s to 7 * excitations, respectively. The experimental values are 1.4 and 1.7 eV, respectively. This shows that the shifts in the XPS (and IPES) are dominantly due to the relaxation effect and the chemical shifts are small. Riihl and Hitchcock [14] also reached the same conclusion in a different way. 3.3. Binding energy of the resonantly excited state The small core excitation energy shifts between the adsorbed and free molecules correspond to the adsorbate/substrate binding energy shifts between the resonantly core excited state and the neutral ground state. For a comparison of the resonant excitation energy between the adsorbed and free molecules, the Uppsala group [1.5] recently recommended the configuration average excitation energy of a free molecule rather than the singlet resonant excitation energy. As the dipole forbidden C 1s to 2~* triplet state transition of free CO was observed at 1.3 eV lower by EELS [16], they suggested 286.4 and 533.8 eV as the appropriate configuration average excitation energy for the carbon and oxygen of free CO, respectively. They argued that in adsorbates the exchange splitting is quenched due to the hybridization of the empty adsorbate orbital (r*) with the substrate band and this exchange splitting quenching is seen in the lack of any exchange splitting in the XPS core hole spectra of NO on Ni(100). Recent studies of NO on Ni(100) by XPS (together with XPD) and UPS [171 show one atomic and two molecular adsorption forms. NO adsorbs molecularly into two different geometries: at low coverage, the molecular one is lying down or highly tilted, while at saturation coverage, the molecular axes are oriented perpendicular to the surface. This agrees with theoretical predictions by Bauschlicher and Bagus [181. They studied the geometries and binding mechanism of NiNO by the CASSCF method and showed that there are two mechanisms for the
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bonding of NO to Ni: one leads to a linear system and the other to a bent system. For both the linear and bent models of the bonding, they showed that there is no open shell character on the NO and, consequently, there will be no multiplet splitting in the NO derived XPS spectra of chemisorbed NO. So the lack of exchange splitting in the XPS spectra of NO/Ni does not indicate the quenching of the exchange splitting in adsorbates. What one can expect upon chemisorption is that the mixing of adsorbate and substrate states delocalizes the free molecular orbitals. As a result, the splitting decreases because the smaller the spatial overlap of 1s and ?r * is, the smaller is the singlet-triplet splitting. Therefore, the splitting is larger for the C 1s excitation than for the 0 1s excitation [18]. We use the singlet excitation energy of a free molecule as reference energy. In table 5 we list the adsorbate/ substrate binding energies of resonantly excited states determined by the resonant excitation energy shifts. As already mentioned, the coupling strength in the N, core excited state is smaller than that in the N, core excited state. For adsorbed CO the coupling strength in the 0 excited state is larger than that in the C excited state. 3.4. DES spectra of adsorbates 3.4.1. Spectral behaviours with excitation energy variation
As there is a significant difference in the core relaxation between the core ionized and resonantly excited states, one may expect significant spectral feature differences between the Auger electron spectroscopy @ES) spectrum and the DES spectrum because their spectral features are expected to be governed by the valence wavefunctions in the core ionized state and the resonantly excited state, respectively. However, the DES spectra of adsorbates resemble the AES spectra much. All the autoionization features are found to move with the photon excitation energy, i.e. have a constant kinetic energy [9,10,19,201. As discussed already by several authors, the delocalization of the excited electron from the adsorbate to the substrate results in a state that could be
considered as locally identical to the XPS lowest energy state before the Auger core hole decay starts [9,10,19,211. This may explain the constant kinetic energy and similarity between the AES and DES spectra. However, Wurth et al. [22] observed a major spectral intensity change in the 0 Is DES spectra of different adsorbate/ substrate systems when the photon excitation energy is increased 6eV above the resonance energy, namely the spectral intensity decrease of the ?r backbonding peak, which they interpreted as due to the hampering of the 7~ CT from the metal by the electron resonantly excited to the Rydberg level. Recently, the Uppsala group [9,10] also studied carefully the DES spectra of several adsorbate/substrate systems with variations of the photon excitation energy. In the 0 1s DES spectra of the CO/Ni(lOO) system they observed notable spectral intensity variations depending on the excitation energy, already at the rr resonance maximum in comparison to the XPS ionization limit. However, no corresponding changes are observed in the Cls DES spectra except for the enhancement of a small structure at 263 eV which occurs at the excitation energy of 291 eV. The major 01s spectral intensity changes observed were the relative intensity variations of the two prominent peaks at the KE of 510 and 513.5 eV. They are interpreted mainly as 40~ ‘lY1 and respectively. The extra l?r -2 configurations, structure which appears at 512 eV at the excitation energy of 535.9 eV seems to remain in all spectra measured at larger excitation energies. Riihl and Hitchcock [14] determined the fvalue for the 1s to r* excitation in transition metal carbonyls. The reduction of the 1s to r* f-value from a free CO molecule to the transition metal carbonyls is smaller for 0 1s than for C 1s. This reflects the smaller coupling with the metal substrate for the oxygen excitation than for the carbon excitation. The Uppsala group reached the same conclusion by considering only the spatial closeness to the substrate of the carbon site in comparison to the oxygen site [9,101. Consequently, there are more significant spectral intensity variations in the 0 1s DES spectrum than in the C 1s DES spectrum, because a localized reso-
M. Ohno, P. Decleua / Resonantly core excited states of N2 /Ni and related system
nantly excited electron may influence the spectral intensity before it delocalizes to the substrate. Before we proceed further in the analysis of the DES spectra of the CO/Ni system, we would like to discuss the DES spectra of the CO/H/Ni(lOO) system. 3.4.2. DES spectrum of the CO/H/ Ni(lO0) system In contrast to the CO/Ni system, the 1s to r* resonant excitation energy of the CO/H/Ni system shows significant variations with different geometries, namely top, bridge and hollow sites [91. The C 1s to r* resonance energy shifts between adsorbed and free molecules are 0.0, -0.2 and -0.8 eV, respectively, for the top, bridge and hollow sites. For the 0 1s they are -0.4, - 1.3 and - 2.3 eV, respectively. To estimate the chemisorption energies of the CO/H/Ni(lOO) system in different geometries, we study the spectral behaviours of the DES spectra of the CO/H/Ni system with different geometries. The 0 1s DES spectrum of CO/H/Ni shows much more significant spectral feature changes than the C 1s DES spectrum with different geometries [93. The 0 1s DES spectra of the CO/H/Ni system of top, bridge and hollow sites resemble those of CO/Cu, CO/Pd and CO/K/Ni systems, respectively. As the chemisorption energies of the CO&u and CO/Pd are known (0.6 and 1.6 eV, respectively [23,24]), the adsorbate/ substrate binding energy of the 0 1s core resonantly excited state can be determined from the available spectroscopic data (for the CO/K/Ni system the chemisorption energy is unknown so we estimate it to be 2.0 eV). They are 1.1, 2.3 and 4.0 eV, respectively. Then, the chemisorption energies for CO/H/Ni top, bridge and hollow sites will be 0.7, 1.0 and 1.7 eV, respectively. Using the C 1s data we obtain 0.5, 0.8 and 1.9 eV, respectively. These results agree with the observation that the chemisorption energy increases with an increase of the coordination number and that coadsorption of H decreases the backdonation (the chemisorption energy of CO/Ni (top or bridge) is 1.3 eV 1251).With a stronger adsorbate/substrate coupling in the order of top, bridge and hollow sites, the bonding orbital is more polarized to-
93
ward the ligand (larger r backdonation). Consequently, the backbonding peak spectral intensity increases. This is noted also by the Uppsala group [9]. We note that with an increase of the adsorbate/substrate coupling the 1~ orbital (which is orthogonal to 2~ orbital) will be polarized more toward the oxygen and that the spectral intensity of the lrr-’ two hole state peak increases dramatically in the 0 1s DES spectra. So the adsorbate/substrate coupling strength in the resonantly excited state (or the ground state) can be monitored by observing the spectral intensity changes of the backbonding peak and the 1~~ peak in the 0 1s DES spectrum. 3.4.3. DES spectrum of the CO/Ni(lOO) system The Uppsala group [9] pointed out a similarity in the spectral intensity changes between 0 1s DES spectra of CO/Ni(lOO) with a photon energy increase from 531.8 to 533.4 eV and those of the CO/H/Ni(lOO) system with a decrease of the adsorbate coordination. The spectral intensity of the l,rr-2 peak decreases relative to that of the 4a- ‘1~~ ’ peak. As also pointed out by them, this similarity indicates that the resonantly excited electron tends to be more localized with the photon energy increase. The presence of localized electrons resonantly excited to the r* or Rydberg levels will hamper the dr to rr* CT from the substrate so that the polarization of the bonding orbital towards the ligand is hindered. Consequently, the spectral intensity for the backbonding peak and the 1Y2 peak will be reduced. In other words, the adsorbate/ substrate coupling strength in the core excited state is weaker than that in the core ionized state. The origin of the 512 eV peak in the 0 1s spectrum and the 263 eV peak in the Cls spectrum of CO/Ni(lOO) is considered as the excitation to the Rydberg level [91. However, it seems to be difficult to. explain why these peaks remain even at the excitation energies far above the Rydberg excitation energy. The two prominent peaks at 513.5 and 510 eV in the 0 1s spectrum are mainly due to the 1~~ and 4(5)a-‘1~~’ double hole configurations, respectively. Their BE is 18.7 and 22.2 eV, respectively. In the Cls spectrum there is a small peak at 271.8 eV (14.1
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M. Ohno, P. Decleva / Resonantly core excited states of N2 / Ni and related system
eV BE) for the CO/Ni system which is not seen for CO/Pd and CO/Cu systems. The Uppsala group interpreted this peak as the dv--r* CT shakeup satellite of 4e-1~;1 [9]. The BE of 4~ ‘rr;i is 11.3 eV (prominent peak seen around 275 eV). Then the r shakeup energy in the presence of 4u-‘7rG1 is 2.8 eV. The origin of the 262 and 512 eV peaks is probably due to the dr-a* shakeup core hole states. Their energies are 288 and 533.8 eV for C and 0, respectively (see the next section). In that case the expected final states are lW27r,‘27r* for 0 and 4(5)(+-l l~-%r,‘27r for C. The BE will be 21.5 and 25.0 eV, respectively; the KE 512 and 263 eV. This agrees well with experiment. These peaks are observed to remain in the AES spectra. This indicates that the origin of these peaks is the incomplete core hole relaxation, namely the drr to v* CT shakeup excitation associated with the main line 1s core ionization. 3.4.4. 0 2s XES spectrum of the CO / NitlOO) system
The recently measured 0 Is X-ray emission spectrum of the CO/Ni(lOO) system shows an interesting spectral feature in comparison to that of free CO [26]. The spectrum shows a strong satellite feature at 1.8 eV at the high energy side of the main line. The main line is observed at 524.8 eV. The satellite intensity is comparable to the main line intensity. The main line is associated with the transition from the 0 1s core hole state (BE is 532.2 eV) to the final 1~ hole state (BE is 7.5 eV). This gives a transition energy of 524.7 eV which agrees well with the experimental value. The authors of ref. [26] suggested that the 1.8 eV satellite is due to the Is core hole shakeup excitation. If the final state for the satellite is the same as that of the main line, the initial shakeup state must be 1.8 eV larger than the initial core hole main line state. The high resolution XPS Cls core hole spectrum of the CO/Ni system shows the satellite at 2.1 eV at the high energy side of the main line, while the 0 Is core hole spectrum does not show such a satellite [271. However, our recent many-body calculation predicts the presence of a rr to rr* shakeup satellite of a small intensity at 1.6 eV at the high energy
side of the main line [4,5]. For the shakeup state to be the initial state, the final state of both main line and satellite transitions must be the same. Many-body calculations show that the one electron picture of the 1~ ionization breaks down due to the strong configuration interaction between the 1~~’ and 1~-‘~;‘2’rr* configurations [28]. The final state of the X-ray emission process which involves the 11~.level is not the single hole state but most likely the 2hlp state, So the main line is due to the transition from Is-’ to 171’-lak127r* and the satellite is from Is-~~;‘27r* to the same final state. This explains at least the energy separation of 1.8 eV between the main line and the satellite line. Despite of a small XPS satellite intensity, how can it be that the satellite originating from the XPS shakeup satellite takes such a large intensity? If the interpretation of the final “lr-“’ state in terms of the lrr-‘x;‘2~* “screened state” is correct, we may observe that transition intensity will be much enhanced from the Is-%T;~~~* initial state (1.8 eV satellite), because of the single electron transition, in contrast to the two electron transition required for transitions from the initial Is-’ state. A different interpretation may be addressed in terms of final state effects, due to a breakdown of the one particle picture for the 1r-l level, as predicted by the calculation [28]. In this case the transition moment from the dominant Is-’ initial state will divide over different final states according to the weight of the la- ’ configuration. Although the calculations clearly show such effects, the quantitative prediction does not appear to fit precisely both the valence photoemission and the XES data. This might be due to the difficulty in obtaining precise numerical values in such a complex system. In any case a decisive choice between the two possible mechanisms could be afforded by XES measurements below and above the XPS shakeup energy. In the initial state mechanism the XES satellite is suppressed below the threshold, while it is not affected in the case of a final state effect. 3.4.5. DES spectrum of the N,/Ni system The spectral features of the N,/Ni system change with an increase of the excitation energy
M. Ohno, P. Decleva / Resonantly core excited states of N2 / Ni and related systems
[93 because of a superposition of the N, and N, DES spectra obtained at different excitation energies. So an analysis of the spectra becomes difficult, although the Uppsala group used the DES spectra to deconvolute the N 1s XAS spectrum into the N, and N, contributions [9,10]. (i) The AES spectrum obtained at the excitation energy far above the N, and N, ionization limit appears to be a superposition of the AES spectrum obtained at the N, and N, ionization limit, respectively, although there may be some extra contributions due to the incomplete core hole relaxation. (ii) With an increase of the excitation energy from the N, XPS ionization limit to the N, 7r* resonance maximum, except for the r backbonding peak (389 eV), the new spectral feature appears at the larger kinetic energy. Here probably the initial state is the resonantly excited state, but the final state is the two hole state. So the kinetic energy increases by the XPS-XAS energy shift for N, (1.0 eV). The spectral intensity of the r backbonding peak must be reduced in comparison to the AES spectrum because of the smaller polarization of the r bonding orbital in the resonantly excited state than in the core ionized state. This may result in the non-shifting of the 389 eV peak (dominance of the AES peak). (iii> With further increase of the excitation energy to the N, XPS ionization limit, the new spectral feature appears at the larger kinetic energy side, except for the rr backbonding peak. The energy shift is most likely due to the N, - N, XPS ionization limit difference (1.3 eV>. The adsorbate/substrate coupling in the N, core ionized state is weaker than that in the N, core ionized state. So the polarization of the bonding peak is less in the N, core ionized state than in the N, core ionized state. So the spectral intensity of the 7 backbonding peak of the N, AES spectrum is smaller than that of the N, AES spectrum. (iv) With a further increase of the excitation energy to the N, r* resonance maximum, a prominent spectral feature appears at smaller kinetic energy (379 eV>. This spectral feature may be interpreted as the autoionization spectrum. The autoionization kinetic energy is shifted from
95
the Auger energy by Uvra + Uvz, - UC,_+. Here U is the effective hole-particle Coulomb interaction, Vl, V2, c and a denote the valence hole Vl, V2, core hole and adsorbate level, respectively. The spectral intensity of the rr backbonding peak decreases because of a weaker adsorbate/ substrate coupling in the N, resonantly excited state than the N, core ionized state. The Uppsala group deconvoluted the N 1s XAS spectrum by monitoring the change of the peak height at 389 eV (for the N, atom) and that at 379 eV (for the N, atom) [9,10]. However, as mentioned above, there seem to be notable spectral feature changes within the r resonant excitation energy region. The Uppsala group assumed that the spectral feature does not change in the r resonant excitation energy region. This may have caused some errors in their deconvolution procedure.
4. Conclusion The 1s to IT* resonant excitation energies and f-values of NiN, are calculated by the lhlp or 2h2p CI method, using the core excited state SCF relaxed orbitals. The excitation energies are well reproduced by the present calculations, including the N, - N, energy difference of 0.6 eV which is due to the inequivalent N atoms. The excitation energy shift between the free and adsorbed molecules shows that the adsorbate/ substrate binding energy in the N, core excited state is the same as that in the ground state and smaller than in the N, core excited state by 0.6 eV. The f-values obtained follow closely the N2p population of the empty r* orbital in the ground state, as expected from the sum rule for the absorption process. The f-value ratio for the outer to the inner N atom excitation is 1.05, while the experimentally determined value is 1.42. This large discrepancy seems to be due to the error associated with the deconvolution procedure of the N 1s XAS spectrum of the NJNi system into the N, and N, contributions, by monitoring the spectral intensity changes in the DES spectra of N,/Ni measured at different excitation energies. The DES spectral intensities of the structures, the
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dominant configurations of which involve the orbitals of the R symmetry, are sensitive to the variations of the photon excitation energies because of the hindrance of the r backdonation due to the presence of the resonantly excited electrons to the V* or Rydberg derived levels. The DES spectra1 intensity changes by different adsorbate/ substrate systems can be used to study the adsorbate/ substrate coupling strength. As such an example, we considered the case of CO/H/Ni(lOO> system in different geometries. Finally, two alternative mechanisms for the appearance of the strong satellite in the XES spectra of the CO/Ni system are proposed, and a discriminating experiment is suggested.
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