Shifts in adsorbate vibrational frequencies due to internal electric fields

22 July 1994

CHEMICAL

PHYSICS t.k'TTrd~ Chemical PhysicsLetters 224 (1994) 576-580

ELSEVIER

Shifts in adsorbate vibrational frequencies due to internal electric fields Paul S. Bagus a, Francesc Illas b • IBMAlmaden Research Center, 650 Harry Road, San Josd, CA 95120-6099, USA Department of Chemical Physics and Material Science Centre, Universityof Groningen, 9747AG Groningen, The Netherlands b Departament de Qulmica Ftsica, Grupde Quimica Qulmtica, Universitatde Barcelona, C/Martt i Franqubs 1, 08028Barcelona, Spain

Received4 February 1994; in final form 10 May 1994

Abstzact

A new physical mechanism is proposed to explain the shifts in vibrational frequency of negatively adsorbed species on a metal surface. Ab initio cluster model calculations for NO adsorbed on Ag( 111 ) in two different orientations, N-down and O-down, suggest that the low-coverage HREELS peak appearing at 1282 cm - i is due to adsorbed NO in an O-down orientation. The large shift in vibrational frequency relative to free NO is shown to be due to the internal electric field produced by the negative charge on the adsorbate and the metal image charge. While external electric fields are known to produce vibrational shifts this is the first evidence that the same effect can be produced by internal electric fields which have their origin in the ionic nature of the chemisorption bond between NO and Ag( 111 ).

Recent high-resolution electron energy loss spectroscopy, HREELS, experiments for NO on Ag( 111 ) at low NO exposures show complex spectra. O f particular interest is the appearance of two peaks at 1153 and 1282 cm -~. These two energy-loss maxima were assigned to N - O stretching vibrations of nitrosyl adsorbed at the threefold site in a bent and a linear configuration respectively [ 1 ]. No peaks were found in the 1300-2000 c m - l region which is characteristic for ligand-related vibrations of transition metal nitrosyl complexes [ 2 ]. Even assuming that these two peaks are related to the N - O stretching it is difficult to understand why the N - O stretch frequency is so low. It is even smaller, by about 60 cm-1, than the vibrational frequency of free N O - [ 3 ]. In this Letter we will show that these vibrational features are consistent with NO adsorbed on Ag( 111 ) in a threefold site but in an O-down orientation. First, we will show that the bond between NO and Ag( 111 ) is highly

ionic regardless of the NO orientation. Then, we will show that only the vibrational frequency of the Odown orientation is consistent with the experimental results. Finally, and more importantly, we will show that the origin of the large shift in vibrational frequency is due to the internal electric field generated by the negatively charged adsorbed NO and the 'image charge' response of the metal surface. While external electric fields are known to strongly modify the vibrational frequencies of adsorbed species [ 4 ] there is no previous knowledge that the same effect may be produced by an internal electric field which arises as a consequence of ionic bonding. The threefold interaction of NO above Ag( 111 ) has been modelled by a finite Ag4 cluster plus an NO molecule. The Ag4 cluster has three metal atoms on the first layer and one on the second layer (Fig. 1 ); these atoms have the unrelaxed Ag( 111 ) geometry. The adequacy of this surface cluster model represen-

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P.S. Bagus, F. lilas / Chemical Physics Letters 224 (1994) 576-580

? Fig. 1. Schematic representation of the cluster model used to represent the threefold interaction of NO on Ag( 111 ).

tation to describe the geometries and frequencies of adsorhates above silver surfaces has been established in a previous study on the interaction of halogens on the Ag( 111 ) surface [ 5 ]. Here, we will use Ag4NO and Ag4ON to indicate N-down and O-down orientations, respectively. The NO is placed normal to the surface and all clusters have C3v symmetry. The Ag4 surface cluster model has a 3A2 electronic ground state with an e2 open shell occupation while the NO ground state is 211 with a n I (or e I ) open shell configuration. Coupling both open shells and assuming that a covalent bond is formed, the electronic ground state of both AgoNO and Ag4ON will be a 2E with a n e 3 open shell configuration. However, the interaction of NO with Ag( 111 ) can also occur through a charge transfer mechanism; an ionic bond is formed [ 6,7 ] and the interaction is between Ag~ and NO-. The electronic configurations involved are e t (2E) for Ag~ and e 2 (3A2) for N O - which may couple to either a 4E or a 2E state for the total cluster. In order to properly take into account both bonding mechanisms, we obtain ab initio multiconfignrational self-consistent field, MCSCF, cluster model wavefunctions for the lowest 2E electronic states of Ag4NO and Ag4ON. This MCSCF wavefunction permits the variational mixing of both ionic and covalent bonding situations. For N and O all electrons have been considered while for the cluster metal atoms, we explicitly consider the 4s24p64dm5s ~electrons and represent the more inner ones with a pseudopotenfial [ 8 ]. Extended basis sets of contracted Gaussian functions were used to describe the atomic orbitals of N, O and Ag, details about basis sets and pseudopotentials are given in Refs. [5,6]. For both Ag4NO and Ag4ON we obtain an energy curve for the perpendicular motion of NO and ON

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above the surface plane. In a first approach the NO distance was fLxed at 2.20 ao, close to the experimental equilibrium distance for free NO. From this potential energy curve we obtain the equilibrium geometry above the surface plane, z,, the vibrational frequency for motion perpendicular to the surface, o3e(Ag-NO), and the interaction energy with respect to the neutral separated fragments, D,. Once these structural parameters are known, the NO equilibrium distance is optimized to permit calculation of the NO stretch vibration. In Table 1 we report these values for both orientations of NO above the surface cluster model. The calculated values of o3c(Ag-NO) are similar for both orientations and are in agreement with the experimental value which is = 230 c m - ~ [ 1 ]. A detailed study of the interaction shows that, for both orientations, the bond is highly ionic although in the N-down case there is a small contribution to De due to ~ covalent bonding. However, all the theoretical measurements suggest that, in both orientations, the bonding mechanism is mainly ionic with NO chemisorbed as N O - [ 9 ]. In order to show that this is the case we will discuss the changes in z, introduced by a uniform external electric field. The idea behind this approach is simple: for a charged adsorhate one expects large changes in ze induced by the field whereas for a neutral adsorhate z~ is hardly affected; see for instance Ref. [5]. When a field of +0.01 au is applied to the Ag4NO cluster z¢ shifts outwards by 0.37 ao whereas it shifts inwards by 0.23 ao when a field of equal intensity but opposite sign is applied. This is a clear indication that the bonding mechanism is essentially ionic. Similar results are obtained when the Ag4ON cluster is considered. A further argument about the large ionicity of the interaction is given by Table 1 Calculated values for the equilibrium geometry above the surface plane, z~, the vibrational frequency for the normal mode perpendicular to the surface, ~,(Ag-NO), and the interaction energy,

Do Property

Ag(NO

A~ON

ze (ao)" oge(Ag-NO ) (cm -~ ) De (eV)

3.54 201 0.45

3.48 216 0.41

• z. is the Ag surface to N distance for AgtNO and the Ag to O distance for Ag40N.

P.S. Bagus, F. lllas / Chemical Physics Letters 224 (1994) 576-580

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the optimum r(N-O) distance for chemisorbed NO which is 2.33 ao or 2.51 ao for the N- and O-down molecular orientations, respectively. This optimum distance is larger than that of free NO (2.20 ao) and closer to that of free N O - (2.45 ao). The fact that the bond is ionic has important consequences; in particular, it is responsible for the shift of the NO stretch below or above that of free N O - depending on the NO orientation. We have computed the vibrational frequency of NO, o3c(N-O), for both N-down and O-down NO above Ag4 b y f i x i n g the NO center of mass near its value at zc and varying the N to O distance. The molecule to surface stretch is ~ 230 c m - ' while the NO internal stretch is in the range 1200-1400 c m - ' [ l ]. Thus, the two modes are only weakly coupled and our use of a coordinate for o3e(N-O) which neglects the mixing of the two Ag-NO and N-O stretches is a good approximation [ 10 ]; this also justifies the two-step approach used in obtaining the optimum z~ and r ( N O) distances. The calculated results for the internal stretch of NO, NO-, Ag4NO and Ag4ON are reported in Table 2. The good agreement between calculated and experimental [ 3 ] values for free NO and N O - suggest that our cluster model wavefunctions will also properly describe the shifts inog~ (N-O) for chemisorbed NO. From the calculated values, we see that only the O-down orientation leads to a value of o3e(N-O) which is consistent with the experimental HREELS data for low NO coverage of NO on Ag( 111 ). Hence, we suggest that the species responsible for the peak at 1280 c m - ' is NO chemisorhed at a threefold site but in an O~down molecular orientation! This point of view is also supported by recent local density functional calculations by Neyman and Rtisch [ 11 ] . Table 2 Calculated and experimental values for the vibrational frequency, We(N-O), of free NO and N O - , and of NO chemisorbed above the Ag4 cluster in the N-down and O-down orientations

calc. ( c m - ' ) exp. (cm -1)

NO

NO-

Ag4NO Ag(ON

2049 1904 •

1351 1363•

1546

1243 1280 b

• Ref. [3]. b From HREELS experiments at low coverage (Re£ [ 1] ).

From the preceding discussion it is clear that our cluster model calculations for NO on Ag4 in the Odown orientation exhibits a low stretching frequency which indeed agrees with the experimental result at low coverage [ 1 ]. Also, we note that at higher coverages of NO above both Cu [ 12 ] and Ag [ 1 ] surfaces a peak around 1800 c m - ' develops. This gives evidence for NO chemisorbed in the N-down orientation at higher coverages. The problem which still remains is the physical origin of this shift with respect to the vibrational frequency of NO-. In order to understand why the calculated vibrational frequency for the N-down orientation has a positive shift of + 195 cm- ' whereas the shift corresponding to the O-down orientation is 108 cm - ' , we notice that, as pointed out above, NO chemisorption on Ag( 111 ) leads to the formation of an ionic bond, NO being chemisorbed as NO-. Consequently, the surface polarizes in response to this charge and an image charge would appear in the metal. Both charges generate an internal electric field which may be responsible for the calculated vibrational shifts. To prove that the internally generated electric field between the polarized metal surface and the anionic, N O - , adsorbate is the mechanism which is the origin of the observed vibrational shifts, we use the simplest model which contains the essential physical features. This is done by replacing the Ag4 cluster model with a point charge of + I, denoted PC+; we consider the NO vibrational shifts for the PC +NO- and PC+ON clusters as models for NO/Ag and ON/Ag. The PC + is placed at 7.0 ao from the nearest atom in NO, either N or O, for R (N-O) = 2.20 ao and, as for the clusters with Ag4, the N and O positions are varied about this point leaving the NO center of mass fixed. The center of charge of NO- is approximately at the center of the molecule; thus with PC + at 7.0 ao from the nearest atom of NO, its distance from the NO- center of charge is ~ 8.1 a0. If we consider that the NO molecule is ~ 3.5 ao from the first, or surface, layer of Ag atoms for either orientation of the NO molecule, see Table 1, the position chosen for PC + corresponds to placing the image plane - 0 . 6 ao above the Ag( 111 ) surface plane. This is consistent with the usual procedure of placing the image plane above the plane of atoms forming the metal surface [ 12 ]; however, we do not wish to determine the precise position of the image plane. Our use of the image charge model is to

P.S. Bagus,F. lllas / ChemicalPhysicsLetters 224 (1994)576-$80 identify the main physical mechanism causing the shift of the NO stretch frequency. The result with the simple model which includes only electric field effects represented by an image charge is that for PC+NO -, we obtain a shift, Ao3c(PC), of + 59 cm - 1 whereas for PC+ON -, Ao3, (PC) = - 72 cm - 1. These shifts are large and comparable to the shifts obtained with the Ag4NO and Ag4ON clusters; they are approximately half of the values for the Ag4 clusters. When the position of the PC + is moved closer to NO, the Ao3,(PC) become larger; in fact, when PC + is placed in the surface layer, or 3.5 ao from NO, the Ao3, (PC) are considerably larger than the shifts obtained for Ag4NO and Ag4ON. The position of the image charge, PC + , and/or that of the image plane can be adjusted to give close agreement between the NO shifts with PC + and Ag4; however, this is not our goal. The simplified models of PC+NO - and PC+ON - clearly show that the driving force for the vibrational shift is the internal electric field generated by the ionic adsorbate, N O - , and the polarized surface, as represented by an 'image charge'. It remains to explain why the electric field shift of o3c depends on the orientation of the NO molecule with respect to the Ag surface. This can be most easily analyzed for the case of a uniform electric field normal to the surface with strength denoted E and to examine the first-order perturbation theory shifts [4,14 ] in the NO stretch frequency. These shifts depend on the dipole moment of NO -,/~, and the Taylor series expansion o f / , as a function of the N - O distance with respect to equilibrium, Ar(N-O), is taken as /z=Mo+MmAr(N-O)+M2Ar(N-O)2+...,

Ml = + 1.1 au and M2 = - 0.2 au. It is clear from Eq. (2) that changing the sign of Ml and M2 changes the sign of the shift and gives Ao~e< 0. The essential physics is direct. The shift of the vibrational frequency depends, to first order, linearly on the dipole moment and the sign of the dipole reverses when the orientation of NO changes by 180 ° between N-down and Odown NO. The non-uniform electric field that arises from the image charge and N O - will lead to a more complicated expression for Ao3ethan in Eq. (2) but still one where, to first order, the sign of Ao3, will depend on the sign of the dipole moment. In this Letter, we have shown that the low frequency assigned to the internal stretch mode of NO on Ag( 111 ) in a threefold site is consistent with NO chemisorbed in the O-down orientation. Hence, shifts in vibrational frequencies can be used as a fingerprint for the molecular orientation of an adsorbed molecule. We have shown that the origin of the vibrational shift is the electric field created by an ionic adsorbate and its image charge. While this effect is known for externally applied fields [4 ] it has not been previously recognized for internal electric fields caused, for example, by ionic adsorbates on a metal surface.

Acknowledgement This work has been carried out in the framework of a Joint Study Agreement between the IBM Almaden Research Center and the University of Barcelona.

(I)

where M~ is the dynamic dipole moment and M2 is the curvature. The first-order perturbation theory shift of o3ecan be written AtD,= C~ M, E + C2M2E+ O ( E 2) ,

579

(2)

where C! and C2 are functions of the constants in the N O - potential curve [4 ]. For an orientation of NO with O at positive z with respect to N which corresponds to N-down NO on a Ag surface, Mt = - 1.1 au and M2= +0.2 au; for these values of MI and M2, A~, > 0. Changing the orientation of NO and placing N at positive z with respect to O corresponds to Odown NO; this changes the sign o f / z and gives

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