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Oxygen vibrations in O–Ag(0 0 1)
Surface Science 530 (2003) 26–36 www.elsevier.com/locate/susc
Oxygen vibrations in O–Ag(0 0 1) David Loffreda a
a,1
, Andrea Dal Corso a,*, Stefano Baroni a, Letizia Savio Luca Vattuone b, Mario Rocca b
b,2
,
SISSA, Via Beirut 2/4, INFM-DEMOCRITOS National Simulation Center, 34014 Trieste, Italy b Dipartimento di Fisica – INFM, Unit a di Genova, Via Dodecaneso 33, 16146 Genova, Italy Received 4 September 2002; accepted for publication 2 March 2003
Abstract The vibrational modes of oxygen on Ag(0 0 1) are studied both theoretically, by density functional perturbation theory (DFPT) within the local density approximation, and experimentally, by means of high resolution electron energy loss spectroscopy (HREELS). The frequencies of the O–Ag stretch mode and of the parallel modes are calculated for the p(1 1), c(2 2), and p(2 2) geometries of oxygen on the non-reconstructed Ag(0 0 1) surface. Increasing the coverage the chemisorption energy as well as the O–Ag stretch frequency. The vibrational frequencies of O in pffiffiffi decreases pffiffiffi the 2 2 2 missing row reconstructed structure, the phase indicated by X-ray photoelectron diffraction as the low temperature phase of this system, are also investigated. The comparison of the DFPT calculations with the HREELS data gives further support to the existence of a missing row structure. 2003 Elsevier Science B.V. All rights reserved. Keywords: Density functional calculations; Chemisorption; Surface relaxation and reconstruction; Adatoms; Oxygen; Silver
1. Introduction Oxygen precovered silver surfaces present a great interest since they play the role of a precursor state for the epoxidation of ethylene [1,2], which is an important industrial catalytic reaction. For this reason, the interaction of oxygen with silver has been extensively studied experimentally and different states of oxygen at the surfaces have been
*
Corresponding author. Tel.: +39-40-378-7428; fax: +39-40378-7528. E-mail address: [email protected] (A. Dal Corso). 1 Present address: IRC-CNRS, 2 Avenue Albert Einstein, F-69626 Villeurbanne, France. 2 Present address: Institut fur Experimental Physik, Freie Universitaet Arnimalle 14, Berlin, Germany.
identified: a physisorbed molecular state, which is very close to the gas phase structure, chemisorbed molecular species, surface and subsurface atomic adsorption. High resolution electron energy loss spectroscopy (HREELS) has been extensively used to characterize these adsorption states together with low energy electron diffraction (LEED) [3] and other spectroscopic techniques. Actually, experimental studies exist for oxygen interacting with Ag(0 0 1) [4–14], Ag(1 1 0) [15–19] and Ag(1 1 1) [16,20–22] surfaces. However, the interpretation of the observed vibrational spectra and the determination of the nature of the adsorption state at given coverage and temperature are not always a simple task. Atomic oxygen adsorption on the Ag(0 0 1) surface is an interesting case, since the determination of
0039-6028/03/$ - see front matter 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0039-6028(03)00382-0
D. Loffreda et al. / Surface Science 530 (2003) 26–36
the oxygen coverage is difficult and the interpretation of the LEED patterns and of the HREEL spectra is still debated. Disordered adsorption into two states has been mentioned 25 years ago [4]; one state being populated at low temperature and the other one at high temperature. More precise HREELS and LEED measurements made by Fang suggested that an ordered c(2 2) superstructure occurs at 180 K, associated with a oxygen stretching mode at 37 meV [5]. Instead, adsorption at room temperature leads to an energy loss peak at 30 meV and to a p(1 1) LEED pattern. Furthermore, Fang reported a structural transition from the c(2 2) to the p(1 1) structures when heating the chemisorbed surface from low to room temperature, while the cooling of the chemisorbed surface from room temperature down to 180 K leads to a transition from the p(1 1) LEED pattern to the coexistence of both p(1 1) and c(2 2) structures. Both LEED patterns and HREELS peaks have been associated with atomic oxygen adsorption at fourfold hollow sites. More recently some of us presented a combined LEED, HREELS, X-rays photoemission spectroscopy (XPS) and photoelectron diffraction (XPD) study of the system [6]. We found that partial dissociation occurs already at a temperature T ¼ 130 K, at which we observe coexistence with molecular adsorption. The adatom–surface vibration reads then 30 meV. When heating the crystal to 190 K, it moves to 36 meV, while the O(1s) binding energy remains unchanged at 530.3 eV (O530 in the following). p XPD ffiffiffi pdata ffiffiffi have been interpreted by assuming a 2 2 2 missing row reconstruction with the oxygen sitting in the former fourfold hollows next to the missing rows. Upon heating above room temperature, the surface converts reversibly into a p(1 1) structure above the fourfold hollows. with O-adatoms 0.6 A The vibrational frequency reads between 28 and 32 meV, depending on coverage, while the O(1s) binding energy is 528.3 eV (O528 phase in the following). Subsurface incorporation of oxygen on Ag(0 0 1) could also occur [13]. Subsurface oxygen has been advocated in order to explain a peak at 130 meV in the HREEL spectra. Initially assigned to a subsurface oxygen vibration, this peak is now
27
thought to be due to an electronic excitation that becomes available thanks to the presence of a subsurface oxygen atoms [8]. The hypothesis of an electronic transition is supported also by the unusual shift from 131 to 134 meV [8,13] of the frequency with the isotopic substitution of 16 O with 18 O. In spite of such a thorough experimental investigation, few theoretical studies have been reported so far for adsorbed oxygen on silver surfaces [23–34], and especially on Ag(0 0 1). In a previous paper [35] some of us have studied within density functional theory [36,37] in the local density (LDA) and the generalised gradient (GGA) approximations, the geometry and energy of several phases of O adsorbed on Ag(0 0 1). In particular pffiffiffi we pffiffiffihave found that the low temperature 2 2 2 missing row reconstructed surface determined by the fitting of the XPD spectra, is competitive in energy with the adsorption on the non-reconstructed surface in the c(2 2) structure. It is the purpose of this paper to investigate the vibrational properties of the different phases of O on Ag(0 0 1) both theoretically and experimentally, with the main goal of identifying the signature of the missing row reconstruction in the vibrational spectra. The missing row reconstruction of the substrate lowers the C4v symmetry of the c(2 2) structure to C2v . The symmetry lowering brings about non-degenerate frustrated translations and makes the out of phase vibration of the oxygen atoms, in the direction parallel to the surface and perpendicular to the missing row, dipole active. The identification of this mode, which is totally symmetric with respect to the C2v group, would be a direct proof of the presence of the missing row reconstruction. Unfortunately, we find that it is hardly visible in HREELS experiments in the dipole scattering regime, indicating a small dynamic dipole moment. On the contrary, out-of-specular measurements (impact scattering regime) allow the detection of other frustrated translations, the frequencies of which agree well with the theoretical predictions for the missing row reconstructed substrate. Therefore our investigation further supports the interpretation of the XPD data. Furthermore, we investigated the vibrational modes of oxygen adsorbed on the hollow site of
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D. Loffreda et al. / Surface Science 530 (2003) 26–36
the non reconstructed surface. Since the oxygen coverage on the non-reconstructed surface is not known precisely, we calculated the dynamics of the p(1 1), the c(2 2) and the p(2 2) structures at coverage 1, 0.5 and 0.25 ML respectively. This paper is organized as follows. In Section 2 we report the methods used for the calculation of the vibrational frequencies; in Section 3 we describe the experimental apparatus. In Section 4 we summarize the main theoretical results regarding the structure and energy of the different phases of O on Ag(0 0 1), in Section 5 we discuss the vibrational properties and in Section 6 we compare them with experiment. Finally, Section 7 contains our conclusions.
2. Computational details The electronic structure calculations discussed here have been carried out within the LDA with the parameterisation of Perdew and Zunger [38]. The ab initio vibrational frequencies have been computed in the framework of density-functional perturbation theory (DFPT) [39–42], as implemented in the PWSCF and PHONON package [43]. Our calculations of the vibrational frequencies are limited to the LDA since, as shown in the previous paper [35] there are not major structural differences if calculations are performed at the GGA level, with the exception of the p(1 1)-O phase for which only LDA gives a stable surface hollow site. We used a plane-wave basis and Vanderbilt ultrasoft pseudopotentials [44] as described in [35]. Kinetic energy cut-offs are 30 Ry for the wave functions and 300 Ry for the charge density. The surfaces have been modelled by 7 layer slabs and an oxygen adlayer on one side of the slab, with four different periodic structures and various coverages (Fig. 1): a full coverage structure p(1 1)-O (1.0 ML), two mediumpcoverage ffiffiffi pffiffiffi structures c(2 2)-O (0.5 ML) and a 2 2 2-2O (0.5 ML), and one low coverage cell p(2 2)-O (0.25 ML). A vacuum of four equivalent ideal bulk layers separates one slab from its periodic image in the direction perpendicular to the surface. The bulk lattice parameter calculated at the LDA level, , has been used as the inand optimised to 4.018 A
Fig. 1. Periodic adsorption structures for O atom on Ag(0 0 1) surface; for the non-reconstructed surface, we show the full coverage p(1 1)-O (1.0 ML), the medium coverage c(2 2)-O (0.5 ML) and the low coverage p(2 2)-O (0.25 ML) structures. For surface, we show the medium coverage pffiffiffi thepreconstructed ffiffiffi 2 2 2-2O (0.5 ML) structure (the oxygen atoms are presented in small dark balls and the Ag atoms close to the missing-row are denoted Agm , while the other surface Ag atoms are denoted Agi ).
plane lattice spacing. The Brillouin zone (BZ) integration has been done with a 12 12 1 Monkhorst–Pack mesh [45] for the p(1 1) cells, corresponding to 21 irreducible k-points in the surface BZ. For thepc(2 ffiffiffi p2) ffiffiffi structure, the p(2 2) structure and the 2 2 2 structure we have used 7 7 1 (10 irreducible k-points), 5 5 1 (6 irreducible k-points) and 3 5 1 (6 irreducible kpoints) meshes respectively. Integration up to the Fermi energy is done with the smearing technique [46] with a smearing parameter of 0.04 Ry. The silver atoms of the six uppermost surface layers have been fully relaxed together with all the oxygen atoms. The minimization of the total energy and the optimisation of the geometries have been performed with the BFGS algorithm [47]. Residual forces do not exceed 104 Ry d/a.u. (3 103 eV/ ). Considering the fact that the highly coordiA nated fourfold hollow site is the most stable one [35], all the optimised structures have been built with oxygen atoms in the fourfold hollow positions. The vibrational frequencies of oxygen adsorbed on Ag(0 0 1) have been calculated by considering extended slabs of more than 75 layers. The interatomic force constants of these slabs have been
D. Loffreda et al. / Surface Science 530 (2003) 26–36
obtained from ab initio phonon calculations in the bulk Ag and from the dynamical matrices of 7 layer slabs. The vibrational modes of the bulk and of the 7 layer slabs have been calculated with DFPT [39–41]. This theory, as implemented in our codes for ultrasoft pseudopotentials, has been described in [42]. For the Ag bulk, the dynamical matrices have been computed in a 4 4 4 q-point mesh in the BZ and a Fourier transform has been used to obtain the interatomic force constants. For the slabs, only parts of the dynamical matrices at the C point have been calculated, including the displacements of the two uppermost Ag layers and O. With this information we have calculated the dynamical matrix at the C point of a finite extended symmetric slab with O adsorbed at both surfaces. We have calculated the total vibrational density of states (DOS) and we have identified the vibrational modes of oxygen from the displacement eigenvectors. The convergence of the frequencies and normal modes has been checked as a function of the slab thickness. About 20 layers are sufficient to reach complete convergence, but we have used much thicker slabs of more than 75 layers in order to obtain a smooth DOS curve calculated as a sum of Gaussian functions with a spreading of 5 cm1 . In the p(1 1)-O hollow structure, we have also calculated the interatomic force constants of the 7 layer slab via frozen phonon calculations with . A comdisplacements between )0.03 and 0.03 A parison between this technique and DFPT showed that the larger difference between the interatomic force constants calculated with the two methods is 2 ), indicating also 4 104 Ry/(a.u.)2 (0.02 eV/A the estimated accuracy of our calculations.
3. Experimental The adsorption experiments were carried out in an ultrahigh vacuum chamber equipped with a commercial HREEL spectrometer (SPECS, IbachÕs design), a LEED and all other typical vacuum facilities. The sample is a Ag monocrystal cut within 0.1 from the (0 0 1) plane and oriented with the [1 0 0] direction parallel to the scattering plane of the spectrometer. The surface was cleaned
29
by sputtering and annealing to 700 K before each experiment, until no trace of contaminants was detected in the HREEL spectrum. O2 was dosed through a shower placed in front of the sample, at a distance of 1 cm, in order to keep the background pressure below 2 107 mbar, while the effective pressure at the surface is at least one order of magnitude larger. HREEL spectra were recorded with primary electron energies Ee ¼ 3:0 and 15.0 eV and with an angle of incidence hi ¼ 60. The off-specular condition was obtained by rotating the analyser towards smaller scattering angles. The typical resolution achieved in-specular is 4 meV.
4. Energy and stability The stability of adsorbed atomic oxygen on Ag(0 0 1) and the geometrical structures of several phases have been presented in a previous paper [35]. Here we summarize the main results of [35] and add a few comments that will be relevant for the following discussion of the vibrational properties. The highly coordinated fourfold hollow site is the preferred adsorption site [35], and we summarize in Table 1 the chemisorption energies and the optimised geometries for various coverages. For the full coverage p(1 1)-O 1.0 ML structure, two different adsorption sites have been reported here: the surface fourfold hollow and the subsurface fivefold hollow. As we reported previously, the surface hollow is the most stable site [35]. The diffusion energy barrier between the surface and subsurface hollow sites has been estimated to 25 meV. At the full coverage, the diffusion of atomic oxygen from the surface hollow site to the subsurface one is therefore very easy. In the diffusion transition state between surface and subsurface sites, the oxygen lies in the Ag surface plane. For the medium coverage 0.5 ML, the two possible adsorption structures, compatible with the LEED pattern that we have considered are the c(2 pffiffi ffi 2)-O pffiffiffi and the missing-row reconstructed 2 2 2-2O structures. In Table 1, the chemisorption energy is reported for both structures. For the missing-row reconstructed structure the chemisorption energy is larger ()1.31 eV/O atom)
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D. Loffreda et al. / Surface Science 530 (2003) 26–36
Table 1 Chemisorption energy Echem per oxygen atom (eV), and optimised geometric parameters Coverage (ML) Echem (eV) ) dO–Ag (A ) DzO–Ag (A
p(1 1)
p(1 1)-Sub
c(2 2)
p(2 2)
1.0 )0.29 2.06 0.47
1.0 )0.27 2.05 )0.42
0.5 )1.11 2.15 0.75
0.25 )1.16 2.20 0.8
pffiffiffi pffiffiffi 2 2 2
0.5 )1.31 2.06–2.04 0.26 pffiffiffi pffiffiffi Full coverage structure p(1 1) (surface and subsurface hollow sites), the medium coverage structures c(2 2) hollow and 2 2 2 hollow, and the low coverage structure p(2 2) hollow; dO–Ag (A) is the distance between the oxygen atom and each equivalent surface ) is the distance between O and the mean position of the surface silver plane. silver atom, DzO–Ag (A
than the one for the c(2 2) configuration ()1.11 eV). However a detailed energy balance [35] shows that the reconstruction energy is very small. Therefore these two structures could both be observed at a coverage of 0.5 ML. The optimised geometries are somewhat different. For the c(2 2) is longer than for the case, the dO–Ag ¼ 2:15 A pffiffiffi pffiffiffi missing-row reconstructed 2 2 2 structure ), and also longer than the surface (2.04–2.06 A pffiffiffi ). For the 2 2 hollow p(1 1) situation (2.06 A pffiffiffi 2 structure the surface silver atoms are not equivalent; the Ag atoms along the missing row ), (Agm ) are closer to the oxygen atoms (2.04 A while the Ag atoms inside the surface layer (Agi ) lie above the medium surface plane and the O–Agi ). As the coverage distances are longer (2.06 A decreases at 0.25 ML, the adsorption energy becomes slightly stronger ()1.16 eV) and the O–Ag optimised distance is once more elongated (2.20 ). The structure determined experimentally by A fitting the XPD data is slightly different, since the lower oxygen atoms in the missing rows are 0.15 A than the neighbouring Ag atoms [6].
Fig. 2. Total DOS calculated within LDA. For the full coverage 1.0 ML p(1 1) non-reconstructed surface we present the case of oxygen adsorbed (a) on a surface hollow site and (b) on a subsurface hollow site. (c) DOS of the medium coverage 0.5 ML c(2 2) non-reconstructed surface with O adsorbed on the hollow site, (d) low coverage 0.25 ML p(2 2) non-reconstructed surface with O adsorbed pffiffiffi on pffiffiffi the hollow site, and (e) medium coverage 0.5 ML 2 2 2-2O missing-row reconstructed surface with O atoms adsorbed on hollow sites.
5. Vibrational modes of O on Ag(0 0 1) In this section we discuss the vibrational modes of the different phases of oxygen on Ag(0 0 1) found in the calculation. We start by discussing the vibrational modes on the non-reconstructed surface as a function of coverage and then compare these modes with those obtained for O on the missing-row reconstructed structure. In the highcoverage p(1 1)-O (1.0 ML) model structure, in LDA, both the surface and subsurface hollow sites are stable, as discussed above. In Fig. 2a, we show
the total density of vibrational states (DOS) of a 151 layer slab. The vibrational modes of the metallic surface are visible and give a continuous DOS between 0 and 180 cm1 . Furthermore, there are three modes with frequencies higher than 180 cm1 which is the maximum frequency for the LDA Ag bulk modes. The frequencies of these modes are reported in Table 2: two modes are degenerate and a mode is non-degenerate. The vibrational eigenvectors clearly identify these modes: the two degenerate modes are in-plane
D. Loffreda et al. / Surface Science 530 (2003) 26–36 Table 2 Vibrational frequencies (cm1 or meV) m1 m2 m3 m4 m5 m6
p(1 · 1)
p(1 · 1)-Sub
c(2 · 2)
p(2 · 2)
611 (76) 611 (76) 210 (26)
647 (80) 647 (80) 225 (28)
438 (55) 438 (55) 279 (35)
338 (42) 338 (42) 306 (38)
pffiffiffi pffiffiffi 2 2· 2 627 616 479 392 246 201
(78) (76) (59) (49) (30) (25)
Values in brackets denote frequencies in meV. Full coverage structure p(1 · 1) (surface and subsurfacephollow ffiffiffi pffiffiffi sites), the medium coverage structures c(2 · 2) and 2 2 · 2 hollow, and the low coverage structure p(2 · 2) hollow; we called m1 and m2 the in plane oxygen frustrated translational frequencies and m3 the oxygen-surface silver stretching frequencies pffiffiffi pffiffiffifor the p(1 · 1), c(2 · 2) and p(2 · 2) structures. For the 2 2 · 2 structure, m1 to m4 are the frequencies of the oxygen frustrated translational modes and m5 , m6 are the oxygen silver surface stretching frequencies. The experimental values are discussed in the text.
oxygen frustrated translations m1 and m2 at 611 cm1 (76 meV) and the other mode is the perpendicular vibration of oxygen against the surface m3 at 210 cm1 (26 meV). The frustrated translations are harder than the perpendicular motion since the oxygen atom sits in a hollow site close to the surface silver atoms (with an oxygen height of 0.47 ). The m3 frequency at 210 cm1 (26 meV) is not A to be compared with any experimental HREEL spectra, since the highest experimentally coverage is about 0.4 ML. The DOS for the Ag(0 0 1)–O system with subsurface hollow oxygen absorption, is shown in Fig. 2b: here the doubly degenerate frustrated translations m1 and m2 at 647 cm1 (80 meV) (see Table 2) are only slightly higher than the ones of the surface hollow case. The stretching oxygen–silver mode m3 is at 225 cm1 (28 meV). This result gives an estimate for the frequency one should expect for a vibration of an oxygen in the subsurface hollow site. Even if in this model system the concentration of subsurface oxygen is extremely high and a clear comparison with experiment is difficult, this result leads to the conclusion that the experimental peak at 130 meV [13] has indeed another origin. Frequencies which can be compared to the experimental HREELS peaks are instead found for the medium c(2 2)-O (0.5 ML) and the low p(2 2)-O (0.25 ML) coverage structures in the
31
non-reconstructed Ag(0 0 1) surface. In Fig. 2c and d we report the vibrational DOS for these two structures. In the c(2 2) structure the DOS due to the Ag(0 0 1) slab is comparable to the sum of the C and M point DOS of the p(1 1) structure, while in the p(2 2) structure (Fig. 2d), the DOS can be compared with the sum of the DOS of the phonons at the C þ 2X þ M points of the p(1 1) BZ. In addition to these Ag modes, in Fig. 2c we can clearly identify three additional peaks. The low frequency peak at 38 cm1 (5 meV) is the Rayleigh mode vibration. The experimental value of the Rayleigh frequency at the M point of the clean surface is 92 cm1 (11.5 meV) 3 and therefore it is strongly weakened in the c(2 2) structure. The weakening of the Rayleigh mode frequency can be explained mainly with the weakening of the Ag bonds between the first and the second layers due to the charge transfer to the electronegative oxygen atoms. In Table 2 we report the frequencies of the two peaks which correspond to the oxygen vibrational modes for the c(2 2) and the p(2 2) structures. The frustrated translational modes m1 and m2 become softer, 438 and 338 cm1 (55 and 42 meV) respectively for 0.5 and 0.25 ML (Fig. 2c and d), and the stretch frequency m3 of oxygen against the surface increases to 279 and to 306 (35 and 38 meV respectively). As the coverage decreases the O–Ag distance increases and the height of the oxygen atom on the surface increases. Therefore the trend in the frequencies is in agreement with what one expects just projecting the Ag–O interatomic forces in the direction parallel or perpendicular to the surface. Let us now consider the effect of the missingrow reconstruction on the vibrational frequencies of O on Ag(0 0p 1).ffiffiffi We pffiffiffishow in Fig. 2e the total DOS for the 2 2 2-2O structure. The DOS due to the Ag surface is comparable with the sum of the C þ 2M 1=2 þ M point of the p(1 1) surface (we called M 1=2 the point 2p=a0 ð0:25; 0:25Þ). Other six modes are visible above the bulk states. The vibrational frequencies of these modes are
3 According to Bunjes et al. [48]. It is 83 cm1 according to Chen et al. [49].
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D. Loffreda et al. / Surface Science 530 (2003) 26–36
c(2 2)) but, due to the missing silver atom in the missing row it is much softer than the m1 and m2 modes of the full coverage p(1 1) structure. The other two modes m1 and m2 at 627 and 616 cm1 (Fig. 3a and b) correspond to in-phase and out-ofphase vibrations of the oxygen atoms in the direction parallel to the missing row, respectively. These frequencies are comparable with the ones calculated for the high coverage 1.0 ML structure, since the oxygen atoms lie close to the surface plane in both cases. Oxygen stretching against the surface gives rise to two separate frequencies since the two oxygen atoms can oscillate in-phase or out-of-phase. The first case corresponds to the frequency m5 at 246 cm1 (30 meV) (Fig. 3e). The out of phase vibration corresponds instead to the frequency m6 at 201 cm1 (25 meV) (Fig. 3f).
6. Comparison with experiment
Fig. 3. Schematic description of the six oxygen vibrational normal modes; four frustrated translation modes m1 to m4 respectively at (a) 627, (b) 616, (c) 479 and (d) 392 cm1 and two stretching O–Ag silver modes m5 and m6 at (e) 246 and (f) 201 cm1 .
reported in Table 2. The analysis of the vibrational eigenvectors (see Fig. 3) shows that these modes are vibrations of the two oxygen atoms contained in the unit cell. Four modes (from m1 to m4 ) are frustrated translations at 627, 616, 479 and 392 cm1 (78, 76, 59 and 49 meV) respectively and two modes are the perpendicular vibration of oxygen against the surface: m5 and m6 at 246 and 201 cm1 (30 and 25 meV) respectively. The softer frustrated translation m4 at 392 cm1 (49 meV) corresponds to an out-of-phase vibration of both oxygen atoms towards the surface silver missing row (Fig. 3d). The corresponding in-phase frustrated translation mode m3 at 479 cm1 (59 meV) (Fig. 3c) is harder than the non-reconstructed c(2 2) case, since the oxygen atoms sit closer to the surface pffiffisilver ffi pffiffiplane ffi respectively for 2 2 2 and (0.26 and 0.75 A
pffiffiffi pffiffiffi In-specular HREEL spectra for the 2 2 2 and for the high temperature p(1 1) phases are reported in Fig. 4. The O530 layer (lower spectrum) was produced by dosing 75 L of oxygen at 87 K and annealing to 190 K. The surface was then cooled down to 90 K to record the spectrum. The O528 layer was produced by exposing the crystal to 5400 L of O2 at T ¼ 320 K. The spectra look quite different: a single, relatively broad, peak at 31 meV has present for O528, while three modes at 29, 39 and 67 meV are observed for O530 (the peak visible at 130 meV is not due to a vibrational exitation [8] and is not discussed in the following). The features at 31 meV (O528) and 39 meV (O530) are dipole active and correspond to the O–Ag stretch vibration for the two phases, respectively. We note that the frequency of 39 meV obtained for the O530 phase is at variance with that of 36 meV reported in [6] for the same surface, probably due to the different coverage. The prediction of the theory for the O530-Ag stretch is 30 meV, lower than the measured one, while it varies from 26 to 38 meV for the O528 depending on the coverage. According to our estimates the coverage in this experiment is quite low and therefore the 31 meV peak has to be compared with the O–Ag stretch calculated for the c(2 2) (35 meV) or the p(2 2)
D. Loffreda et al. / Surface Science 530 (2003) 26–36 2.0
31
E e=15.0 eV O528
1.5
Intensity (Arb. Units)
x1000 39 1.0
O530 0.5
29 67
130
x250 0.0 0.00
0.04
0.08
0.12
Energy loss (eV) Fig. 4. In-specular HREEL spectra of the missing row reconstructed (bottom spectrum) and of the unreconstructed oxygen phases (top spectrum). We note the presence of a peak at 130 meV for O530, the nature of which is discussed in the text.
(38 meV) structures. Therefore, the quantitative comparison of the O–Ag stretch frequencies shows only a fair agreement. Slight discrepancies were also found in [35] comparing the theoretical geometry with the one determined by the experimental XPD data. Possible reasons for these discrepancies are discussed in more detail in [35] and will require further investigations, possibly with the inclusion of additional effects (such as temperature or disorder) in the calculation. The variation of the O–Ag stretch frequency with the coverage is instead qualitatively correct since in experiments in which the O528 was produced by dosing at low temperature and annealing, we actually found lower stretch frequencies the higher the initial coverage. In-specular, the spectra show also two additional peaks (29 and 67 meV) for the O530 phase that are not present in the O528 phase. The theory
33
predicts a doublet in the low energy range for the reconstructed surface and the 29 meV peak is explained as the out of phase stretch of oxygen against the surface. The frequency of this mode is calculated at 25 meV. The 67 meV is not dipole active and is well explained by theory, as we will discuss below. On the Cu(0 0 1) the presence of the oxygen induced missing row reconstruction can be inferred from the HREEL specular spectra [50]. The out-of-phase in-plane vibration of the oxygen atoms perpendicular to the missing row (m4 mode) becomes dipole active due to the symmetry lowering induced by the missing row. In our spectra, this peak is hardly visible. A very weak peak is visible at 59 meV in-specular for Ee ¼ 3:0 eV (see Fig. 6) which is not detected out-of-specular. If we assume that this is the m4 mode, this would mean that the dynamic dipole moment is strongly screened. A high screening is in agreement with the geometry inferred by the XPD results, which predict a slightly subsurface location of the O-adatoms. This view is not supported by the theoretically calculated structure where the oxygen is higher on the surface [35]. The discrepancy in the structure can also explain the fact that the theoretically calculated frequency (49 meV) is lower than in experiment. Out-of-specular data are shown in Fig. 5 for the O528 and in Fig. 6 for the O530 phases. In the former case (Fig. 5), besides the O–Ag stretch at 30–32 meV, a very weak and broad feature is present at 69 meV, corresponding to the frustrated translation. The low intensity is due both to the small coverage and to non-optimal scattering conditions. Attempts to find a better scattering cross-section were however unsuccessful. The frequency range of the in-plane vibration is in-between the values predicted for the p(1 1) and the c(2 2) structures. pffiffiffi pffiffiffi In the latter case (2 2 2 reconstruction, Fig. 6), besides the O–Ag stretch at 39 meV, three modes are much more evident in the impact scattering regime: at 29, 67 and 83 meV. In the measurements the scattering plane is parallel to the [1 0 0] direction and since two reconstructed phase domains with the missing rows oriented along or across the scattering plane are present on the surface, we expect to see the oxygen vibrations both parallel and perpendicular to the missing
34
D. Loffreda et al. / Surface Science 530 (2003) 26–36 3.0
O528 o
Intensity (Arb. Units)
30
Ee=3.0 eV
67
2.5
off-specular Intensity (Arb. Units)
10
83
2.0 59
1.5
80
10 off x1000 -1 0.097 A
29
1.0
0 off 128
39
0.5
x1000
x250
0.0 3.0
E e (eV)
32
69
15.0 69
3.0
Intensity (Arb. Units)
2.5
0.04
0.06
20 off -1 0.44 A
2.0 x1000
10 off -1 0.20 A
1.0 x250
0.08
Energy loss (eV)
row. The 29 meV peak has been already discussed and is identified with the m6 mode (out-of-phase stretch) predicted at 25 meV. The 83 meV peak, observed only for Ee ¼ 3:0 eV, has a frequency close to the m1 and m2 modes. We assign it to the inphase oxygen vibration parallel to the missing rows, expected at 78 meV (m1 mode), since the outof-phase vibration (m2 mode) is odd with respect to the scattering plane and cannot be excited in this experimental configuration. Finally, we assign the 67 meV peak to the mode m3 predicted at 59 meV, i.e. to the in-phase oxygen vibration towards the missing rows. In fact, since the intensity of this mode is higher out-of-specular than in-specular
x1000 67
29
129
39
0 off
0.0 0.00
Fig. 5. HREEL spectra of the O–Ag(0 0 1) unreconstructed phase recorded 10 out-of-specular for different primary energies. At this scattering angle Ee ¼ 3:0 eV corresponds to a 1 , Ee ¼ 15:0 eV to parallel momentum transfer qk ¼ 0:097 A 1 . O528 was produced by exposing the crystal at qk ¼ 0:020 A 320 K to 5400 L (top spectrum) and 700 L (bottom spectrum) of O2 , respectively.
67.5
1.5
0.5
0.02
Ee=15.0 eV
0.04
0.08
0.12
Energy loss (eV) pffiffiffi pffiffiffi Fig. 6. HREEL spectra of the O–Ag(0 0 1) 2 2 2 missing row reconstructed surface in different experimental conditions. Panel A: Ee ¼ 3:0 eV; the upper spectrum, recorded 10 offspecular, corresponds to a parallel momentum transfer 1 . Panel B: Ee ¼ 15:0 eV; 10 off-specular correqk ¼ 0:097 A 1 and 20 off-specular to qk ¼ 0:044 sponds to qk ¼ 0:020 A 1 . The spectra are normalised with respect to the elastic peak A intensity of the corresponding in-specular spectrum.
(see Fig. 6), this mode is not dipole active. The presence of these three modes and the agreement of their frequencies with the theoretical calculations are a further indication that the missing row reconstruction is indeed present on the surface, as deduced from XPD measurements [6]. 7. Conclusions We have reported a LDA DPFT study of the vibrational modes of atomic oxygen interacting
D. Loffreda et al. / Surface Science 530 (2003) 26–36
with the Ag(0 0 1) surface and we have compared the results with HREELS experiments. The theoretical vibrational frequencies of oxygen in a subsurface hollow site show that this oxygen is not responsible for the 130 meV loss peak observed in the energy loss spectra. A study of the coverage dependence of the oxygen vibrational frequencies for an unreconstructed substrate shows that decreasing the coverage increases the vibrational O–Ag stretch frequency in agreement with the increased distance of oxygen from the surface. This trend is also in accord with experiment but the small dependence of the O–Ag stretch frequency on the adsorption geometry has not allowed us to discriminate between different structures using only the frequency of this mode. We made a complete theoretical analysis of the vibrational frequencies of oxygen adsorbed on the missing-row reconstructed surface. From the analysis of the frustrated translations, it is possible to gain information on the geometrical structure. Indeed, HREEL spectra show energy losses at 67 and 83 meV, which are excited in the impact scattering regime, in accord with theoretical predictions. The presence of these peaks and the agreement of their frequencies with the calculated ones give further support to the existence of a missing row reconstruction as deduced by XPD experiments. No strong assignment is possible, on the contrary, for the frustrated translation which should become visible in the dipole-scattering regime thanks to the symmetry breaking induced by the reconstruction, and predicted by theory to be at a frequency of 49 meV. We hope that this work will stimulate an STM investigation on this system which could provide complementary information on the structure and definitively confirm the presence of the missing row reconstruction.
Acknowledgements This work was sponsored by MURST (PRINCOFIN01), by INFM (Sezioni F e G and ‘‘Iniziativa Transversale calcolo parallelo’’). Calculations have been performed on the CRAY T3E
35
at CINECA in Casalecchio (Bologna) using the PWSCF package [43]. References [1] M.A. Akita, S. Hiramoto, N. Osaka, K. Itoh, J. Phys. Chem. B 103 (1999) 10189. [2] M.A. Akita, S. Hiramoto, N. Osaka, K. Itoh, Surf. Sci. 427–428 (1999) 374. [3] S. Dorel, S. Pesty, P. Garoche, Surf. Sci. 446 (2000) 294. [4] H.A. Engelhardt, D. Menzel, Surf. Sci. 57 (1976) 591. [5] C.S. Ares Fang, Surf. Sci. Lett. 253 (1990) L291–L294. [6] M. Rocca et al., Phys. Rev. B 61 (2000) 213. [7] L. Savio, L. Vattuone, M. Rocca, Phys. Rev. B 61 (2000) 7324. [8] G. Benedek, F. Buatier de Mongeot, U. Valbusa, M. Rocca, Europhys. Lett. 53 (2001) 544. [9] F. Buatier de Mongeot, M. Rocca, U. Valbusa, Surf. Sci. 363 (1996) 68. [10] F. Buatier de Mongeot, A. Cupolillo, U. Valbusa, M. Rocca, J. Chem. Phys. 106 (1997) 9297. [11] L. Vattuone, P. Gambardella, U. Valbusa, M. Rocca, Surf. Sci. 377–379 (1997) 671. [12] F. Buatier de Mongeot, A. Cupolillo, U. Valbusa, M. Rocca, Chem. Phys. Lett. 270 (1997) 345. [13] F. Buatier de Mongeot, A. Cupolillo, M. Rocca, U. Valbusa, Chem. Phys. Lett. 302 (1999) 302. [14] M.A. Barteau, R.J. Madix, Chem. Phys. Lett. 97 (1) (1983) 417. [15] C. Backx, C.P.M. de Groot, P. Biloen, Surf. Sci. 104 (1981) 300. [16] B. Pettinger, X. Bao, I.C. Wilcock, M. Muhler, G. Ertl, Phys. Rev. Lett. 72 (1994) 1561. [17] A. Raukema, D.A. Butler, A.W. Kleyn, J. Phys.: Condens. Matter 8 (1996) 2247. [18] A.A. Dzhurakhalov, V.Kh. Ferleger, S. Kakimov, Surf. Sci. 433–435 (1999) 188. [19] D. Sekiba, H. Nakamizo, R. Ozawa, Y. Gunji, H. Kukutani, Surf. Sci. 449 (2000) 111. [20] C.T. Campbell, Surf. Sci. 173 (1986) L641; C.T. Campbell, Surf. Sci. 157 (1985) 43. [21] F. Buatier de Mongeot, U. Valbusa, M. Rocca, Surf. Sci. 339 (1995) 291. [22] V.I. Bukhtiyarov, V.V. Kaichev, I.P. Prosvirin, J. Chem. Phys. 111 (1999) 2169. [23] K. Wu, X. Wei, Y. Cao, D. Wang, X. Guo, Catal. Lett. 26 (1994) 109. [24] G.J. Millar, M.L. Nelson, P.J.R. Uwins, Catal. Lett. 43 (1997) 97. [25] S.S. Nandra, J. Bandekar, A.L. Cholli, Appl. Spectrosc. 45 (1991) 1183. [26] P.A. Gravil, J.A. White, D.M. Bird, Surf. Sci. 352–354 (1996) 248. [27] P.A. Gravil, D.M. Bird, J.A. White, Phys. Rev. Lett. 77 (1996) 3933. [28] D.M. Bird, P.A. Gravil, Surf. Sci. 377–379 (1997) 555.
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Oxygen vibrations in O–Ag(0 0 1) David Loffreda a
a,1
, Andrea Dal Corso a,*, Stefano Baroni a, Letizia Savio Luca Vattuone b, Mario Rocca b
b,2
,
SISSA, Via Beirut 2/4, INFM-DEMOCRITOS National Simulation Center, 34014 Trieste, Italy b Dipartimento di Fisica – INFM, Unit a di Genova, Via Dodecaneso 33, 16146 Genova, Italy Received 4 September 2002; accepted for publication 2 March 2003
Abstract The vibrational modes of oxygen on Ag(0 0 1) are studied both theoretically, by density functional perturbation theory (DFPT) within the local density approximation, and experimentally, by means of high resolution electron energy loss spectroscopy (HREELS). The frequencies of the O–Ag stretch mode and of the parallel modes are calculated for the p(1 1), c(2 2), and p(2 2) geometries of oxygen on the non-reconstructed Ag(0 0 1) surface. Increasing the coverage the chemisorption energy as well as the O–Ag stretch frequency. The vibrational frequencies of O in pffiffiffi decreases pffiffiffi the 2 2 2 missing row reconstructed structure, the phase indicated by X-ray photoelectron diffraction as the low temperature phase of this system, are also investigated. The comparison of the DFPT calculations with the HREELS data gives further support to the existence of a missing row structure. 2003 Elsevier Science B.V. All rights reserved. Keywords: Density functional calculations; Chemisorption; Surface relaxation and reconstruction; Adatoms; Oxygen; Silver
1. Introduction Oxygen precovered silver surfaces present a great interest since they play the role of a precursor state for the epoxidation of ethylene [1,2], which is an important industrial catalytic reaction. For this reason, the interaction of oxygen with silver has been extensively studied experimentally and different states of oxygen at the surfaces have been
*
Corresponding author. Tel.: +39-40-378-7428; fax: +39-40378-7528. E-mail address: [email protected] (A. Dal Corso). 1 Present address: IRC-CNRS, 2 Avenue Albert Einstein, F-69626 Villeurbanne, France. 2 Present address: Institut fur Experimental Physik, Freie Universitaet Arnimalle 14, Berlin, Germany.
identified: a physisorbed molecular state, which is very close to the gas phase structure, chemisorbed molecular species, surface and subsurface atomic adsorption. High resolution electron energy loss spectroscopy (HREELS) has been extensively used to characterize these adsorption states together with low energy electron diffraction (LEED) [3] and other spectroscopic techniques. Actually, experimental studies exist for oxygen interacting with Ag(0 0 1) [4–14], Ag(1 1 0) [15–19] and Ag(1 1 1) [16,20–22] surfaces. However, the interpretation of the observed vibrational spectra and the determination of the nature of the adsorption state at given coverage and temperature are not always a simple task. Atomic oxygen adsorption on the Ag(0 0 1) surface is an interesting case, since the determination of
0039-6028/03/$ - see front matter 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0039-6028(03)00382-0
D. Loffreda et al. / Surface Science 530 (2003) 26–36
the oxygen coverage is difficult and the interpretation of the LEED patterns and of the HREEL spectra is still debated. Disordered adsorption into two states has been mentioned 25 years ago [4]; one state being populated at low temperature and the other one at high temperature. More precise HREELS and LEED measurements made by Fang suggested that an ordered c(2 2) superstructure occurs at 180 K, associated with a oxygen stretching mode at 37 meV [5]. Instead, adsorption at room temperature leads to an energy loss peak at 30 meV and to a p(1 1) LEED pattern. Furthermore, Fang reported a structural transition from the c(2 2) to the p(1 1) structures when heating the chemisorbed surface from low to room temperature, while the cooling of the chemisorbed surface from room temperature down to 180 K leads to a transition from the p(1 1) LEED pattern to the coexistence of both p(1 1) and c(2 2) structures. Both LEED patterns and HREELS peaks have been associated with atomic oxygen adsorption at fourfold hollow sites. More recently some of us presented a combined LEED, HREELS, X-rays photoemission spectroscopy (XPS) and photoelectron diffraction (XPD) study of the system [6]. We found that partial dissociation occurs already at a temperature T ¼ 130 K, at which we observe coexistence with molecular adsorption. The adatom–surface vibration reads then 30 meV. When heating the crystal to 190 K, it moves to 36 meV, while the O(1s) binding energy remains unchanged at 530.3 eV (O530 in the following). p XPD ffiffiffi pdata ffiffiffi have been interpreted by assuming a 2 2 2 missing row reconstruction with the oxygen sitting in the former fourfold hollows next to the missing rows. Upon heating above room temperature, the surface converts reversibly into a p(1 1) structure above the fourfold hollows. with O-adatoms 0.6 A The vibrational frequency reads between 28 and 32 meV, depending on coverage, while the O(1s) binding energy is 528.3 eV (O528 phase in the following). Subsurface incorporation of oxygen on Ag(0 0 1) could also occur [13]. Subsurface oxygen has been advocated in order to explain a peak at 130 meV in the HREEL spectra. Initially assigned to a subsurface oxygen vibration, this peak is now
27
thought to be due to an electronic excitation that becomes available thanks to the presence of a subsurface oxygen atoms [8]. The hypothesis of an electronic transition is supported also by the unusual shift from 131 to 134 meV [8,13] of the frequency with the isotopic substitution of 16 O with 18 O. In spite of such a thorough experimental investigation, few theoretical studies have been reported so far for adsorbed oxygen on silver surfaces [23–34], and especially on Ag(0 0 1). In a previous paper [35] some of us have studied within density functional theory [36,37] in the local density (LDA) and the generalised gradient (GGA) approximations, the geometry and energy of several phases of O adsorbed on Ag(0 0 1). In particular pffiffiffi we pffiffiffihave found that the low temperature 2 2 2 missing row reconstructed surface determined by the fitting of the XPD spectra, is competitive in energy with the adsorption on the non-reconstructed surface in the c(2 2) structure. It is the purpose of this paper to investigate the vibrational properties of the different phases of O on Ag(0 0 1) both theoretically and experimentally, with the main goal of identifying the signature of the missing row reconstruction in the vibrational spectra. The missing row reconstruction of the substrate lowers the C4v symmetry of the c(2 2) structure to C2v . The symmetry lowering brings about non-degenerate frustrated translations and makes the out of phase vibration of the oxygen atoms, in the direction parallel to the surface and perpendicular to the missing row, dipole active. The identification of this mode, which is totally symmetric with respect to the C2v group, would be a direct proof of the presence of the missing row reconstruction. Unfortunately, we find that it is hardly visible in HREELS experiments in the dipole scattering regime, indicating a small dynamic dipole moment. On the contrary, out-of-specular measurements (impact scattering regime) allow the detection of other frustrated translations, the frequencies of which agree well with the theoretical predictions for the missing row reconstructed substrate. Therefore our investigation further supports the interpretation of the XPD data. Furthermore, we investigated the vibrational modes of oxygen adsorbed on the hollow site of
28
D. Loffreda et al. / Surface Science 530 (2003) 26–36
the non reconstructed surface. Since the oxygen coverage on the non-reconstructed surface is not known precisely, we calculated the dynamics of the p(1 1), the c(2 2) and the p(2 2) structures at coverage 1, 0.5 and 0.25 ML respectively. This paper is organized as follows. In Section 2 we report the methods used for the calculation of the vibrational frequencies; in Section 3 we describe the experimental apparatus. In Section 4 we summarize the main theoretical results regarding the structure and energy of the different phases of O on Ag(0 0 1), in Section 5 we discuss the vibrational properties and in Section 6 we compare them with experiment. Finally, Section 7 contains our conclusions.
2. Computational details The electronic structure calculations discussed here have been carried out within the LDA with the parameterisation of Perdew and Zunger [38]. The ab initio vibrational frequencies have been computed in the framework of density-functional perturbation theory (DFPT) [39–42], as implemented in the PWSCF and PHONON package [43]. Our calculations of the vibrational frequencies are limited to the LDA since, as shown in the previous paper [35] there are not major structural differences if calculations are performed at the GGA level, with the exception of the p(1 1)-O phase for which only LDA gives a stable surface hollow site. We used a plane-wave basis and Vanderbilt ultrasoft pseudopotentials [44] as described in [35]. Kinetic energy cut-offs are 30 Ry for the wave functions and 300 Ry for the charge density. The surfaces have been modelled by 7 layer slabs and an oxygen adlayer on one side of the slab, with four different periodic structures and various coverages (Fig. 1): a full coverage structure p(1 1)-O (1.0 ML), two mediumpcoverage ffiffiffi pffiffiffi structures c(2 2)-O (0.5 ML) and a 2 2 2-2O (0.5 ML), and one low coverage cell p(2 2)-O (0.25 ML). A vacuum of four equivalent ideal bulk layers separates one slab from its periodic image in the direction perpendicular to the surface. The bulk lattice parameter calculated at the LDA level, , has been used as the inand optimised to 4.018 A
Fig. 1. Periodic adsorption structures for O atom on Ag(0 0 1) surface; for the non-reconstructed surface, we show the full coverage p(1 1)-O (1.0 ML), the medium coverage c(2 2)-O (0.5 ML) and the low coverage p(2 2)-O (0.25 ML) structures. For surface, we show the medium coverage pffiffiffi thepreconstructed ffiffiffi 2 2 2-2O (0.5 ML) structure (the oxygen atoms are presented in small dark balls and the Ag atoms close to the missing-row are denoted Agm , while the other surface Ag atoms are denoted Agi ).
plane lattice spacing. The Brillouin zone (BZ) integration has been done with a 12 12 1 Monkhorst–Pack mesh [45] for the p(1 1) cells, corresponding to 21 irreducible k-points in the surface BZ. For thepc(2 ffiffiffi p2) ffiffiffi structure, the p(2 2) structure and the 2 2 2 structure we have used 7 7 1 (10 irreducible k-points), 5 5 1 (6 irreducible k-points) and 3 5 1 (6 irreducible kpoints) meshes respectively. Integration up to the Fermi energy is done with the smearing technique [46] with a smearing parameter of 0.04 Ry. The silver atoms of the six uppermost surface layers have been fully relaxed together with all the oxygen atoms. The minimization of the total energy and the optimisation of the geometries have been performed with the BFGS algorithm [47]. Residual forces do not exceed 104 Ry d/a.u. (3 103 eV/ ). Considering the fact that the highly coordiA nated fourfold hollow site is the most stable one [35], all the optimised structures have been built with oxygen atoms in the fourfold hollow positions. The vibrational frequencies of oxygen adsorbed on Ag(0 0 1) have been calculated by considering extended slabs of more than 75 layers. The interatomic force constants of these slabs have been
D. Loffreda et al. / Surface Science 530 (2003) 26–36
obtained from ab initio phonon calculations in the bulk Ag and from the dynamical matrices of 7 layer slabs. The vibrational modes of the bulk and of the 7 layer slabs have been calculated with DFPT [39–41]. This theory, as implemented in our codes for ultrasoft pseudopotentials, has been described in [42]. For the Ag bulk, the dynamical matrices have been computed in a 4 4 4 q-point mesh in the BZ and a Fourier transform has been used to obtain the interatomic force constants. For the slabs, only parts of the dynamical matrices at the C point have been calculated, including the displacements of the two uppermost Ag layers and O. With this information we have calculated the dynamical matrix at the C point of a finite extended symmetric slab with O adsorbed at both surfaces. We have calculated the total vibrational density of states (DOS) and we have identified the vibrational modes of oxygen from the displacement eigenvectors. The convergence of the frequencies and normal modes has been checked as a function of the slab thickness. About 20 layers are sufficient to reach complete convergence, but we have used much thicker slabs of more than 75 layers in order to obtain a smooth DOS curve calculated as a sum of Gaussian functions with a spreading of 5 cm1 . In the p(1 1)-O hollow structure, we have also calculated the interatomic force constants of the 7 layer slab via frozen phonon calculations with . A comdisplacements between )0.03 and 0.03 A parison between this technique and DFPT showed that the larger difference between the interatomic force constants calculated with the two methods is 2 ), indicating also 4 104 Ry/(a.u.)2 (0.02 eV/A the estimated accuracy of our calculations.
3. Experimental The adsorption experiments were carried out in an ultrahigh vacuum chamber equipped with a commercial HREEL spectrometer (SPECS, IbachÕs design), a LEED and all other typical vacuum facilities. The sample is a Ag monocrystal cut within 0.1 from the (0 0 1) plane and oriented with the [1 0 0] direction parallel to the scattering plane of the spectrometer. The surface was cleaned
29
by sputtering and annealing to 700 K before each experiment, until no trace of contaminants was detected in the HREEL spectrum. O2 was dosed through a shower placed in front of the sample, at a distance of 1 cm, in order to keep the background pressure below 2 107 mbar, while the effective pressure at the surface is at least one order of magnitude larger. HREEL spectra were recorded with primary electron energies Ee ¼ 3:0 and 15.0 eV and with an angle of incidence hi ¼ 60. The off-specular condition was obtained by rotating the analyser towards smaller scattering angles. The typical resolution achieved in-specular is 4 meV.
4. Energy and stability The stability of adsorbed atomic oxygen on Ag(0 0 1) and the geometrical structures of several phases have been presented in a previous paper [35]. Here we summarize the main results of [35] and add a few comments that will be relevant for the following discussion of the vibrational properties. The highly coordinated fourfold hollow site is the preferred adsorption site [35], and we summarize in Table 1 the chemisorption energies and the optimised geometries for various coverages. For the full coverage p(1 1)-O 1.0 ML structure, two different adsorption sites have been reported here: the surface fourfold hollow and the subsurface fivefold hollow. As we reported previously, the surface hollow is the most stable site [35]. The diffusion energy barrier between the surface and subsurface hollow sites has been estimated to 25 meV. At the full coverage, the diffusion of atomic oxygen from the surface hollow site to the subsurface one is therefore very easy. In the diffusion transition state between surface and subsurface sites, the oxygen lies in the Ag surface plane. For the medium coverage 0.5 ML, the two possible adsorption structures, compatible with the LEED pattern that we have considered are the c(2 pffiffi ffi 2)-O pffiffiffi and the missing-row reconstructed 2 2 2-2O structures. In Table 1, the chemisorption energy is reported for both structures. For the missing-row reconstructed structure the chemisorption energy is larger ()1.31 eV/O atom)
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D. Loffreda et al. / Surface Science 530 (2003) 26–36
Table 1 Chemisorption energy Echem per oxygen atom (eV), and optimised geometric parameters Coverage (ML) Echem (eV) ) dO–Ag (A ) DzO–Ag (A
p(1 1)
p(1 1)-Sub
c(2 2)
p(2 2)
1.0 )0.29 2.06 0.47
1.0 )0.27 2.05 )0.42
0.5 )1.11 2.15 0.75
0.25 )1.16 2.20 0.8
pffiffiffi pffiffiffi 2 2 2
0.5 )1.31 2.06–2.04 0.26 pffiffiffi pffiffiffi Full coverage structure p(1 1) (surface and subsurface hollow sites), the medium coverage structures c(2 2) hollow and 2 2 2 hollow, and the low coverage structure p(2 2) hollow; dO–Ag (A) is the distance between the oxygen atom and each equivalent surface ) is the distance between O and the mean position of the surface silver plane. silver atom, DzO–Ag (A
than the one for the c(2 2) configuration ()1.11 eV). However a detailed energy balance [35] shows that the reconstruction energy is very small. Therefore these two structures could both be observed at a coverage of 0.5 ML. The optimised geometries are somewhat different. For the c(2 2) is longer than for the case, the dO–Ag ¼ 2:15 A pffiffiffi pffiffiffi missing-row reconstructed 2 2 2 structure ), and also longer than the surface (2.04–2.06 A pffiffiffi ). For the 2 2 hollow p(1 1) situation (2.06 A pffiffiffi 2 structure the surface silver atoms are not equivalent; the Ag atoms along the missing row ), (Agm ) are closer to the oxygen atoms (2.04 A while the Ag atoms inside the surface layer (Agi ) lie above the medium surface plane and the O–Agi ). As the coverage distances are longer (2.06 A decreases at 0.25 ML, the adsorption energy becomes slightly stronger ()1.16 eV) and the O–Ag optimised distance is once more elongated (2.20 ). The structure determined experimentally by A fitting the XPD data is slightly different, since the lower oxygen atoms in the missing rows are 0.15 A than the neighbouring Ag atoms [6].
Fig. 2. Total DOS calculated within LDA. For the full coverage 1.0 ML p(1 1) non-reconstructed surface we present the case of oxygen adsorbed (a) on a surface hollow site and (b) on a subsurface hollow site. (c) DOS of the medium coverage 0.5 ML c(2 2) non-reconstructed surface with O adsorbed on the hollow site, (d) low coverage 0.25 ML p(2 2) non-reconstructed surface with O adsorbed pffiffiffi on pffiffiffi the hollow site, and (e) medium coverage 0.5 ML 2 2 2-2O missing-row reconstructed surface with O atoms adsorbed on hollow sites.
5. Vibrational modes of O on Ag(0 0 1) In this section we discuss the vibrational modes of the different phases of oxygen on Ag(0 0 1) found in the calculation. We start by discussing the vibrational modes on the non-reconstructed surface as a function of coverage and then compare these modes with those obtained for O on the missing-row reconstructed structure. In the highcoverage p(1 1)-O (1.0 ML) model structure, in LDA, both the surface and subsurface hollow sites are stable, as discussed above. In Fig. 2a, we show
the total density of vibrational states (DOS) of a 151 layer slab. The vibrational modes of the metallic surface are visible and give a continuous DOS between 0 and 180 cm1 . Furthermore, there are three modes with frequencies higher than 180 cm1 which is the maximum frequency for the LDA Ag bulk modes. The frequencies of these modes are reported in Table 2: two modes are degenerate and a mode is non-degenerate. The vibrational eigenvectors clearly identify these modes: the two degenerate modes are in-plane
D. Loffreda et al. / Surface Science 530 (2003) 26–36 Table 2 Vibrational frequencies (cm1 or meV) m1 m2 m3 m4 m5 m6
p(1 · 1)
p(1 · 1)-Sub
c(2 · 2)
p(2 · 2)
611 (76) 611 (76) 210 (26)
647 (80) 647 (80) 225 (28)
438 (55) 438 (55) 279 (35)
338 (42) 338 (42) 306 (38)
pffiffiffi pffiffiffi 2 2· 2 627 616 479 392 246 201
(78) (76) (59) (49) (30) (25)
Values in brackets denote frequencies in meV. Full coverage structure p(1 · 1) (surface and subsurfacephollow ffiffiffi pffiffiffi sites), the medium coverage structures c(2 · 2) and 2 2 · 2 hollow, and the low coverage structure p(2 · 2) hollow; we called m1 and m2 the in plane oxygen frustrated translational frequencies and m3 the oxygen-surface silver stretching frequencies pffiffiffi pffiffiffifor the p(1 · 1), c(2 · 2) and p(2 · 2) structures. For the 2 2 · 2 structure, m1 to m4 are the frequencies of the oxygen frustrated translational modes and m5 , m6 are the oxygen silver surface stretching frequencies. The experimental values are discussed in the text.
oxygen frustrated translations m1 and m2 at 611 cm1 (76 meV) and the other mode is the perpendicular vibration of oxygen against the surface m3 at 210 cm1 (26 meV). The frustrated translations are harder than the perpendicular motion since the oxygen atom sits in a hollow site close to the surface silver atoms (with an oxygen height of 0.47 ). The m3 frequency at 210 cm1 (26 meV) is not A to be compared with any experimental HREEL spectra, since the highest experimentally coverage is about 0.4 ML. The DOS for the Ag(0 0 1)–O system with subsurface hollow oxygen absorption, is shown in Fig. 2b: here the doubly degenerate frustrated translations m1 and m2 at 647 cm1 (80 meV) (see Table 2) are only slightly higher than the ones of the surface hollow case. The stretching oxygen–silver mode m3 is at 225 cm1 (28 meV). This result gives an estimate for the frequency one should expect for a vibration of an oxygen in the subsurface hollow site. Even if in this model system the concentration of subsurface oxygen is extremely high and a clear comparison with experiment is difficult, this result leads to the conclusion that the experimental peak at 130 meV [13] has indeed another origin. Frequencies which can be compared to the experimental HREELS peaks are instead found for the medium c(2 2)-O (0.5 ML) and the low p(2 2)-O (0.25 ML) coverage structures in the
31
non-reconstructed Ag(0 0 1) surface. In Fig. 2c and d we report the vibrational DOS for these two structures. In the c(2 2) structure the DOS due to the Ag(0 0 1) slab is comparable to the sum of the C and M point DOS of the p(1 1) structure, while in the p(2 2) structure (Fig. 2d), the DOS can be compared with the sum of the DOS of the phonons at the C þ 2X þ M points of the p(1 1) BZ. In addition to these Ag modes, in Fig. 2c we can clearly identify three additional peaks. The low frequency peak at 38 cm1 (5 meV) is the Rayleigh mode vibration. The experimental value of the Rayleigh frequency at the M point of the clean surface is 92 cm1 (11.5 meV) 3 and therefore it is strongly weakened in the c(2 2) structure. The weakening of the Rayleigh mode frequency can be explained mainly with the weakening of the Ag bonds between the first and the second layers due to the charge transfer to the electronegative oxygen atoms. In Table 2 we report the frequencies of the two peaks which correspond to the oxygen vibrational modes for the c(2 2) and the p(2 2) structures. The frustrated translational modes m1 and m2 become softer, 438 and 338 cm1 (55 and 42 meV) respectively for 0.5 and 0.25 ML (Fig. 2c and d), and the stretch frequency m3 of oxygen against the surface increases to 279 and to 306 (35 and 38 meV respectively). As the coverage decreases the O–Ag distance increases and the height of the oxygen atom on the surface increases. Therefore the trend in the frequencies is in agreement with what one expects just projecting the Ag–O interatomic forces in the direction parallel or perpendicular to the surface. Let us now consider the effect of the missingrow reconstruction on the vibrational frequencies of O on Ag(0 0p 1).ffiffiffi We pffiffiffishow in Fig. 2e the total DOS for the 2 2 2-2O structure. The DOS due to the Ag surface is comparable with the sum of the C þ 2M 1=2 þ M point of the p(1 1) surface (we called M 1=2 the point 2p=a0 ð0:25; 0:25Þ). Other six modes are visible above the bulk states. The vibrational frequencies of these modes are
3 According to Bunjes et al. [48]. It is 83 cm1 according to Chen et al. [49].
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D. Loffreda et al. / Surface Science 530 (2003) 26–36
c(2 2)) but, due to the missing silver atom in the missing row it is much softer than the m1 and m2 modes of the full coverage p(1 1) structure. The other two modes m1 and m2 at 627 and 616 cm1 (Fig. 3a and b) correspond to in-phase and out-ofphase vibrations of the oxygen atoms in the direction parallel to the missing row, respectively. These frequencies are comparable with the ones calculated for the high coverage 1.0 ML structure, since the oxygen atoms lie close to the surface plane in both cases. Oxygen stretching against the surface gives rise to two separate frequencies since the two oxygen atoms can oscillate in-phase or out-of-phase. The first case corresponds to the frequency m5 at 246 cm1 (30 meV) (Fig. 3e). The out of phase vibration corresponds instead to the frequency m6 at 201 cm1 (25 meV) (Fig. 3f).
6. Comparison with experiment
Fig. 3. Schematic description of the six oxygen vibrational normal modes; four frustrated translation modes m1 to m4 respectively at (a) 627, (b) 616, (c) 479 and (d) 392 cm1 and two stretching O–Ag silver modes m5 and m6 at (e) 246 and (f) 201 cm1 .
reported in Table 2. The analysis of the vibrational eigenvectors (see Fig. 3) shows that these modes are vibrations of the two oxygen atoms contained in the unit cell. Four modes (from m1 to m4 ) are frustrated translations at 627, 616, 479 and 392 cm1 (78, 76, 59 and 49 meV) respectively and two modes are the perpendicular vibration of oxygen against the surface: m5 and m6 at 246 and 201 cm1 (30 and 25 meV) respectively. The softer frustrated translation m4 at 392 cm1 (49 meV) corresponds to an out-of-phase vibration of both oxygen atoms towards the surface silver missing row (Fig. 3d). The corresponding in-phase frustrated translation mode m3 at 479 cm1 (59 meV) (Fig. 3c) is harder than the non-reconstructed c(2 2) case, since the oxygen atoms sit closer to the surface pffiffisilver ffi pffiffiplane ffi respectively for 2 2 2 and (0.26 and 0.75 A
pffiffiffi pffiffiffi In-specular HREEL spectra for the 2 2 2 and for the high temperature p(1 1) phases are reported in Fig. 4. The O530 layer (lower spectrum) was produced by dosing 75 L of oxygen at 87 K and annealing to 190 K. The surface was then cooled down to 90 K to record the spectrum. The O528 layer was produced by exposing the crystal to 5400 L of O2 at T ¼ 320 K. The spectra look quite different: a single, relatively broad, peak at 31 meV has present for O528, while three modes at 29, 39 and 67 meV are observed for O530 (the peak visible at 130 meV is not due to a vibrational exitation [8] and is not discussed in the following). The features at 31 meV (O528) and 39 meV (O530) are dipole active and correspond to the O–Ag stretch vibration for the two phases, respectively. We note that the frequency of 39 meV obtained for the O530 phase is at variance with that of 36 meV reported in [6] for the same surface, probably due to the different coverage. The prediction of the theory for the O530-Ag stretch is 30 meV, lower than the measured one, while it varies from 26 to 38 meV for the O528 depending on the coverage. According to our estimates the coverage in this experiment is quite low and therefore the 31 meV peak has to be compared with the O–Ag stretch calculated for the c(2 2) (35 meV) or the p(2 2)
D. Loffreda et al. / Surface Science 530 (2003) 26–36 2.0
31
E e=15.0 eV O528
1.5
Intensity (Arb. Units)
x1000 39 1.0
O530 0.5
29 67
130
x250 0.0 0.00
0.04
0.08
0.12
Energy loss (eV) Fig. 4. In-specular HREEL spectra of the missing row reconstructed (bottom spectrum) and of the unreconstructed oxygen phases (top spectrum). We note the presence of a peak at 130 meV for O530, the nature of which is discussed in the text.
(38 meV) structures. Therefore, the quantitative comparison of the O–Ag stretch frequencies shows only a fair agreement. Slight discrepancies were also found in [35] comparing the theoretical geometry with the one determined by the experimental XPD data. Possible reasons for these discrepancies are discussed in more detail in [35] and will require further investigations, possibly with the inclusion of additional effects (such as temperature or disorder) in the calculation. The variation of the O–Ag stretch frequency with the coverage is instead qualitatively correct since in experiments in which the O528 was produced by dosing at low temperature and annealing, we actually found lower stretch frequencies the higher the initial coverage. In-specular, the spectra show also two additional peaks (29 and 67 meV) for the O530 phase that are not present in the O528 phase. The theory
33
predicts a doublet in the low energy range for the reconstructed surface and the 29 meV peak is explained as the out of phase stretch of oxygen against the surface. The frequency of this mode is calculated at 25 meV. The 67 meV is not dipole active and is well explained by theory, as we will discuss below. On the Cu(0 0 1) the presence of the oxygen induced missing row reconstruction can be inferred from the HREEL specular spectra [50]. The out-of-phase in-plane vibration of the oxygen atoms perpendicular to the missing row (m4 mode) becomes dipole active due to the symmetry lowering induced by the missing row. In our spectra, this peak is hardly visible. A very weak peak is visible at 59 meV in-specular for Ee ¼ 3:0 eV (see Fig. 6) which is not detected out-of-specular. If we assume that this is the m4 mode, this would mean that the dynamic dipole moment is strongly screened. A high screening is in agreement with the geometry inferred by the XPD results, which predict a slightly subsurface location of the O-adatoms. This view is not supported by the theoretically calculated structure where the oxygen is higher on the surface [35]. The discrepancy in the structure can also explain the fact that the theoretically calculated frequency (49 meV) is lower than in experiment. Out-of-specular data are shown in Fig. 5 for the O528 and in Fig. 6 for the O530 phases. In the former case (Fig. 5), besides the O–Ag stretch at 30–32 meV, a very weak and broad feature is present at 69 meV, corresponding to the frustrated translation. The low intensity is due both to the small coverage and to non-optimal scattering conditions. Attempts to find a better scattering cross-section were however unsuccessful. The frequency range of the in-plane vibration is in-between the values predicted for the p(1 1) and the c(2 2) structures. pffiffiffi pffiffiffi In the latter case (2 2 2 reconstruction, Fig. 6), besides the O–Ag stretch at 39 meV, three modes are much more evident in the impact scattering regime: at 29, 67 and 83 meV. In the measurements the scattering plane is parallel to the [1 0 0] direction and since two reconstructed phase domains with the missing rows oriented along or across the scattering plane are present on the surface, we expect to see the oxygen vibrations both parallel and perpendicular to the missing
34
D. Loffreda et al. / Surface Science 530 (2003) 26–36 3.0
O528 o
Intensity (Arb. Units)
30
Ee=3.0 eV
67
2.5
off-specular Intensity (Arb. Units)
10
83
2.0 59
1.5
80
10 off x1000 -1 0.097 A
29
1.0
0 off 128
39
0.5
x1000
x250
0.0 3.0
E e (eV)
32
69
15.0 69
3.0
Intensity (Arb. Units)
2.5
0.04
0.06
20 off -1 0.44 A
2.0 x1000
10 off -1 0.20 A
1.0 x250
0.08
Energy loss (eV)
row. The 29 meV peak has been already discussed and is identified with the m6 mode (out-of-phase stretch) predicted at 25 meV. The 83 meV peak, observed only for Ee ¼ 3:0 eV, has a frequency close to the m1 and m2 modes. We assign it to the inphase oxygen vibration parallel to the missing rows, expected at 78 meV (m1 mode), since the outof-phase vibration (m2 mode) is odd with respect to the scattering plane and cannot be excited in this experimental configuration. Finally, we assign the 67 meV peak to the mode m3 predicted at 59 meV, i.e. to the in-phase oxygen vibration towards the missing rows. In fact, since the intensity of this mode is higher out-of-specular than in-specular
x1000 67
29
129
39
0 off
0.0 0.00
Fig. 5. HREEL spectra of the O–Ag(0 0 1) unreconstructed phase recorded 10 out-of-specular for different primary energies. At this scattering angle Ee ¼ 3:0 eV corresponds to a 1 , Ee ¼ 15:0 eV to parallel momentum transfer qk ¼ 0:097 A 1 . O528 was produced by exposing the crystal at qk ¼ 0:020 A 320 K to 5400 L (top spectrum) and 700 L (bottom spectrum) of O2 , respectively.
67.5
1.5
0.5
0.02
Ee=15.0 eV
0.04
0.08
0.12
Energy loss (eV) pffiffiffi pffiffiffi Fig. 6. HREEL spectra of the O–Ag(0 0 1) 2 2 2 missing row reconstructed surface in different experimental conditions. Panel A: Ee ¼ 3:0 eV; the upper spectrum, recorded 10 offspecular, corresponds to a parallel momentum transfer 1 . Panel B: Ee ¼ 15:0 eV; 10 off-specular correqk ¼ 0:097 A 1 and 20 off-specular to qk ¼ 0:044 sponds to qk ¼ 0:020 A 1 . The spectra are normalised with respect to the elastic peak A intensity of the corresponding in-specular spectrum.
(see Fig. 6), this mode is not dipole active. The presence of these three modes and the agreement of their frequencies with the theoretical calculations are a further indication that the missing row reconstruction is indeed present on the surface, as deduced from XPD measurements [6]. 7. Conclusions We have reported a LDA DPFT study of the vibrational modes of atomic oxygen interacting
D. Loffreda et al. / Surface Science 530 (2003) 26–36
with the Ag(0 0 1) surface and we have compared the results with HREELS experiments. The theoretical vibrational frequencies of oxygen in a subsurface hollow site show that this oxygen is not responsible for the 130 meV loss peak observed in the energy loss spectra. A study of the coverage dependence of the oxygen vibrational frequencies for an unreconstructed substrate shows that decreasing the coverage increases the vibrational O–Ag stretch frequency in agreement with the increased distance of oxygen from the surface. This trend is also in accord with experiment but the small dependence of the O–Ag stretch frequency on the adsorption geometry has not allowed us to discriminate between different structures using only the frequency of this mode. We made a complete theoretical analysis of the vibrational frequencies of oxygen adsorbed on the missing-row reconstructed surface. From the analysis of the frustrated translations, it is possible to gain information on the geometrical structure. Indeed, HREEL spectra show energy losses at 67 and 83 meV, which are excited in the impact scattering regime, in accord with theoretical predictions. The presence of these peaks and the agreement of their frequencies with the calculated ones give further support to the existence of a missing row reconstruction as deduced by XPD experiments. No strong assignment is possible, on the contrary, for the frustrated translation which should become visible in the dipole-scattering regime thanks to the symmetry breaking induced by the reconstruction, and predicted by theory to be at a frequency of 49 meV. We hope that this work will stimulate an STM investigation on this system which could provide complementary information on the structure and definitively confirm the presence of the missing row reconstruction.
Acknowledgements This work was sponsored by MURST (PRINCOFIN01), by INFM (Sezioni F e G and ‘‘Iniziativa Transversale calcolo parallelo’’). Calculations have been performed on the CRAY T3E
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